Introductory Botany BOT 1010
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This 59 page Class Notes was uploaded by Magnus Ankunding on Monday October 12, 2015. The Class Notes belongs to BOT 1010 at Florida International University taught by Suzanne Koptur in Fall. Since its upload, it has received 50 views. For similar materials see /class/221740/bot-1010-florida-international-university in Botany at Florida International University.
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Date Created: 10/12/15
Pellicle Eyespot Co ntractilevacuole FlagellarGroove RedTide Plasmodium Phagocytosis Cellulose Spora ngium Ca rote nes Xa ntho phylls Phycobilin WhiplashFlagellum TinselFlagellum Hete rokont Chrysolaminarin Fucoxa nthin Frustule Silica Zoos pore Antheridium Oogonium Cell membrane in Euglenas A small pigmented structure in flagellate unicellular organisms that is sensitiveto light also called a stigma A clear fluidfilled vacuole in some groups of protists that takes up water within the cell and then contracts expelling its contents from the cell A groove containing the flagella in some protists Seawater that is discolored by large numbers of certain dinoflagellates that produce saxitoxin Stage in life cycle of myxomycetes a multinucleate mass of protoplasm surrounded by a membrane The uptake of materials into a cell by invagination The chief component of the cell wall in plants and some protists A hollow unicellular or multicellular structure in which spores are produced A yellow or orange pigment belonging to the carotenoid group A yellow chloroplast pigment a member of the carotenoid class A group of watersoluble accessory pigments including phycocyanins and phycoerythrins which occur in the red algaae and cyanobacteria Shorter smooth flagellum found on heterokonts Long ornamented flagellum found on heterokonts Organisms with one tinsel flagellum and one whiplash flagellum includes oomycetes chrysophytes diatoms brown algae and certain other groups The storage product of the chrysophytes and diatoms A brownish carotenoid found in brown algae and chrysophytes Two parts of diatom cells that fit together like a shoebox Compound of which cell walls of diatoms are made A motile spore found amont algae oomycetes and chytrids A spermproducing structure that may be multicellular or unicellular A unicellular female sex organ that contains one or several eggs Oospore Coenocytic Algin ZygoticMeiosis GameticMeiosis SporicMeiosis Isomorphic Heteromorphic Sporophyte Gametophyte Phragmoplast Desmid Matrotro phic Embryophyte Archegonium The thickwalled zygote characteristic of the oomycetes A term used to describe an organism or part of an organism that is multinucleate the nuclei not separated by walls or membranes An important polysaccharide component of brown algal cell walls used as a stabilizer and emulsifier for some foods and for paint Meiosis by a zygote to form four haploid cells which divide by mitosis to produce either more haploid cells or a multicellular individual that eventually gives rise to gametes Meiosis resulting in the formation of haploid gametes from a diploid individual the gametes fuse to form a diploid zygote that divides to form another diploid individual Meiosis resulting in the formation of haploid spores by a diploid individual or sporophyte the spores give rise to haploid individuals or gametophytes which eventually produce gametes that fuse to form diploid zygotes the zygotes in turn develop into sporophytes this kind of life cycle is known as alternation of generations A term used to describe a life cycle in which the haploid and diploid generations are identical in form A term used to describe a life cycle in which the haploid and diploid generations are dissimilar in form The sporeproducing diploid 2n phase in a life cycle characterized by alternation of generations In plants that have an alternation of generations the haploid n gamete producing generation or phase A spindleshaped system of fibrils which arises between two daughter nuclei at telophase and within which the cell plate is formed during cell division or cytokinesis found in all green algae except the members of the class Chlorophyceae and in plants Freshwater green algae Gains nutrients through the maternal gametophyte The bryophytes and vascular plants both of which produce embryos A multicellular structure in which a single egg is produced found in the bryophytes and some vascular plants Protonema Capsule Stomata Cuticle Hydroid VascularTissue Lignin Homosporous Heterosporous Prostele Siphonostele Eustele Xylem Phloem Pith Enation Micro phyll Mega phyll Embryo So rus The first stage in development of the gametophyte of mosses and certain liverworts The sporangium of bryophytes A minute opening bordered by guard cells in the epidermis of leaves and stems through which gases pass Waxy or fatty layer on outer wall of epidermal cells formed of cutin and wax The waterconducting cells of the moss hadrom Tissue that conducts water and nutrients through the plant body in higher plants One of the most important consituents of the secondary wall of vascular plants although not all secondary walls contain lignin Having only one kind of spore Having two kinds of spores 39 39U 39 as 39 r and g r The simplest type of stele consisting of a solid column of vascular tissue A type of stele containing a hollow cylinder of vascular tissue surrounding a pith A stele in which the primary vascular tissues are arranged in discrete strands around a pith typical of gymnosperms and angiosperms A complex vascular tissue through which most of the water and minerals of a plant are conducted The foodconducting tissue of vascular plants which is composed of sieve elements various kinds of parenchyma cells fibers and sclerids The ground tissue occupying the center of the stem or root within the vascular cylinder A natural projection or outgrowth from a plant body or organ A small leaf with one vein and one leaf trace not associated with either a leaf gap or leaftrace gap A generally large leaf with several to many veins its leaf trace or traces isare associated with a leafgap in ferns and a leaftrace gap in seed plants A young sporophytic plant before the start of a period of rapid growth A group or cluster of sporangia or spores Prothallus Seed Progym nos perm Micros pore Megaspore Strobilus Pollen OvuliferousScale Bract Integument Micropyle Megaspo rocyte Microsporangium Micros poro phyll Cotyledon Seedcoat In homosporous vascular plants such as ferns the more or less independent photosynthetic gametophyte A structure formed by the maturation of the ovule of seed plants following fertilization An ancestral fossil type from which modern gymnosperms are thought to have derived ln heterosporous plants a spre that develops into a male gametophyte ln heterosporous plants a haploid spore that develops into a female gametophyte A reproductive structure consisting of a number of modified leaves or ovule bearing scales grouped terminally on a stem The fine spores that contain male gametes and that are borne by an anther in a flowering plant In certain conifers the appendage or scalelike shoot to which the ovule is attached A modified usually reduced leaflike structure The outermost layer or layers of tissue enveloping the nucellus of an ovule develops into the seed coat In the ovules of seed plants the opening in the integuments through which the pollen tube usually enters A diploid cell in which meiosis will occur resulting in the production of four megaspores A sporangium within which microspores are formed A leaflike organ bearing one or more microsporangia Seed leaf generally absorbs food in monocotyledons and stores food in other angiosperms The outer layer of the seed developed from the integuments of the ovule Hybrid 39239 In general usage hybrid is synonymous with heterozygous any offspring resulting from the mating of two genetically distinct individuals Monohybrids 39239 A cross between two varieties that differ in only one trait 39239 A monohybrid cross is a breeding experiment between P generation parental generation organisms that differ in one trait 0 o 0 o o o 000 00 O 0 O Gregog Mendel Austrian monk conducted experiments in his garden to study inheritance of characters in the garden pea 185060 s from his results Mendel predicted the existence of genes and their occurrence in pairs in organisms Mendel studied selected traits in peas that didn t blend by making crosses handpollinations These were traits controlled by single genes Developed Laws of Inheritance Mendel made crosses between plants with the same or different traits then selfed the progeny of these crosses P F1 F2 parental first and second generations Chose common garden pea PisumSativum Peas are fast and easy to grow Segregating 39239 Separating out in the offspring of the hybrid generation CrossFertilization SelfFeItiliza on 39239 A experimenter can transfer pollen from two difference variety of peas that has different traits to the stigma the receptive part of the carpel v Normally occurs 1f the ower ls um dlsmrbed by of the ower whose anthers were removed This process is called poumatlon crosspollination results in crossfertilization Dominant amp Recessive F1 Generation rst filial generation 39239 Mendel referred to the trait expressed in the F1 pea plants as dominant and the unexpressed form of the trait recessive 39239 Filius is Latin for son and filia is Lation for daughter 39239 Essentially the dominant plant is the plant when crossed that 39239 The resulting hybrid offspring are referred to as the F1 produces more in the next generation and the plant that produces less generation is the recessive form Punnett Squares Diagram what happens in formation of gametes haploid egg and sperm then in fusion of gametes to form diploid progeny This example two homozygotes mate to produce heterozygote Fl39 the heterozygotes cross to produce genotypes in ratio 121 phenotypes 31 Named after its founder Reginald Punnett Alleles 6390 4 fp Ii 5p pp 4 P a 0 Figure 144 Planl Biology We 7 P in PD PP 39 5 F generation F P PP PJ f f n Pp up PP L p w 7 F2 generation 2006 Pearson Education 3 An allele is an alternative form of a gene one member of a pair that is located at a speci c position on a speci c chromosome These DNA coding s determine distinct traits that can be passed on from parents to offspring The process by which alleles are transmitted was discovered by Gregor Mendel and formulated in what is known as Mendel s law of segregation 9 99 9 99 F2 Generation second f11ial generation Mendel allowed the pea plants in his F1 generation to selfpollinate and then planted the seeds to see which form of the traits would appear in F2 For each trait some of the F2 individuals exhibited the recessive form of the trait In F2 generations about 3A of the offspring had the dominant form of the trait and 1A had the recessive or a 31 ratio Law of Segregation Segregationalleles separate Happens in meiosis Alleles of a particular gene are segregated Homologous chromosomes each with a different allele are separated segregated during the rst meiotic division 0 0 0 0 Homologous Chromosomes 39239 Pairs of chromosomes that have genes involved with the same traits Heterozygous Heterozygous refers to having two different alleles for a single trait The gene for seed shape in pea plants exists in two forms one form or allele for round seed shape R and the other for wrinkled seed shape r A heterozygous plant would contain the following alleles for seed shape Rr Diploid 39239 Pea plants are Diploids 39239 Have two genes for each trait 39239 Nucleus of each cell has two copies of each type of chromosome one from each parent Homozygous 0 quot Homozygous refers to having identical alleles for a single trait The gene for seed shape in pea plants exists in two forms one form or allele for round seed shape R and the other for wrinkled seed shape r A homozygous plant would contain the following alleles for seed shape RR or rr 0 O Incomplete Dominance 39239 Incomplete dominance is a form of intermediate inheritance in which one Lek for a speci c trait is not completely dominant over the other Locus allele This results in a third phenotvpe in which the expressed physical trait is a combination of the dominant and recessive 39239 A gene coding for a particular trait is located at the same phenotypes site or locus on each of the homologous chromosomes 0 39 Incomplete dominance is similar to but different from codominance In codominance an additional phenotype is produced however both alleles are expressed completely Codominance is exempli ed in AB blood type inheritance Linked Genes aka Linkage Polygenes aka Polygenic Inheritance 0 0 0 O 39 Linked genes don t segregateiare inherited together on same Traits that are controlled by a number of genes chromosome Polygenic inheritance in Wheat kernel color ve different 39239 Crossingover can break up linkage groups recombination colors depending on the number of R alleles 39239 Recombination happens in prophase of Meiosis I Color distribution resembles a bellshaped curve Fl dihybrids crossed yield varied F2 0 O o o o 0 0 0 0 0 0 Ind endent Assortment Djhybrid Cross 39239 A dihybrid cross is a breeding experiment between P generation parental generation organisms that differ in two traits Genes on different chromosomes separate independently Chromosomes assort independently when 2 or more traits are examined in a single cross I39ftbezare 011 different chromosomes then they are inherited independently of each other Independent assortment happens in metaphase of Meiosis l homologues separate Epistasis 39239 Genes interact to produce the phenotype absence of one stops eXpresssion of phenotype Hybrid Vigor heterosis 39239 Heterosis hybrid vigor or outbreeding enhancement is the improved or increased function of any biological quality in a hybrid offspring The adjective derived from heterosis is heterotic v An offspring exhibits heterosis if its traits are enhanced which is often de ned in terms of evolutionary tness as a result of mixing the genetic contributions of its parents v These effects can be due to Mendelian or nonMendelian inheritance Gem 6 amp Pheno e Mendel s Model of Inheritance 1 Discrete units of inheritance inherited traits are transmitted by factors genes 2 Genes occur in pairs diploidy 2N haploid IN or N 3 Genes of a pair can differ alleles are different forms of the same gene 4 Alleles remain discreteidon t blend don t alter each other 5 Genes are not always expressed phenotype can differ from genotype 0 v Genotype actual genes in an organism 39239 Phenotype the expression of the genes sometimes in uenced by the environment and relationships between genesalleles such as dominance recessive and codominance o 99 Chromatids A chromatid contains the replicated DNA of each individual chromosome which are joined by a centromere for the process of cell division mitosis or meiosis They are normally identical quothomozygousquot but may have slight differences in the case of mutations in which case they are heterozygous They are called sister chromatids so long as they are joined by the centromeres When they separate during anaphase of mitosis and anaphase 2 of meiosis the strands are called daughter chromosomes although having the same genetic mass as the individual chromatids that made up its parent the daughter quotmoleculesquot are still referred to as chromosomes much as one child is not referred to as a single twin Before replication one chromosome is composed of one DNA molecule and after there are two DNA molecules This is because DNA replication increases the amount of DNA and does not increase the number of chromosomes Plasticity 3 Environment and development affect the expression of genes ie same genotype can produce different phenotypes 3 Soil acidity can affect ower color in Hydrangea 7 acidic soil blue neutralbasic pink Meosis Z The cell division by which gametes are formed pair of homologous Pair 039 h0m l 9 5 chromosomes 7 chromosomes 1 alleles for seed shape R or r centromere o O locus alleles for seed color Y or y Figure in Plant Biology we a zone Faunaquot Enucallan Co Dominance 3 Codominance both alleles of a gene are expressed 3 Carnations have alleles for red and white owers the red are homozygous red the white are homozygous white and the pink are heterozygous o 0 o o o 0 o GMO s Genetically Modi ed Organisms Genetic engineering has produced new forms of crops that yield more are easier to grow can make it with less waterfertilizer Some GM foodplants have been modified to be more nutritious I I All is not wonderful between F 39 ofN n 39 1 quot Inheritance engineers and food activists ecologists some envuonmental concegiiiglglgfl 51215 39239 linkage of genes on chromosomes o A I u I 1 39 I I I Evolution of resistance to incorporated pest control measures eg Bt toxin 2 r DNA K 14 1 Bacrllus thurz genszs caterpillar killer bacterium that naturally produces those V mumtlonsi maracliers appear that were HOt Present 111 toxins either parentieg transposable elements Bt can be sprayed on plants to protect them Crops have been transformed to produce their own Bt Nontarget effects can be problematic Example GMO com plants produce Bt toxrh resistant to catcrprllm their pollen blows onto a zer plants par39sms butterflies GROWTH DIFFERENTIATION DEVELOPMENT NUTRIENTS VITAMINS HORMONES GROWTH REGULATORS HOW HO RMONES WORK AUXINS GIBBERELLINS CYTOKININS ABSCISIC ACID ETHYL ENE OLIGOSACCHARINS BRASSINOSTERIOUDS SENESCENCE NUTATION IRREVERSIBLE INCREASE IN MASS DUE TO DIVISION AND ENLARGEMENT OF CELLS CELLS DEVELOP DIFFERENT FORMS ADAPTED TO SPECIFIC FUNCTIONS COORDINATION OF GROWTH AND DIFFERENTIATION OFASINGLE CELL INTO TISSUES AND ORGANS SUBSTANCES THAT CHANGE ELEMENTS AND ENERGY TO PRODUCE ORGANIC MOLECULES OBTAINED FROM AIR AND SOIL ORGANIC MOLECULES THAT PARTICIPATE IN CATALYZING REACTIONS USUALLY AS ELECTRON ACCEPTORS AND DONORS REQUIRED FOR NORMAL GROWTH AND DEVELOPMENT FACTORS PRODUCED IN ONE REGION THAT CAUSE CHANGE IN THE ORGANISM COMPOUNDS THAT AFFECT PLANT DEVELOPMENT SIMILAR TO THOSE OF HORMONES AND VITAMINS FACTORS CHEMICALLY BIND TO SPECIFIC REACTORS TO INITIATE SIGNAL TRANSDUCTION PATHWAYS TO TURN GENES ON OR OFF FOUND IN ACTIVELY DIVIDING PARTS OF PLANTS INFLUENCE GROWTH DELAY FRUIT DEVELOPMENT AND INHIBIT LATERAL BRANCHING POLAR MOVEMENT DRAMATICALLY INCREASES STEM GROWTH AND RATE OF GROWTH NONPOLAR MOVEMENT FOUND IN ROOT TIPS AND GERMINATING SEEDS REGULATE CELL DIVISION NON POLAR MOVEMENT FOUND IN PLASTID CAROTENOID PIGMENTS INHIBITS STIMULATORY EFFECTS OF OTHER HORMONES amp PREVENTS GERMINATION NONPOLAR MOVEMENT FOUND IN ROOTS FLOWERS FRUITS SEEDS AND LEAVES USED TO RIPEN FRUITS AND CAUSE LEAF ABSCISSION FOUND IN CELL WALL ENZYMES INFLUENCE CELL DIFFERENTIATION REPRODUCTION AND GROWTH INFLUENCE PLANT STEM ELONGATION BREAKDOWN OF CELL COMPONENTS AND CELL DEATH SPIRALING GROWTH MOVEMENTS NOT VISIBLE TO THE EYE NODDING MOVEMENTS TWINING MOVEMENTS CONTRACTION MOVE MENTS NASTICMOVEMENTS TROPIS MS PHOTOTROPISM GRAVITROPISM TURGORMOVEMENTS PULVINI CIRCADIANRHYTHMS SOLARTRACKING TAXICMOVEMENT PHOTOPERIODISM PHYTOCHROMES CRYPTOCHROMES THERMOPERIOD DORMANCY QUIESCENCE STRATIFICATION 0F LIVING SPECIES OF FLOWERING PLANTS SPECIES PROVIDE THE 6 MAOR PLANTS THE WORLD RELIES ON 0F SIDE TO SIDE GROWTH MOVEMENTS THAT ANCHORS PLANT INTO SOIL VISIBLE SPIRALING IN GROWTH CONTRACTILE ROOTS THAT PULL ROOTS DEEPER NONDIRECTIONAL GROWTH MOVEMENTS GROWTH OF A PLANT TOWARD OR AWAY FROM EXTERNAL STIMULI GROWTH OFA PLANT TOWARD OR AWAY FROM LIGHT GROWTH OFA PLANT IN RESPONSE TO GRAVITY RESULTS FROM CHANGES IN INTERNALWATER PRESSURES INITIATED BY CONTACT WITH OBJECTS SPECIAL SWELLINGS AT THE BASE OF LEAVES THAT MOVE THE LEAF REGULAR 24 HOUR CYCLES INFLUENCES BY TURGOR LIGHT AND TEMPERATURE WHEN LEAVE TWIST IN RESPONSE TO ILLUMINATION AND BECOME PERPENDICULARLY ORIENTED TO LIGHT SOURCE MOVEMENT THAT INVOLVES THE ENTIRE PLANT THAT MOVES BY FLAGELLA OR CILIA TOWARD ORAWAY FROM STIMULUS LENGTH OF DAY OR NIGHT DIRECTLY AFFECTS TIIE ONSET OF FLOWERING SHORT DAY VS LONG DAY PIGMENTS THAT CONTROL PHOTOPERIODISM FOUND IN MERISTEMATIC TISSUE BLUE LIGHT SENSITIVE PIGMENTS THAT PLAY ROLES IN CIRCADIAN RHYTHMS OPTIMUM NIGHT AND DAY TEMPERATURES PERIOD OF GROWTH INACTIVITY IN PLANTS STATE WHERE SEED CANNOT GERMINATE UNLESS CONDITIONS ARE RIGHT ARTIFICIAL BREAKING OF DORMANCY 250000 6 SPECIES PROVIDE 80 OF CALORIES CONSUMED BY HUMAN WHEAT RICE CORN POTATO SWEET POTATO CASSAVA DOMESTICATEDPLANT PLANT WHOSE REPRODUCTIVE SUCCESS DEPENDS ON HUMAN INTERVENTION OUTOF AFRICA THEORY OUR HUMAN ANCESTORS EVOLVED IN AFRICAAND THEN SPREAD AROUND THE WORLD HOW CAN YOU IDENTIFY WHAT CROPS ANCIENT PEOPLE ATE GRASS HAS CERTAIN C12 TO C13 ISOTOPE RATIOS THAT GETS INCORPORATED INTO SKELETONS PLANT BREEDING ACCELERATED EVOLUTION GUIDED BY HUMANS RATHER THAN NATURE CROSSPOLLINATION PLANT MUST BE FERTILIZED FROM OTHER INDIVIDUALS INBREEDING DEPRESSION INCREASED EXPRESSION OF DELETERIOUS RECESSIVE ALLELES GERMPLASM SUM TOTAL OFA PLANT39S GENES TRANSGENETICPLANT TAKING GENES FROM ONE ORGANISM USINGA CLONING VECTORAND RESTRICTION ENZYMES AND INCORPORATING IT INTO ANOTHER ORGANISM PLASMID SMALL CIRCULAR DNA CAPABLE OF INDEPENDENT REPLICATION GRAFTING SEGMENTS OF DIFFERENT PLANTS CONNECTED AND INDUCED TO GROW TOGETHERAS ONE PLANT TOTIPOTENCY CAPACITY OF A CELL TO GIVE RISE TO ANY STRUCTURE OF A MATURE ORGANISM EVOLUTION CHANGE OVER TIME NATURALSELECTION TENDENCY OF ORGANISMS WITH FAVORABLE GENOTYPES TO SURVIVE AND PRODUCE OFFSPRING BEHAVIORALEVOLUTION STUDY OF DEVELOPMENT OF BEHAVIORAL TRAITS EVOLUTIONARYDEVELOPMENTALBIOLOGY STUDY OF EFFECT OF GENETIC VARIATION ON TRAITS THAT EFFECT SURVIVAL AND REPRODUCTION EVOLUTIONARYECOLOGY STUDY OF HOW ECOLOGICAL FEATURES EFFECT EVOLUTION OS SPECIES AND ECOSYSTEMS EVOLUTIONARYGENETICS USE OF MOLECULAR AND TRANSMISSION GENETICS TO UNDERSTAND ORIGINS OF GENERIC VARIABILITY EVOLUTIONARYPALEONTOLOGY USE OF FOSSIL RECORD TO EXAMINE LARGESCALE EVOLUTIONARY CHANGE EVOLUTIONARYPHYSIOLOGYANDMORPHOLOGY STUDY OF ADAPTATION THROUGH BIOCHEMICAL PHYSIOLOGICAL AND ANATOMICAL CHANGES HUMANEVOLUTION USE OF ALL DISCIPLINES OF EVOLUTION TO STUDY GENETIC VARIATION IN HISTORICAL AND MODERN HUMANS MOLECULAREVOLUTION STUDY OF EVOLUTIONARY CHANGES IN DNAIN RELATION TO BE GENE STRUCTURE ORGANIZATION AND CONTROL OF EXPRESSION SYSTEMATICS NAMING OF SPECIES AND DETERMINATION OF THEIR EVOLUTIONARY RELATIONSHIPS ARTIFICIALSELECTION EVOLUTION DIRECTED BY HUMANS EVOLUTIONARYMEDICINE APPLIES EVOLUTIONARY PRINCIPLES TO THE WAY IN WHICH WE TREAT ILLNESSES PARACELSUS DOCTRINE OF SIGNATURES CONSERVATIONBIOLOGY USES EVOLUTIONARY PRINCIPLES TO UNDERSTAND SPECIES EXPANSION OR CONTRACTION IN RESPONSE TO CHANGING ENVIRONMENT COUNT DE BUFFON DESCRIBED ALL KNOWN PLANTS AND ANIMALS PRESENTED EVIDENCE THAT ORGANISMS CHANGE ACROSS GENERATIONS IEAN BAPTISTE LAMARCK CHARACTERISTICS ACQUIRED DURING LIFE WERE PASSED ON AND BECAME CUMULATIVE IDEA OF EVOLUTION WAS RIGHT BUT MECHANISM WAS WRONG HOMEOBOX GENES REGULATORY GENES THAT ACT AS DEVELOPMENTAL SWITCHES ALFRED WALLACE CAME UP WITH THE GEOGRAPHIC REALMS HOMOLOGY CHARACTERISTIC SHARED BY DIFFERENT ORGANISMS WITH COMMON ANCESTRY CONVERGENTEVOLUTION PLANTS ADAPTED IN SIMILARWAYS BECAUSE OF COMMON ENVIRONMENTAL CONDITIONS IN DIFFERENT PARTS OF THE WORLD MICROEVOLUTION EVOLUTION WITHIN SPECIES MUTATION CHANGE IN A GENE OR CHROMOSOME MIGRATION GENE FLOW BETWEEN POPULATIONS WHEN INDIVIDUALS OR GAMETES MIGRATE FROM ONE POPULATION TO ANOTHER GENETICDRIFT CHANGES IN THE GENETIC MAKEUP OFA POPULATION DUE TO RANDOM EVENTS PUNCTUATEDEVOLUTION MAOR CHANGES OCCUR IN SPURTS BASED ON FOSSIL RECORD MACROEVOLUTION HOW SPECIES EVOLVE GEOGRAPHICISOLATION ISOLATION OF TWO POPULATIONS PREVENTS GENE FLOW ECOLOGICALISOLATION ECOLOGICAL FACTORS PLAY A ROLE IN ISOLATION MECHANICALISOLATION ORGANISMS CANNOT PHYSICALLY BREED POSTZYGOTICEVOLUTION FAILURE OF EMBRYOS TO DEVELOP OR OF HYBRIDS TO SURVIVE AND BREED HYBRIDS OFFSPRING PRODUCED BY PARENTS THAT DIFFER IN ONE OR MORE CHARACTERISTICS POLYPLOIDY OCCURRENCE OF DOUBLE THE NORMAL CHROMOSOME NUMBER Gregory Mendel Hybrids Monohybrids Segregating Scl fcrtilization CrossFcrtilization F Generation Dominant amp H r51 filial generation chcssivc 2 Generation r Funnett Square second tilial generation Mendel s First Law 01 Heredity the Law Oi chregation Alleles Diploid Homologous Chromosomes Homozygous Heterozygous Locus Incomplete Dominance Folggencs Linked Genes Mcndcl s Second Law of Hereditary Cross Law o Indcpcndcnt Assortment Epistasis Hybrid Vigor Mendel s Model of Inheritance heterosns Genotype 8w Fl39lenotgpe Mcosis Chromaticls C0Dominancc Flasticity Examples 01C NonMendelian Inheritance GMO S Genetically Modi ed Organisms Chapter 12 Plant Behavior and Hormones Regulating Growth Plant Hormones Plant cells are in constant chemical communication with one another and with their environment They recognize and respond to stimuli of many kinds using a system of chemical messengers that receive and transmit the stimuli via ordinary body cells unlike the highly specialized cells of animal nervous systems Control of the plant system apparently resides in the genes of each cell which are turned on and off by the chemical messages they receive The response may be stimulatory initiating cellular division and enlargement for example or inhibitory such as stopping a metabolic process The chemical messengers are hormones organic substances manufactured in small amounts in one tissue and usually transported to another where they initiate a response A few act in the tissues where they are produced The hormone molecule itself carries little information and produces a reaction only when it binds to appropriate receptor molecules at the response site Plants in comparison to animals have both fewer hormones and fewer kinds of responses Plant hormones however usually act in combination thus producing more varied responses than if acting individually The same hormone also can produce different effects when acting in different tissues or in different concentrations in the same tissue The developmental stage of the plant additionally determines what effects the hormone activates Growth and development depend upon a successful coordination of the activities of hormones not just the presence or absence of individual ones Types of Plant Hormones There are five general classes of hormones auxins cytokinins gibberellins ethylene and abscisic acid Auxins An auxin indole3acetic acid 1AA was the first plant hormone identified It is manufactured primarily in the shoot tips in leaf primordia and young leaves in embryos and in parts of developing owers and seeds Its transport from cell to cell through the parenchyma surrounding the vascular tissues requires the expenditure of ATP energy IAA moves in one direction onlyithat is the movement is polar and in this case downward Such downward movement in shoals is said to be basipetal movement and in roots it is acropetal Auxins alone or in combination with other hormones are responsible for many aspects of plant growth IAA in particular Activates the differentiation of vascular tissue in the shoot apex and in calluses initiates division of the vascular cambium in the spring promotes growth of vascular tissue in healing of woun s Activates cellular elongation by increasing the plasticity of the cell wall Maintains apical dominance indirectly by stimulating the production of ethylene which directly inhibits lateral bud growth Activates a gene required for making a protein necessary for growth and other genes for the synthesis of wall materials made and secreted by dictyosomes Promotes initiation and growth of adventitious roots in cuttings Promotes the growth of many fruits from auxin produced by the developing seeds Suppresses the abscission separation from the plant of fruits and leaves lowered production of auxin in the leaf is correlated with formation of the abscission layer Inhibits most owering but promotes owering of pineapples Activates tropic responses Controls aging and senescence dormancy of seeds Synthetic auxins are extensively used as herbicides the most widely known being 24D and the notorious 245T which were used in a 11 combination as Agent Orange during the Vietnam War and sprayed over the Vietnam forests as a defoliant Cytokinins Named because of their discovered role in cell division cytokinesis the cytokinins have a molecular structure similar to adenine Naturally occurring zeatin isolated first from corn Zea mays is the most active of the cytokinins Cytokinins are found in sites of active cell division in plantsifor example in root tips seeds fruits and leaves They are transported in the xylem and work in the llPage presence of auxin to promote cell division Differing cytokininauxin ratios change the nature of organogenesis If kinetin is high and auxin low shoots are formed39 if kinetin is low and auxin high roots are formed Lateral bud development which is retarded by auxin is promoted by cytokinins Cytokinins also delay the senescence of leaves and promote the expansion of cotyledons Gibberellins The gibberellins are widespread throughout the plant kingdom and more than 75 have been isolated to date Rather than giving each a specific name the compounds are numberedifor example GAl GA2 and so on Gibberellic acid three GA3 is the most widespread and most thoroughly studied The gibberellins are especially abundant in seeds and young shoots where they control stem elongation by stimulating both cell division and elongation auxin stimulates only cell elongation The gibberellins are carried by the xylem and phloem Numerous effects have been cataloged that involve about 15 or fewer of the gibberellic acids The greater number with no known effects apparently are precursors to the active ones Experimentation with GA3 sprayed on genetically dwarf plants stimulates elongation of the dwarf plants to normal heights Normal height plants sprayed with GA3 become giants Ethylene Ethylene is a simple gaseous hydrocarbon produced from an amino acid and appears in most plant tissues in large amounts when they are stressed It diffuses from its site of origin into the air and affects surrounding plants as well Large amounts ordinarily are produced by roots senescing owers ripening fruits and the apical meristem of shoots Auxin increases ethylene production as does ethylene itselfismall amounts of ethylene initiate copious production of still more Ethylene stimulates the ripening of fruit and initiates abscission of fruits and leaves In monoecious plants those with separate male and female owers borne on the same plant gibberellins and ethylene concentrations determine the sex of the owers Flower buds exposed to high concentrations of ethylene produce carpellate owers while gibberellins induce staminate ones Abscisic acid Abscisic acid ABA despite its name does not initiate abscission although in the 1960s when it was named botanists thought that it did It is synthesized in plastids from carotenoids and diffuses in all directions through vascular tissues and parenchym a lts principal effect is inhibition of cell growth ABA increases in developing seeds and promotes dormancy If leaves experience water stress ABA amounts increase immediately causing the stomata to close Responsive Growth Movements Tropisms Responsive growth movements toward or away from an external stimulus are called tropisms If the plant movement is toward the stimulus it is a positive tropismaway from the stimulus a negative tropism Phototropism The tropic response to unidirectional light is called phototropism In general shoots grow toward light and hence are positively phototropic roots grow away from light and are negatively phototropic Wellknown and oftenrepeated experiments with oat seedlings have shown that the auxin TAA which causes elongation of cells migrates to the shaded side of oat coleoptiles The subsequent differential growth on the two sides causes the coleoptiles to bend toward the light Although green stems also bend and grow toward the light in this case an TAA inhibitor prevents cells from elongating on the lighted side while those on the shaded side continue to elongate39 the stem bends toward the light as a consequence of the differential growth Different wavelengths of light cause differing growth responses The blue end of the spectrumiwavelengths less than 500umiis most effective in producing a growth response Gravitropism Gravitropism is the plant response to gravity The mechanism of how gravity is sensed by plants is as yet unexplained None of the numerous hypotheses is fully adequate Over the eons plants probably developed several methods to cope with this environmental factor Shoots are negatively gravitropic because they grow upward39 roots are positively gravitropicithey grow downwards IAA calcium ions Ca2 and possibly ABA are involved in instigating growth and curvature in many plants Still to be proven is the long held belief that starch grains migrating from upper to lower sides in the root cap of a horizontally held root initiate the growth response 2Page Thigmotropism The growth response of a plant or a plant part to the touch of a solid object is called thigmotropism Tendrils of climbing plants wrapping around a support is a common thigmotropic response accomplished by cells on the side touching the support shortening and those on the opposite side elongating IAA and ethylene are two hormones probably involved in the response Other Plant Movements N astic movements Although anchored in place by root systems plants move their organs in response to many kinds of external stimuli These movements are called nastic movements and differ from tropic movements in that they are not directed toward or away from the stimulus Movements triggered by touch such as closing the traps of insectivorous plants are called thigmonastic or seismonastic movements The changing daily cycles of light and darkness produce sleep nyctinastic movements in leaves of many species Most of the actual nastic movements can be explained by changes in the turgor pressure of specially located parenchyma cells after a stimulus has been received Thigmomorphogenesis The growth response to generalized mechanical quot is called quot 39 39 39 Plants in their natural environment are subject to all manner of jarring touching and shaking by the wind passing animals rain and the like The general response of most plants to such disturbances results in decreased height increased diameter and more supportive tissues in the shoots The change in the general form of the plant apparently results from activation of genes one of which carries the code for calmodulin a calcium binding protein Undoubtedly the thigm omorphic response mechanism is similar to other calmodulingrowth responses of plant cells Ethylene important in growth regulation also is important here Solar tracking Some plants move their leaves and owers toward the sun and track its movement from east to west during the day The common sun ower got its name because of this trait In the morning all of its owers face the east at noon they lie horizontally facing the zenith while in the late afternoon and evening they face west toward the setting sun The phenomenon is called heliotropism although it is not a true tropic response since it does not involve growth Turgor pressure changes in parenchyma cells account for the movements Circadian Rhythms Many plants exhibit a rhythmic behavior on about a 24hour cycle such as the owers that open in late afternoon every day This regular repetition of growth or activity on approximately a 24hour cycle is called a circadian rhythm All sorts of metabolic processes are circadian such as cell divisions in root tips and protein or hormone synthesis Sleep movements of leaves are well known circadian rhythms as are the opening and closing of nightblooming or dayblooming owers Circadian rhythms are endogenous meaning they are controlled by an internal timing mechanism called the biological clock of the plant Although circadian rhythms are not triggered by an external stimulus the environment does set and keep the biological clock in harmony with external changes such as darkness and light The resetting of the biological clock is called entrainment Entrainment for example keeps the circadian periodicity of owering in sequence with light and dark periods even as the day lengths change seasonally The biological clock keeps the plant responding appropriately for each season by measuring the changing day lengths The mechanism by which it does this involves the pigment phytochrome Photoperiodism Photoperiodism is a biological response to a change in the proportions of light and dark in a 24hour daily cycle Plants use it to measure the seasons and to coordinate seasonal events such as owering 3Page Phytochrome Plants make such adjustments by utilizing the pigment phytochrome which exists in two forms P which absorbs red light and P which absorbs farred light Each can convert to the other when they absorb light During the day the two forms convert back and forth CP becomes P and vice versa until they reach an equilibrium of 6040 P P in plant tissues During the night Pf slowly converts to P or else disintegrates P is stable in the dark P is the biologically active form acting as the switch that turns on such plant responses as owering or seed germination When the threshold concentration of P is attained the response is stimulated Thus it is the length of the night period not the day period that determines the response Short nights meaning long days favor activities that require large amounts of P conversely if the night is long and the day short more P is converted back to P and responses triggered by small amounts of P are favored P synthesized from amino acids is the inactive form Photoperiodic responses Photoperiodism was first studied in relation to owering Plants can be described in relation to their photoperiod responses as short day longday dayneutral and intermediateday plants Plants that ower in late summer and fall are shortday they have a critical period of light exposure of less than about 16 hours Longday plants are summer owering and have a critical period of longer than 9 to 16 hours Dayneutral plants ower in photoperiods of any length while interm ediateday plants ower only in periods neither too long nor too short for the particular plant that amount of time is different for each plant studied to date but not classifiable as either longday or shortday Other photoperiodic responses involving the phytochrome system include seed germination and the early growth of seedlings Florigen Because hormones control so many metabolic activities in plants owering has long seemed likely to be under the control of one or more hormones Early experiments sought to determine which part of a plant is sensitive to the light that initiates owering The results suggested the presence of a substance that moved from the leaves to the ower buds Although the substance was not identified theninor has it been isolated nowiit was named origen Florigen is the hypothetical owering hormone it may or may not actually exist Note that owering most likely is not controlled by a single hormone but is the result of a combination of internal and external signals and responses Dormancy Shoot dormancy Rarely do all factors of the environment remain suitable indefinitely for plant growth In the temperate latitudes for example breaks in the growing season occur when seasons change bringing reduced temperatures and shorter days in the autumn and winter In the subtropics and tropics where temperatures and day lengths remain equitable all year water availability may uctuate between a wet and a dry season Plants have developed mechanisms to survive during such adverse periods One effective mechanism used by annual plants is to produce a photosynthetic and owering structure rapidly and then sink the resources derived from photosynthesis into seed production and distribution The plant body is no longer useful and is abandoned after protected embryos are produced The seeds withstand the changes of the next unfavorable growth period and germinate when environmental stimuli indicate favorable growth conditions Perennial owering plants also use the seed mechanism but some retain their photosynthetic and root structures merely dropping the most vulnerable parts leaves during the unfavorable growth period en one or more of the plant organs undergoes a period in which the growth processes are slowed down or suspended that state is termed dormancy The grth is reactivated when environmental stimuli are received that in effect inform the plant that conditions are again suitable for growth The signals to break dormancy are extraordinarily precise External stimuli combine with internal signals to ensure that renewal of growth will occur at the most favorable time Many plants have internal growth inhibitors that decay slowly over time such as ABA Until the inhibitor has dropped to a certain low level no growth will take place despite external stimuli39 both external and internal signals must be correct 4Page Seed dormancy Almost all seeds undergo some period of dormancyiif they did not they would start to grow in the fruits on the mother plant and defeat their principal purposes dispersal and survival of the germplasm The period between the formation of the seed and the time when it will germinate is called the afterripening period which may be a few days or months depending on the plant Seeds of plants native to regions with cold winters almost all require an afterripening period of cold temperatures before they will germinate This requirement can be met in horticultural and crop varieties by refrigerating the moist seed for a period of time This procedure is called strati cation Dormancy of seeds with hard seed coats often can be broken artificially by scarifying the seedi mechanically thinning the seed coat with a file or nicking it with a knife allowing water and oxygen to penetrate to the embryo Bud dormancy Woody and herbaceous perennials produce dormant overwintering buds in habitats with cold winters The buds are miniature shoots with apical meristems leaf primordia and axillary buds the whole enclosed by several modified leaves called bud scales The scales protect the embryonic tissues of the bud from mechanical injury and insulate them In many climates the scales prevent the formation of ice crystals in the young tissues Bud scales also restrict gas exchange and prevent desiccation They often accumulate growth inhibitors as well Buds start their growth early in the growing season and by midsummer are completely formed They then undergo a series of physical and physiological changes in preparation for winter The process is called acclimation and is triggered primarily by the shorter days of late summer Plants that have acclimated to winter are said to be coldhardy Dormancy is broken in the spring in tree buds by the lengthening days The buds are the photoperiod receptors Senescence Senescence is the orderly ageinduced breakdown of cells and their components leading to the decline and ultimate death of a plant or plant part The timing of senescence is speciesspecific and varies among the organs of individual plants Some species of plants produce shortlived flowers whose petals last for only a few hours before shriveling and dropping off while the leaves of deciduous plants last through long growing seasons before senescing Senescence is a metabolic process therefore it requires energy It is not simply the ending of growth Leaves for example move the products of photosynthesisiand their own structural substanceswut of leaf tissue into stem and root tissue during senescence and before their vascular connections are severed at abscission One of the first materials to degrade is the energyconverting pigment chlorophyll As the bright green color of chlorophyll fades the yelloworange colors of the carotenoids become prominent and combine with the redblue anthocyanins to produce the vivid colors of autumn in the trees and shrubs of the northern deciduous forest The role of hormones in senescence is not clear Not only the kinds but the proportions of each are important Ethylene promotes abscission of leaves owers and fruits while 1AA retards senescence and abscission When days shorten in autumn 1AA production decreases and ethylene production increases hastening changes in the cells of the abscission zone When the degradation of the cell wall materials is complete nothing remains to hold the leaf to the stem and with any slight disturbance the leaf falls Some evidence indicates that a senescence factor presumably an unknown hormone exists in some plants like soybeans but it has yet to be isolated or synthesized 5Page Chapter 13 Reproduction Meiosis Life Cycles Prokaryote Cell Division The continuity of life depends upon the ability of cells to reproduce In the prokaryotes cellular reproduction is by binary ssion an asexual division of the contents of a single cell into two new cells of approximately equal size The process is fast and relatively simple The circular bacterial chromosome replicates and the two new genomes move toward opposite ends of the cell A new plasma membrane is added between them dividing the cytoplasm roughly in two and the cell splits Each of the two daughter cells formed has a complete set of genes and some materials with which to begin an independent life During periods of active growth the new cells acquire and metabolize nutrients grow replicate their bacterial chromosome and reproduce once more In a favorable environmentibathed in the warm rich nutrients of the small intestine for exampleithe bacterial cells can divide every 20 to 30 minutes Eukaryote Cell Divison The Cell Cycle When compared with prokaryotic cell division the process isn39t as simple in eukaryotes where linear chromosomes that are contained within a membranebound nucleus have to be apportioned equally between two daughter cells If something goes wrong and they aren39t distributed equally chances are the daughter cells will die for lack of instructions on how properly to conduct the business of life The eukaryote cell is also filled with organelles and other cytoplasmic materials that must be divided Small wonder then that the process not only is more highly orchestrated but that it takes much longer to accomplish Sexual Reproduction Meiosis A second type of cell division called meiosis takes place in multicellular eukaryotes This is a reduction division in which the daughter cells receive exactly half the number of chromosomes of the mother cells Meiosis occurs in the production of gametesithe sperm of the males and the eggs of the females When a sperm fertilizes an egg a zygote is produced with the appropriate number of chromosomes for the speciesiin humans and potatoes the zygote and the somatic body cells produced from it have 46 chromosomes This is the diploid 2 n number of chromosomes half of which have come from the sperm nucleus half from the egg The sperm and egg are haploid n they carry half the number of chromosomes of the body cells in humans 23 in each sperm and egg Meiosis thus makes it possible to maintain a constant number of chromosomes in a species that reproduces sexually by halving the number of chromosomes in the reproductive cells Meiosis uses many of the same mechanisms as mitosis and is assumed to have been derived from mitosis after the latter procedures were in place in some early organisms millenia ago Figure 1 shows the stages of mitosis and Figure g shows the stages of meiosis Note that the names for the stages are the same as those of mitosis with the addition of a numeral to designate either the first or the second divisional stage Bath divisions are part of meiosis not until the final four daughter cells are produced is the process complete 6Page INTERPHASE PROPHASE METAPHASE ANAPHASE TELDPHASE I ll I y Lt 39c MITOSIS Figure 1 cell wall 3rx ilr4 nucloolus l V l 77r 73 nucluus Q chromosome composed of Wu chromatids celmomere spmule hbeys chromaml two daughter cell nuclei p cull plow PROPHASE one pair at homologous chromosomes in synopsis four chromatidsl spindle liIJEIS METAPHASE canirnmeres ANAPHASEI homologous nhlomosomes separate and move to opposale poles cell plate TELOPHASEI daughwr nucleus two daughter cells PHDPHASE II chromosome with two chromatids METAPHASE II cluomands sepalale ANAPHASE and move In upposne Dales om daughter cells each TELQPHASE H with hallmu chromosome number oflhe parent cell Meiosis and mitosis have many similarities There are however several fundamental differences Compare Figure l mitosis with Figure g meiosis In meiosis In Prophase I homologous chromosomes come together in synapsis and form pairs called bivalents or tetrads because there are four chromatids in the pair each bivalent has two chromosomes and four tetra s In Metaphase I bivalents align randomly on the equatorial plane which means that each daughter cell has an equal chance of getting either the chromosome from the sperm or one from the egg cells is now haploid n I There is no S phase and the chromosomes line up immediately in Metaphase II their chromatids separate in Anaphase II and in Telophase II new cell walls form around the four haploid cells Events of the second division are similar to those of mitosis In Anaphase I the chromosomes separate each with two chromatids and move to opposite poles each of the two daughter Synapsis in Prophase I is a decisive interval in determining the inheritance of the daughter cells At this time genetic recombination can occur39 that is daughter cells may receive combined traits of their two parents rather than simply the trait from one or the other This is possible because the phenomenon called crossing over often occurs when the chromatids lie togetherisegments containing similar alleles break apart and rejoin to the corresponding segment of the opposite chromatid thus mixing the traits from individual parents 7Page Chapter 14 amp 15 Inheritance Genetics Genetic Engineering Mendelian Genetics The breeding experiments of the monk Gregor Mendel in the mid18005 laid the groundwork for the science of genetics He published only two papers in his lifetime and died unheralded in 1884 The significance of his paper published in 1866 on inheritance in peas which he grew in the monastery garden apparently went unnoticed for the next 34 years until three separate botanists who also were theorizing about heredity in plants independently cited the work in 1900 During the next 30 years the universality of his findings was confirmed and breeding programs for better livestock and crop plantsiand the science of geneticsiwere well under way At the time of Mendel39s work scientists widely believed that offspring blended the characteristics of their parents but Mendel39s painstaking experimentation suggested this was not so Remember no one had yet heard of genes chromosomes or meiosis but Mendel concluded from his breeding experiments that particles or factors that passed from the parents to the offspring through the gametes were directly responsible for the physical traits he saw first lost in the offspring39s generation then repeated in the next Closer still to the actual truth Mendel even hypothesized that two factors probably one from each parent interacted to produce the results His factors were of course the genes which do indeed come in pairs or alleles for each trait Some say Mendel was lucky others that his reported results are too good to be true that he or someone else must have fudged the data to make them come out right His choice of garden peas was fortuitous Peas are selfpollinated and the seven traits he chose to measure are inherited as single factors so Mendel could establish truebreeding lines for each trait Thus he was able to select the parent traits pollinate the owers and count the results in the offspring with no complicating elements He was mathematically trained kept accurate records and applied mathematical analyses and was among the first to do so with biological materials Mendel39s rst law Law of Segregation Mendel did not formulate his conclusions as laws or principles of genetics but later researchers have done so Restating and using modern standardized terminology this is the information that developed and expanded from his early experiments I Inherited traits are encoded in the DNA in segments called genes which are located at particular sites loci singular locus in the chromosomes Genes are Mendel39s factors I Genes occur in pairs called alleles which occupy the same physical positions on homologous chromosomes both homologous chromosomes and alleles segregate during meiosis which results in haploid gametes I The chromosomes and their alleles for each trait segregate independently so all possible combinations are present in the gametes I The expression of the trait that results in the physical appearance of an organism is called the phenotype in contrast to the genotype which is the actual genetic constitution I The alleles do not necessarily express themselves equally one trait can mask the expression of the other The masking factor is the dominant trait the masked the recessive I If both alleles for a trait are the same in an individual the individual is homozygous for the trait and can be either homozygous dominant or homozygous recessive I If the alleles are differentithat is one is dominant the other recessiveithe individual is heterozygous for the trait Animal and plant breeders often use the term truebreeding for homozygous individuals Geneticists use a standard shorthand to express traits using letters of the alphabet upper case for dominant lower case for recessive Red color for example might be R or r so a homozygous dominant individual would be R a homozygous recessive individual rr and a heterozygous individual Rr Crosses between parents that differ in a single gene pair such as those that Mendel made are called monohybrid crosses usually TT and tt Crosses that involve two traits are called dihybrid crosses Symbols are used to depict the crosses and their offspring The letter P is used for the parental generation and the letter F for the filial or offspring generation F1is the first filial generation thhe second and so forth What kinds of crosses did Mendel make to conclude that factorsgenes segregate First of all he made certain that the plants that he planned to use in the experiment were pure line for the traitithat is that they bred true for the trait for two or more years Peas are selfpollinated so he simply grew the plants and examined their offspring Other experimenters omitted this step which confounded their results Mendel then made a series of monohybrid crosses for each of the seven traits he had identified using parents of opposite 8Page main TT vs dwazfm yellvw seed vs geenyy seed xvund 5226RR vs wnnkled x and SD mm He af cwurse a Flam m dwarfswauld always pxaduce dwarf plants and SD an cm39nmon r phnmlwn mu P1 punNM pcucmlwu mumw I2 3 mm II plmmhyn mm mm 7 1 I a Inumbcn 1x7 mm bur m n AhUul 1 I n 1 1 md 1 1 I n ILKI I MIL dnm 1 1 uulx I s Page parent what would the ratio have been Right all tall39 that39s why breeders today make test crosses back to the homozygous recessive parent to see if their phenotypically dominant individuals are homozygous or heterozygous Mendel39s second law Law of Independent Assortment Mendel also worked with crosses involving two traitsithis is where his luck really entered in The traits he picked are on separate chromosomes though of course he didn39t know this Had they been on the same chromosomes the ratios he obtained would not have been possible because the traits would always go together in the same gamete unless some cellular tinkering took place The mechanisms for figuring out the possible gametes with two traits filling out the Punnettsquare and counting the possibilities are the samegonly with more variations possible see Table l for potential numbers TABLE 1 Possible TwoTrait Genetic Variations of different kinds of gametes 2 4 2quot recessive F2 14 116 of different F2 2 4 8 2quot of different genotypes in F 2 3 9 27 3n Here39s what the cross looks like for two of Mendel39s traits combined ower color and pod characteristics One allele for each goes in each gamete39 purple color P is dominant over white p owers and in ated pods I are dominant over constricted i I Immiml urncmnon I Pll s pmi iumciu Pl only pi ml 1 pennant Ppli Fl phunnlypu purple inllmud Self pollinate the F1 purple owered in ated pod plants and what is the F2 ratio Not 31 anymore Fill out a Punnett square and see the possibilities Each gamete gets one allele of each trait so a dominant purple P can have either a dominant in ated pod I or a recessive constricted pod i39 ditto the white p Thus four kinds of gametes are possible PT Pi pT pi and 4 X 4 combinations are possible from the two parents l l l39i pl pi Pl I Pll l l li I pll I pli Pi PPli I39l ii Pp39ll Ppii pl 1 le l39pli mall ppl i l1i PpIi PPll ppll ppii The phenotypic dihybrid ratio is 933179 purple in ated 3 purple constricted 3 white in ated and 1 white constricted Geneticists now test their results statistically to see if they approach the theoretical 9331 and usually use the X2 chisquare test 10Page me alleles afathzx gems parent m mmmsa pawsuh mm mums PI mm W m an The Pmnzu square far we backcmss laaks m Lhs whnte cansfnttedi whnch mdmates that me halts have separated and xecambmed mdependm y af am anmhex lnlrincizs uflnhexilance mmcacxes Liaahge 2nd cmssing aver alas mgemea mme same chxmnasame cmssaver 5 called me chhmz plural chizsmzlz and than may be several m each pa appxaxxmale lacaums af genes Mum kl luminance whnte aha express their angnal kmtmthe hnmazygnus eanauan 11 p a g a Mutations A mutation is defined as any change in the DNA of an organismia sufficiently broad definition to include all manner of changes deletions a piece of the chromosome breaks off and is lost translocations pieces of material are exchanged between two nonhomologous chromosomes inversions two breaks occur and the segment in between rotates and reattaches with its gene sequence in opposite direction to the original base substitutions a different base is substituted for the original duplications gene sequences are repeated and added to the chromosome and other changes Point mutations gene mutations are changes in DNA that are limited to one base pair39 the gene changes and becomes different from its allele Chromosome mutations occur when parts of a chromosome or whole chromosomes change Polyploidy A cell or an organism containing more than two sets of chromosomes is called a polyploid which most often forms when homologs do not separate at anaphase I in meiosis nondisjunction Gametes produced in this fashion will be diploid 2 n rather than haploid l n If two of the diploid gametes unite the resulting individual will be tetraploid 4 n Tetraploids are able to reproduce because there is an even number of chromosomes to pair at meiosisithere39s simply one set too many If an odd number triploid pentaploid and so forth results through only partial disjunction or some other deviation the individual is usually sterile because the extra set of chromosomes lacks a partner homolog during meiotic division Polyploidy in animals is rare because of this Because plants commonly reproduce vegetatively however polyploidy is common in many plant families and is especially prevalent in the arctic ora A particular kind of asexual reproduction termed apomixis permits transmission of polyploids through seeds Apomictic plants form embryos and seeds without fertilization Dandelions are apomictic as are many grass taxa Polyploids that form within individuals of the same species are called autopolyploids Those that are produced when two different species cross are allopolyploids and interspeci c hybrids Other variations Numerous other varieties of interactions occur Epistasis epi upon for example results when the action of one gene masks the expression of a different gene Some plants have multiple alleles for a specific gene Others have polygenic inheritance in which many genes combine to express a trait The differences in the trait show a continuous variation because none of the genes have a clear dominance over the others Genes that in uence several phenotypic characteristics are termed pleiotropic genes lZlPage Chapter 16 Evolution Darwin39s Theory of Evolution Evolution as understood by biologists is the change through time that occurs in populations of organisms in response to changing environments The changes coded in the molecules of DNA are transmitted from generation to generation and over the history of the Earth have resulted in progressively more complex life forms The name of Charles Darwin and his theory of natural selection are inexorably attached to evolution and together with the mechanisms of genetics form the basis of the modern theory of evolution Simplifying and paraphrasing from Darwin39s book On the Origin of Species by Means of Natural Selection and adding current interpretations the main points of his theory are all life came from one or a few kinds of simple organisms new species arise gradually from preexisting species the result of competition among species is extinction of the less fit39 gaps in the fossil record account for the lack of transitional forms These assertions set the stage for the next part of the theory why life evolves the number of individuals increases at a geometric rate39 populations of organisms tend to remain the same size because the resources are limited and only the fittest survive the survivors are variable and those that survive reproduce perpetuating the favorable traits Natural selection according to Darwin is similar to arti cial selection The environment acted as the selecting force in natural selection Unlike the relatively rapid selection pressures instituted by breeders however natural selection took long periods of time to accomplish change Darwin was familiar with the then new conclusions of geologists that the Earth was far older than previously thought which gave his theory of natural selection sufficient time in which to work A major problem was an explanation for how the favorable selections were perpetuated In the 1860s the idea that offspring were blends or mixtures of the traits of their parents the socalled blending theory of inheritance was unable to accommodate transmittal of favorable adaptations from one generation to the next With botanist Gregor Mendel39s ideas and the development of genetics the inheritance portion of Darwin39s theory no longer posed a problem Modern Theory of Evolution The neeDarwin view of evolution incorporates modern understanding of population genetics developmental biology and paleontology to which is being added knowledge of the molecular sequencing of DNA and the insights it provides concerning the phylogeny of life The major premises of the genetic synthetic theory of evolution are evolution is the change of gene allele frequencies in the gene pool of a population over many generations species and their gene pools are isolated from one another and the gene pool of each species is held together by gene flow an individual has only a portion of the pool which came from two different parents and the portions are different in each individual the alleles the individual receives are subject to chromosomal or gene mutations and recombinations natural selection will favor some individuals who will then contribute a larger portion of their gene combinations to the gene pool of the next generation changes of allele frequencies come about primarily by natural selection but migration gene flow and chromosomal variations are contributing factors isolation and restriction of gene flow between subpopulations and their parent populations are necessary for the genetic and phenotypic divergence of the subpopulations lSlPage Chapter 17 Plant Diversity Modern Taxonomy Includes Phylogenetics Systematics is the name for the branch of biology concerned with the study of the kinds of organisms their relationships to one another and their evolutionary history Taxonomy a term often used interchangeably with systematics is the part of systematics involved in the description naming and classification of organisms Phylogenetics another part of systematics is the study of the phylogeny or evolutionary history of an organism or a group of organisms Two underlying goals of plant systematics thus are to I Find describe give unique names to and organize into categories the species of plants of the world a goal of taxonomy I Organize plants and plant groups to re ect their evolutionary relatedness and their descent from a common ancestor a goal of phylogenetics Systematics today is a vigorous and exciting field that has been given great impetus by the discoveries of molecular biologists who now are describing organisms at their most fundamental levelithe DNA sequences of the cellsiand providing the system atists new data on which to base their phylogenetic trees Phylogenetic trees are the graphic representation of the evolutionary divergences of organisms that put together on the same branches the organisms most closely related with oldest ancestors near the base youngest descendants near the top The trees obtained from the DNA sequences basically trace the history of how the genes have changed through time Naming Plants Biologists around the world use today a single method with standardized rules to name plants and animals the bionomial system of nomenclature The bionomial system of nomenclature The binomial system in use today gives a single name recognizable throughout the world to each individual kind of organism The scientific name consists of two parts in Latin the name of the genus plural genera plus the name of the particular species The system originated with Carl Linnaeus in the middle of the eighteenth century as a shortcut to the cumbersome polynomial system then in use that required lZword descriptions to be written as part of the name In the binomial system the scientific name is italicized in print and the genus is capitalized but the species is not Lay people often ridicule scientific names as unpronounceable atrocities but these same scoffers use many genera names with little complaint Geranium chrysanthemum aster asparagus primula begonia and rhododendron as well as hundreds of others are not only common plant names but genera names as well Other common names are recognizable as anglicized versions of such genera names as Pinus Juniperus Cypems Rosa Hyacinlhus Tulipa Lilium and others Scientific names are important because they are exact one kind of plant one name Common names vary from place to place and language to language but scientific names in Latin remain the same and are recognizable anywhere in the world 14Page Taxonomic hierarchy The Linnaean hierarchical system see Table l is a means to group similar organisms together in levels of increasing inclusiveness from the species at the bottom to the most inclusiveikingdomiat the top Genera are groups of species families are groups of genera and so on up the hierarchy Taxon plural taxa is a general name given to the members of any level in the hierarchy in Table l Aster is a taxon or Anthophyta is a taxon or speclabilis is a taxon TABLE 1 Linnaean Hierarchical System Types of Classi cations Classifications are orderly ways to present information and depending upon their obj ectives can be artificial natural or phylogenetic phyletic which includes phenetic and cladistic Artificial and natural classi cations Classifications that use single or at most only a few characteristics to group plants usually are artificial classificationsithat is all the plants in a single group share the same characteristics but they are not closely related to one another genetically Popular floras books to identify plants of a certain area sometimes group plants using color of their owers or their growth form trees shrubs herbs and so on Although such books are useful in finding the names of taxa they provide few clues about relationships among the taxa and hence are not predictive which means that you can deduce nothing more about the plant other than that it exhibits the characteristics used to classify it Natural classifications group together plants with many of the same characteristics and are highly predictive That is by enumerating the characteristics of a plant one can predict the natural group to which it belongs Taxonomic floras for example identify species genera and families by listing as many characteristics as possible concerning anatomy morphology cytology ecology biochemistry genetics and distribution Phylogenetic phyletic classi cations Phyletic classifications are natural classifications that try to identify the evolutionary history of natural groups When botanists accepted Darwin39s theory of evolution near the end of the last century the reasons why some groups of plants looked alike became clear They were related to one another by a common ancestry The mission of taxonomy since Darwin has become a quest for evolutionary relationships not just at the lower levels of the hierarchy but at the upper levels as well The evolutionary history of a taxon is called its phylogeny To establish phylogenies decisions must be made concerning which characteristics are primitive and which advanced that is which taxon is the ancestor of the others Early phylogenetic classifications were based prim arily upon plant morphology and anatomy with great emphasis upon reproductive morphology which is more stable and less in uenced by the environment than is vegetative A T 39 quot 39 the techniques of biochemistry and molecular biology to add details of internal organization and mechanisms to the classifications But phylogenies no matter how carefully constructed are dependent upon som eone s interpretation of data and herein lies the problem Systematists frequently differ in their interpretations of relationships A phylogenetic classification is a hypothesis a scientific explanation of the data and like any hypothesis is subject to further testing lSlPage Certain assumptions are necessary in phylogenetic classifications A taxon should be monophyletic all of the members of the taxon should be descendants of a single common ancestor The characters or features used to identify the taxa must be homologous which means that they must have a common origin but not necessarily a common function For example all the parts of a floweripetals sepals stamens and carpelsioriginate in the same way as leaves from primordia in meristems Although they now have different functions in the ower they39re not photosynthetic some sepals and petals structurally resemble leaves Leaves and the parts of the ower are homologous structures Some features that look alike do not have a common origin and are said to be analogous An example of analogous structures is the prickles on two groups of succulent desert plants the cacti and the euphorbs Cacti have spines that are modified leaves euphorbs have thorns that are modified branches Spines and thorns look alike and are functionally similar in that both keep animals from eating the plants Spines and thorns are analogous This example of analogy is also an example of convergent evolution The cactus family and the euphorb family both developed the same morphology in response to a desert environmentithe cacti in North and South America the euphorbs in Africa and Asia The families are not related and have no recent common ancestor Numerical taxonomy pheneticsSystematists have tried many ways to make phyletic classifications more subjective When computers became readily accessible in the 1960s numerical taxonomy or phenetics became a popular approach In practice measurements were made of a large number of characters of a taxon at least 60 per plant and often 100 or more No special importance was attributed to any one of the characters After the measurements were complete on hundreds of individuals the data were analyzed statistically with computer programs and cluster analysis or other methods to show purported natural groupings of plants with overall similarities Systematists interpretations were thought to be minimized in this fashion Cladistics Cladistics is the most popular method of classifying organisms today In contrast to phenetics in which similarities are sought using as many characters as possible cladists look for patterns using derived character states that is features that have evolved from an ancestral character group The intent is to find groups of organisms that share a common ancestor and to diagram the relationship of the groups called clades in a cladogram see Figure l The branching points nodes separate groups that have diverged in the evolutionary past from a common ancestor All the taxa below the node lack the character state all those above it retain it Homologous inherited characters are chosen to categorize an organism and its character states The states are hypothesized to be either ancestral or derived evolved and the cladogram is a test of the hypothesis MOSS FERN CONIFER FLOWERlNG Pl a Vascular tissue Figure 1 Molecular biology and phylogenyThe most promising developments in formulating a phylogeny for the entire tree of life come today from molecular biology where new tools and techniques allow researchers to use as character states the molecular sequences of amino acids as well as those of nucleotides in nucleic acids The latter is the most fundamental of comparisons for of course the genes control the structure of life itself The closer the similarity in sequences of molecules among groups of organisms the closer the relationship of the groups Widely different sequences indicate a different evolutionary history and ancestry Some assumptions made by the users of molecular sequencing include I Phenotypic outward appearance evolutionary changes accompanied by genetic hereditary changes occur over time in organisms I Long time intervals result in the accumulation of more changes Organisms that have the most similarities in their gene sequences are more closely related than those with fewer39 they have had a shorter time in which to evolve different phenotypes and genotypes I The groups with widely different sequences must have separated at an earlier time in the evolutionary past 16Page Plants Among the Diversity of Organisms Classification schemes are in a state of flux because of the availability of large volumes of data generated by molecular sequencing of DNA and RNA As might be expected disagreements among biologists are common For example not all biologists believe Widely differentappearing and behaving organisms should be grouped together just because they have similar DNA base pair sequences But the cladists do and are Willing to debate the doubters Major groups and current ways of grouping of organisms In the middle of the eighteenth century Linnaeus39 ideas transformed biological classification In the latter half of the nineteenth century Darwin revolutionized biology with an irrefutable theory of evolution At the end of the twentieth century molecular sequencing is changing the phylogeny of the entire tree of life Appropriately enough a major adjustment has already been made at the roots of the tree There appear to be three main lines of development from the primitive milieu The hodgepodge of prokaryotes unicellular nonnucleated organisms clearly belong to two separate groups the Bacteria and the Archaea Archaebacteria The nucleated organisms eukaryotesi plants animals and so forthi fit in a separate lineage the Eukarya Figure l The Linnaean hierarchy is modified and a new name added for these three super kingdoms the Domain L 1 mm w m Figure 1 Controversial as this change has been shifts among groupings of the Eukarya are even more controversial not because the data are suspect but because biologists differ on how best to organize the new information with the old Organisms in the fivekingdom approach of the recent past are now distributed among four kingdoms of the Domain Eukarya and the two domains of prokaryotes Domain Bacteria and Domain Archaea This change among groupings brings up a problem for botanists What do you do if the organisms you study are evicted from the plant kingdom Are you still a plant scientist if you no longer study plants Many of the ousted groups are included in plant biology textbooks with the justification that because the groups share many of the features of plants it39s appropriate for botanists to study them Classifications are based on current knowledge which is constantly changing so rearrangements are bound to occur along with differences of opinion about What belongs Where Rarely do all parties agree Some of the old group names survive the advent of new classification schemes and are useful ways to discuss informally some groups Classifying Groups of Organisms Biologists use the following features of organisms to identify the major groupings of current classifications This book does not discuss animals and animallike protists beyond placing them in general perspective Presence or absence of a defined nucleus Unicellular or multicellular with specialized organelles Mode of nutrition Presence or absence of a cell wall Composition of the cell wall Motility Mode of reproduction Kind of life cycle 17Page Nucleus The most basic division of organisms separates the living world into two groups on the basis of those possessing and those lacking a defined nucleus plural nuclei The nucleus is an organelle which contains the major portion of the genetic material DNA of the cell and is surrounded by a nuclear membrane The genetic material of Prokaryotes is not contained within a membranebounded nucleus Eukaryotes all have nuclei Cellularity The form morphology of an organism can be unicellular onecelled or multicellular manycelled Some unicellular organisms form filaments strings of cells others form sheets of cells held together by pectins and still others form colonies that give a superficial resemblance to multicellularity Unicellular organisms do not form tissues similar cells organized into a functional unit nor organs groups of tissues organized for a particular function Some organisms alternate a unicellular stage with a multicellular stage in their life cycles Eukaryotic organisms have organelles membranebounded structures within their cells specialized to perform certain functions Nutrition All organisms need a source of energy to fuel their metabolism the chemical processes that maintain life Organisms obtain their nutrients for metabolism in one of two basic ways 1 Autotrophs are able to make the organic compounds they use for metabolism directly from inorganic materials and 2 Heterotrophs are unable to do this and obtain their nutrients from the organic materials manufactured by autotrophs Some autotrophs are photoautotrophs They use radiant energy from the sun in the process of 39 t quot 39 to organic 1 d quot t t r39 use chemical energy in chemosynthesis oxidizing inorganic compounds to manufacture organic nutrients Chloroplasts are present in the photoautotrophs absent in the chem oautotrop Animals are heterotrophs they ingest swallow their food and then digest it internally Fungi are heterotrophs which release digestive enzymes into their surroundings and then absorb the nutrients into their cells Many protists use phagotrophy a type of nutrition in which single cells ingest food particles Some fungi and other organisms are saprophages heterotrophs that break down the organic materials of dead organisms Cell wall Animals and the animallike protists have no cell walls but most other organisms with a few exceptions have some kind of wall made from a variety of materials Almost all of the prokaryote cells have walls and a major distinction between the Bacteria and the Archaea is the presence of peptidoglycans glycoprotein polymers in the Bacteria and their absence in the Archaea cell walls Fungi cell walls are made of chitin the substance that makes the exoskeletons of lobsters crabs cockroaches and other arthropods hard The basic material of plant cells and those of many algae is cellulose Lignin suberin waxes and many other substances may be deposited additionally Motility Plants in general and some animals don39t move around39 they are sessile attached to a substrate But many plant and sessile animal cells are motile and they move using a variety of techniques There are motile organisms in all of the kingdoms so motility per se does not distinguish groups but the kind and location of the devices employed for movement do determine groups The organelle that propels most cells is the agellum plural flagella or in the terminology of some biologists the undulipodium plural undulipodia A smaller shorter agellum is a cilium plural cilia The flagella are long threads of protoplasm that extend outside of the cell and have the capability for limited movement The prokaryotes have a singlefiber agellum that rotates the agella of eukaryotes are bundles that consist of nine pairs of microtubules wrapped around a central pair a 9 2 configuration A sliding action moves the microtubules Type of reproduction Reproduction is the creation of new individuals from existing ones and can be either asexualiwithout special sex cells gametesi or sexual in which gametes fuse to produce new individuals Gametes are usually haploid with a single set of chromosomes and their fusion fertilization results in a diploid with two sets of chromosomes zygote the cell formed by the fusion of two gametes Variations of both sexual and asexual reproduction are legion throughout the living world Asexual reproduction occurs in some members of all the kingdoms whereas sexual reproduction is present in all but the Archaea Many types of asexual reproduction exist Fission a splitting in two of the cell is one type of asexual reproduction In prokaryotes division of the genetic material accompanies lSlPage fission whereas it does not accompany fission in the eukaryotes Yeasts and some other organisms bud simply by pushing out and breaking off pieces of the cell Sporeformation is a widespread method of asexual reproduction in which singlecelled spores formed in specialized structures called sporangia are produced in large numbers They may undergo a resting stage first or produce new individuals directly Sexual spores are produced in some organisms See Figure l GAMETOPHYTE SFOROPHYTE 2n 59f Spore Mather Cells Mllosls q f 1 1 I lt l2n G p lfv E 03 s A rowlh quot k Dullerenllanun l Sporangla Mature Spurnphyle 2n Mature Gameloplwle u Game a 939a Mllosls Gmwsn Dmerenlialiun A l Embryo I Mllosls Growlll Dl erenlialioll if FERTILI 39ION 7 Zygule leNGAMY Femllzcd Egg Figure 1 p Anmerldlum Arcnagonlum l Gameles l Sperm Egg n n Life cycle Three basic types of life cycles differentiate major groups of organisms see Figure g All are variations on a general theme in which haploid cells alternate with diploid in the stages of the life cycle Thus meiotic reduction cell divisions alternate with fertilization fusion of gametes The three life cycles are Zygole W 39CAZEIOSTSD Wgt Haploid CellsIndividuals 1 ZYGOTIC v FERTILIZATION r Gametes Zygote 7gtDlploid gt GWEIOS 7 Gameles 2 GAMETIC quot Individual IEERTIL39IzATlON i Zygote gt Diplond gt IBA EIOSISD gt Spores Individual 3 SPORIC FERTILIZATION 1 39 4 Gameles lt Haploid V Individuals Figure 2 Zygotic meiosis The individual organisms are haploid and only the zygote is diploid The zygote produced by fertilization immediately undergoes meiosis producing more haploid individuals This life cycle appears in all fungi and some algae Gametic meiosis The mature common individuals are diploid and produce haploid gametes that fuse The zygote divides by ordinary mitosis producing the adult diploid individuals Animals some brown and green algae and many other organisms maintain this type of life cycle Sporic meiosis Also called alternation of generations because during the life cycle two kinds of individuals switch or alternate as the comm on individual one diploid one haploid In plants the diploid individual called the sporophyte produces spore mother cells that divide by meiosis producing haploid spores The spores germinate and produce haploid gametophytes The latter then produce the haploid gametes which fuse in fertilization forming the diploid zygote that matures into the adult sporophyte In addition to plants this form of life cycle is present in many algae 19Page Chapter 18 Prokaryotes and the Origin of Lifeamp Chapter 19 Protists and Eukaryotes General Characteristics of Prokaryotes The prokaryotes are the most abundant organisms on Earth and their biomass undoubtedly outweighs all the rest of the organisms together Although they are too small to be seen individually without powerful magnification they and the results of their activities are everywhere without them life on Earth would cease They have persisted for 35 billion years exploiting every possible inorganic and organic habitatithe first 2 billion years alone with no other kinds of organisms In so doing they have evolved ways to make a living in each They manage by being metabolically diverse morphologically small cellularily simple and genetically versatile They are the dispersers and the recyclers of the Earth39s materials and great parts of the human economy depend upon either finding ways to make use of the prokaryotes or ways to get rid of them Table 1 summarizes the basic features that separate the three domains of life TABLE 1 Distinguishing Features of the Three Domains of Life 107100 um 107100 um 7 single bacterial chromosome lipids with phytanol side chains on cell membranes 7 cell wall methane of RNA polymerases i bacterial of rotate i idirect39 fission or no microtubules 7 7 sexual recombination transfer from donor to anaerobes amp aerobes 7 most diverse of all organismsi see text The division of the prokaryotes into two domains poses many problems not the least of which is the inclusiveness of the name bacteria Technically bacteria aren39t all of the old bacteria39 when used appropriately today the fascinating extremophiles are excluded by the term Some microbiologists suggest the use of Eubacteria eu true as a domain and common name to distinguish one specific group but the practice is not universally accepted Some clarification may be necessary therefore in usingian interpreting others39 useiof the word bacteria 20Page Structure Until light microscopes with better lenses and electron microscopes with higher magnifying capabilities were developed microbiologists knew little about the structure and considerably more about the chemistry of the organisms they studied Atypical bacterial cell is illustrated in Figure l with the major features named The cell is most of the features found in eukaryote cells Bylth Iasm Cell membrane 1 lecocalyx capsule i Cell wall Metalchrumatic granules Figure 1 When growth conditions become unfavorableiwhen nutrients become scarce for example or the environment driesimany bacteria produce endospores adding thick walls around the circular DNA together with a bit of cytoplasm The spores resist high temperatures desiccation chemical disinfectants ultraviolet radiation Xrays boiling for several hours and are the reason bacteria sometimes survive even in sterile hospital environments Bacteria are basically unicellular with simple shapes short rods or bacilli singular bacillus spheres or cocci or spiral elongated cells spirilla The single cells often are linked together into ribbonlike laments or beadlike chains of cells some taxa form at sheetlike colonies others produce stalked branching ones All except the mycoplasmas have a cell wall composed of disaccharides and peptides amino acids together with a unique compound not found in eukaryotes peptidoglycan The latter substance is present in the Domain Bacteria and absent in the Domain Archaea making it a good diagnostic feature The gram stain a dye that reacts with peptidoglycan and proteins of the cell walls effectively divides the Domain Bacteria individuals into two major groups grampositive and gramnegative members In the days of light microscopes when not much beyond general rodcoccusspirilla shape was discernable bacteriologists struggled to find good diagnostic features the chemistry of the walls proved to be one Reproduction 21Page The principal mode of reproduction is an asexual separation of one bacterium into two There are in addition several mechanisms that make possible the exchange of genetic material the DNA among and between bacterial cells None however are as elaborate as the mitosisimeiosis choreography of gene exchange in the eukaryotes Asexual Three common types of asexual reproduction are present I Binary ssion the most common The chromosome replicates and the two genomes move to opposite ends of the cell The old cell walls then grow inward between the two pinching the cell apartino mitosis no microtubules The whole process is over within 30 minutes to three hours I Fragmentation This occurs when filaments of cells break into separate pieces or agmenls Budding An outgrowth bud pushes out from the cell pinches off and then enlarges into a new cell Gene exchange With no nuclei there can be no sexual reproduction in the prokaryotes but there is an exchange of DNA In one type conjugation conjugation sex pili bridges of cytoplasm form between cells and some DNA is passed from the donor to the recipient cell In bacterial communities some DNA exists outside of cells presumably left when the cells die or more probably excreted into the environment by living cells This free DNA is picked up bacterial cells in another kind of gene exchange transformation A third type of exchangei transductioniresults when bacteriophages special kinds of viruses bring into bacterial cells the DNA from their previous viral host Researchers in biotechnology use the same method to introduce new genes into host organisms Mutation Random changes in the DNA are common These mutations of the genetic code alter the response of the individual to its environment If the mutation is deleterious the individual dies but favorable mutations spread rapidly as the cells divide repeatedly and often Prokaryote Metabolism 22Page The prokaryotes are the most metabolically diverse of all organisms and have some exotic ways to obtain and channel their needs Organisms need carbon for building cells and energy to fuel the process eukaryotes in general all follow the same basic metabolic pathways whereas prokaryotes use a variety of materials and pathways some employed by no other organisms The terms for energy and carbon acquisition are not standardized among biologists and microbiologists and to make matters worse are inconsistently used concerning the separation of energy and carbon acquisition Table 1 lists some basic terms frequently used by plant biologists Photosynthesis TABLE 1 Metabolic Requirements and Terms source sun chemical source carbon dioxide C02 from saprotrophs saprophages symbiotrophs 02 Source free 02 live in the absence offree OZ facultative strict 23Page Cyanobacteria The bluegreen algae or cyanobacteria n n a pigment chlorophyll a 39 39 carotenoius 39 39 39 L 39 w the other a dark red nhvrmrvlhrin L 39 39 39 fmninn them a 39 The n p r s r e r r r photosynthesi s nnnnnh 1 rue quotfa net 39 r 39 pr nan r 39 quot oxygenrich paving the way for arrival of eukaryote oxygen users ol gre n nuu een nonsulfur bacterial green uuui 39 ucmu c uicii H i L quot nieii 39 39 A U as lithnt 39 39 Photosystern 11 of plants that in the green bacteria a forerunner of Photosysteni I Nitrogen cycle The gaseous N2 4 son harteria live free or That is 39 a 39 ecosystem for use by all When the n uni i an in r piani 39 lake aminnnri in the oceans with fungi and plants in lichen bodies for example or with cycaols anol fems 24Page Nitri cation Nitrification is the twostep conversion of NH to nitrite NO and then to nitrate 037 The energy released inthe process is used by the chemolithoautotrophs to reduce C02 Plants are able to assimilate nitrate but nitrites are toxic to them A 1 439 1 1 A Nitrogen 39 39 39 or 39 is the name ofthe process that the organisms of decayichiefly saprophytic bacteria and fungiiuse to decompose nitrogencontaining organic molecules which release nitrogen as ammonia in the process lLineralization is the term for the conversion of organically bound nutrients into plantavailable inorganic forms while decomposition is a more general term for the breakdown of organic matter Without mineralization the world would soon run out of raw materials for organisms and life would cease The nitrogen cycle turns because decay organisms exist Denitri cationDenitrification is the opposite of nitrogen fixing and nitrification denitrifying bacteria return nitrogen to the atmosphere as NZO nitrous oxide or gaseous nitrogen N 2 Anaerobic bacteria carry out denitrification when oxygen diffuses too slowly through the soil to meet the microbial respiration demand Nitrate replaces oxygen as the needed electron acceptor in these instances Extreme environments The Archaea are divided into three large groups based on their physiological diversity The methanogens are strict anaerobes that produce methane CH4 halophiles are chemoorganotrophs that require 12723 percent salt to grow and one group of thermophiles grows best at temperatures over 80 C the other group at 0 C and lower Pyralabus grows at 113 C the hottest known temperature to maintain an organism Thermaplasma species combine low acidity and high temperature and live at pH 172 and temperatures of 60 C The methanogens are the only organisms that produce methane They use ammonium NHX as a nitrogen source and get their carbon from C02 The supplies of natural gas used today come from the metabolism of methanogens of the past and all of the methane in the air today around 16 ppm comes from the metabolism of current methanogens The Archaea are more closely related to the eukaryotes than to the Bacteria and probably evolved later than the Bacteria Our understanding of how many there are and where they live is limited By virtue of their metabolic versatility many live in remarkable environmentsihot springs hydrothermal vents at the bottom of the ocean in the gut of cows in vats of sulfuric acid in sewa e disposal tanks Recent discoveries in soils of rRNA fragments with molecular sequences similar to those in the Archaea suggest that the Archaea may be more common than we now know Systematics The oldest known fossils are suspected to be bacteria ancestors of the modern prokaryotes Fossil record Prokaryotes were the first organisms when the atmosphere of the new Earth was anoxic without oxygen and there was extensive volcanic activity Bacteriumlike filaments have been found in 35 billionyearold rocks in western Australia and bacteriumlike spheroids of the same age occur in South Africa Fossil stromatolites are present in many ancient sedimentary rocks worldwide Stromatolites are layered columns or mushroomshaped domes a few inches wide and a foot or more in height that formed when bacterial mats composed primarily of cyanobacteria growing in primeval ponds trapped sediments Together sediment and bacteria solidified into rock over the ensuing eons Stromatolites are forming today in many places in the same way by descendants of the same organisms Phylogeny Data from molecular sequencing of DNA has changediand continues to changeithe ideas concerning bacterial relationships At this time there is no clear consensus among microbiologists concerning the lineages among the prokaryotes With only an estimate 10percent of the bacteria named and the majority of those identified not yet studied in detail the task appears formidable Stay tuned Ecology Attributing lifeordeath importance to organisms too small to be seen without great magnification is difficult but consider that the prokaryotes I Decompose complex organic molecules and return to the soil and air the elements needed for growth of all organisms 25Page Participate in complex biogeochemical webs that concentrate mineralsiiron manganese copper and others I Maintain soil fertility by fixing atmospheric nitrogen thus assuring the supply of available nitrogen for protein and nucleic acid synthesis by all organisms I Are the base of food webs on land and in the oceans I Are crucial links in the sulfur phosphorus carbon oxygen and nitrogen cycles I Using novel metabolic pathways both discharge into the atmosphere and extract from it all of the major reactive gases nitrogen oxygen carbon dioxide carbon monoxide sulfurcontaining gases hydrogen methane and ammonia Human Interest Humans have a loveihate relationship with the prokaryotes They cause terrible diseases but by recycling the indispensable materials that all organisms need make life possible on Earth Plant pathology Among the other accomplishments of the prokaryotes is their ability to parasitize all manner of plants and animals Human bacterial diseases include tuberculosis Lyme disease black plague cholera botulism pneumonia and hundreds of others Bacteria cause particularly vicious disease in plants of all sorts The will diseases are produced by bacteria that live within the xylem vessels plugging them and preventing water from reaching the upper part of the plant which causes wilting After filling the vessel openings the bacteria next attack the cell walls rupturing the cells causing the plant to collapse Crowngall a cancerous tumor on stems is caused by bacteria that inject DNA plasmids into the host cell nuclei thereby taking control over hormone synthesisiand acquiring a home in the process Within the number of pathogens are the mycoplasmasibacteria that lack cell walls They are the smallest organisms able to live independently They are 02703 pm in diameter live in the sugar solutions carried in the phloem of vascular plants cause over 200 extremely destructive diseases of plants and were completely unknown for decades because they passed through standard laboratory filters and were invisible in ordinary light microscopes Researchers grew old and gray trying to determine what was happening in their supposedly cellfree solutions before the existence of mycoplasmas was known Genetic engineering Genetic engineers use recombinant DNA techniques learned from manipulating bacterial genomes to cut and splice packets of genes Bacteriophages viruses that infect bacteria often are used to carry genes between organisms Plasmids with genes of interest can be isolated and the genes duplicated by the billions in industrial applications to produce vaccines and hormones or in agricultural applications to develop plants and animals with desirable characteristics Opinions vary concerning the ethics of doing so Proponents minimize the risks to the environment but the potential to wreak havoc remains a disturbing possibility The prokaryotes are a mish mash of contradictions Some are the smallest of living organisms but they outnumber all others on Earth They are the most abundant organisms but can39t be seen with the naked eye and even light microscopes reveal only gross details they are structurally the simplest of organisms but they alone live and thrive in the most extreme environments imaginable They have been on Earth the longest of all organisms but we have only begun to identify the kinds extant around us They cause the deadliest diseases and the most efficacious antibiotic drugs We probably could survive without cheese and yogurt or pickles and sauerkraut which are possible because bacteria produce lactic acid and vinegar but life itself could not survive without the bacteria Viruses Although often studied by plant biologists viruses are not living organisms because they Are not cellular and have no cytoplasm membranes nor organelles Can39t metabolize39 they lack the enzymes necessary for protein synthesis and energy transfer Don39t increase in size they don39t grow Don39t respond to external stimuli Aren39t motile 26Page But they are able to reproduce within a host cell by using the metabolic equipment of the host Outside of a cell they are simply a collection of inert molecules of nucleic acids and proteins which can be denatured easily They are extremely small between 20 nm7 400 nm the size of large molecules Viruses are obligate intracellular parasites that live within the cells of all kinds of organisms frequently to the detriment and ultimate death of the host cells Their presence may trigger a response within the host that produces disease symptoms Each virus can enter only the cells of hosts with receptors specific for it39 hum ans don39t get canine distemper and dogs don39t get polioiboth virus caused diseasesibecause the proteins don39t match Viruses attack hosts whose genomes are most like their own Bacteriophages or simply phages are viruses that invade bacteria They have very small genomes consisting of only a few genes A still smaller replicating particle the viroid is a small bit of RNA without a protein coat Viroids are about a tenth the size of small viruses and replicate like viruses using the mechanisms of the host cell Viroids have been identified as causing some infectious plant diseases and probably are responsible for many animal diseases as well Plant diseases The viruses that invade plants do so by entering an open wound or other breaks in the surface or from the actions of an animal invader The first virus to be isolated and described was the tobacco mosaic virus which earned its investigator a Nobel prize in 1946 Over 1000 plant diseases are attributed to viruses and viroids and more than 400 kinds of viruses are involved Some plants harbor several kinds of viruses and show no effects while others such as Rembrandt tulips owe their unique ower colors to the presence of a virus in their cells The symptoms of viral diseases usually are systemic rather than localized because the virus spreads throughout the plant in the phloem and from cell to cell through the plasmodesmata There is no evidence that viruses can penetrate cell walls In plants infected by some kinds of viruses only the actively dividing cells of the growing points seem to be virusfree presumably because the meristem cells divide faster than the virus can move through the plasm odesmata into tip cells The universal plant response to a viral infection is reduction in sizeistunting of the whole plant small leaves and decreased flower and seed productiongdue to upset of normal metabolism and interference with translocation of materials in the phloem Symptoms include yellowing of leaves and mottling leaf spots wilting and tumors resulting in abnormal flowers and leaves Virusinfected plants are weakened and more susceptible to other diseases Except for tobacco mosaic disease there is no evidence however that viruses are spread from plant to plant by direct contact of plant parts Most are spread by insects nematodes or other animals such as slugs and snails or by soilinhabiting fungi Humans contribute to the distribution by making and growing cuttings from diseased plants There are no vaccines with which to inoculate plants against viral diseases Control at present consists of stopping the spread such as burning diseased crop plants and potential weed hosts sterilizing tools used to make cuttings destroying the seeds of infected plants and controlling insect and nematode vectors by insecticides and soil fumigation 27Page Chapter 20 Fungi and Lichens A Kingdom Separate from Plants The fungi singular fungus once were considered to be plants because they grow out of the soil and have rigid cell walls Now they are placed independently in their own kingdom of equal rank with the animals and plants and in fact are more closely related to animals than to plants Like the animals they have chitin in their cell walls and store reserve food as glycogen Chitin is the polysaccharide that gives hardness to the external skeletons of lobsters and insects They lack chlorophyll and are heterotrophic Familiar representatives include the edible mushrooms molds mildews yeasts and the plant pathogens smuts and rusts Most fungi are terrestrial multicellular eukaryotes the body soma of which is a mass of threadlike filaments called hyphae singular hypha which collectively form a mycelium plural mycelia When the fungus reproduces specialized hyphae pack together tightly and form distinctive fruiting bodies or sporocarps from which sexual spores are released The ordinary edible mushrooms are the fruiting bodies of fungi Fruiting bodies are temporary structures in the life cycle39 the primary body of all fungi is in reality the diffuse widespreading mycelium The fungi reproduce by spores both asexual and sexual and the details and structures of the sexual process separate the kingdom into four phyla see Table l The zygote is the only diploid phase in the life cycle39 meiosis occurs shortly after the zygote is formedi hence the life cycle is an instance of zygotic meiosis Chemical signals pheromones are exchanged among fungi especially between pairs preparatory to sexual reproduction TABLE 1 The Four Phyla of the Fungi Kingdom water molds molds zygomycetes Rhizapus Pilabalus fungi truf es morels bluegreen molds Saccharamyces Marchella mildew chestnut mushrooms rusts smuts puffballs Puccinia Uslilaga Palyparus Amanita Fungi are heterotrophs which release digestive enzymes into their surroundings and absorb nutrients back Some fungi are saprobes saprophytes as important in decomposition as the bacteria39 others are symbiotrophs living in symbiotic association with plants animals protists and cyanobacteria Wellknown symbioses are lichens that are associations of fungi and green algae or cyanobacteria39 mycorrhizae associations of fungi and plant roots and endophytes fungi and plant leaves and stems Some fungi are parasites fungal pathogens and responsible for diseases of both plants and animals Complex life cycles involving one or more hosts have developed between fungal pathogens and their hosts The Earth39s largest living organism may be a fungus either the mycelium reported from Washington state that covers 1500 acres but probably is disjointed and broken or the one in Michigan that covers 37 acres and is estimated to weigh 110 tonsithe weight of a blue whale 28Page Characteristics of Fungi All fungi have some features in common but other special structural and reproductive features separate the four phyla see Table l TABLE 1 Characteristics of the Fungi Phyla Specialize d Cell Where Asexual Nuclear Habita Flagellat Plant Wall Reproduct Fusion Phylum t ed Cells DiseasesPathogens s Chitin Hyphae ion Occurs Sexual Spore lacki Chytridiomyco black wart of potato aseptate None water yes ng in yes zoospores none ta brown spot of com some coenocyt1c occurs mostl fugion Of 2 os ore in y aseptate nonm otile two yg p Zygomycota terrestri no soft rot of many taxa yes yes zygosporangi a1 coenocyt1c spores gametangi um a mos H a few budding Ascom cota terresti39li no Dumh elm disease es With se tate conidia ascus eight y al chestnut blight y cellulo p fragmentati ascospores se on mostly black stem rust of buddmg con1d1a four bas1dio Bas1diomycota terrestri no wheat white pine yes yes septate bas1d1um fragmentati spores al blister rust on Structure The fungi are eukaryotic and have membranebound cellular organelles and nuclei They have no plastids of any kind and no chlorophyll The hyphae of the fungi are of two general kinds Some are septate and are divided by septa walls that separate the cylindrical hypha into cells in the nonseptate fungi the hypha is one long tube The septa are perforated however permitting the cytoplasm to flow throughout the length of the filament Mitosis occurs in the nonseptate hyphae but there is no accompanying cytokinesis division of the cytoplasm so the hyphae are multinucleate with many nuclei The special name for this conditionian organism or part of an organism with many nuclei not separated by walls or membranesiis coenocytic and the organism is a coenocyte A few fungiicalled by the general name yeastsiare singlecelled and nonfilamentous much of the time The only flagellated cells in the kingdom are the flagellated gametes of the chytrids Metabolism The fungi are all heterotrophic but unlike animals and many other heterotrophs that ingest their nutrients as bits or bites of food the fungi secrete digestive enzymes into their surroundings in effect digesting their food outside of their bodies They then can absorb the smaller particles and incorporate the nutrients into their own cells Some are parasites obtaining nutrients from living organisms but more are saprobes saprotrophs that digest and recycle materials from dead organisms In addition to potent digestive enzymes some fungi manufacture powerful alkaloids that when ingested by humans assail the nervous system causing hallucinations and even death The death angel Amanila is one such wellknown poisonous fungus ergot Clavicepspmpurea is another 29Page Fungal hyphae like the roots of vascular plants grow primarily at the tip elongating and branching repeatedly The filaments are in direct contact with their environment obviating in the fungal body the need for separate absorbing and conducting systems and precluding the need for storage tissues Materials readily pass through the plasma membrane and cell walls of the hyphae along their entire length although the most active metabolism and material exchange is concentrated near the hyphal tips Most of the cytoplasm is located at the tips also Fungi Reproduction Nonmotile sexual and asexual sporesimicroscopic in sizeiare the common means of reproduction and the primary agents of fungal dispersal They are readily carried in air or attached to the bodies of insects and other animals and are not resistant structures like bacterial endospores Although they can withstand desiccation they are killed by heat Sexual spores often require a period of dormancy after they are formed but asexual spores usually germinate and produce new hyphae whenever and wherever moisture is available Asexual spores are produced in special hyphae called sporangia in the zygomycetes and conidia in the ascomycetes and basidiomycetes Unlike many organisms that produce embryos the fungal spores form hyphae directly with no immature or embryonic stage between spore and adult Sexual reproduction Among fungi there are no female and male individuals and no eggs and sperm Physiological differences among the hyphae do exist however and result in different mating types only compatible strains fuse In the zygomycetes the strains are designated simply as and 7 Haploid n gametes are produced by mitotic division from haploid n parent nuclei in specialized hyphae called gametangia In the and 39 quot tart with hyphae from two mating strains fusing but the nuclei remain independent within the merged cytoplasm The name for this process is plasmogamy and the cells with the two genetically distinct haploid nuclei are called dikaryons In genetic shorthand dikaryotic cells are n n rather than the 2 n of diploid cells At some point the nuclei combine mixing the DNA from the two separate mating types This type of fertilization is called karyogamy the union of nuclei following plasmogamy Mycologists frequently use the term syngamy for the process of fertilization both syngamy and fertilization however mean the same thing the union of two haploid gametes to form a diploid zygote In most fungi karyogamy is followed almost immediately by a reduction division meiosis that restores the haploid chromosome number to the resultant spores and the new hyphae that are produced when the spores germinate Nuclear division Both meiosis and mitosis in the fungi are different from nuclear division in most other organisms Before the nuclear material is divided in plants and vertebrates the nuclear membranes nuclear envelopes disintegrate and the DNA condenses into discrete chromosomes that divide and move into new cells In the fungi the nuclear membranes do not disintegrate and in many taxa no discrete chromosomes appear nor do centrioles develop In some fungi spindles form outside of the nucleus and move into it while in others a spindle apparatus forms within the nucleus Lacking the elaborate apparatus of more advanced organisms the nuclei of fungi simply elongate constrict near the middle and pinch or tear apart into two daughter nuclei Life Cycle The predominant phase in the life cycle of fungi is haploid the zygote is the only diploid cell in the entire cycle This is called a zygotic life cycle and is the type prevalent in algae and some protists in addition to the fungi Systematics of Fungi Fungi are separated into phyla on the basis of their reproductive structures Because some fungi have never been observed to reproduce sexually they have no place in the classification Until their sexual reproduction is identified they are placed in Fungi Imperfecti Deuteromycetes DNA sequencing is giving the mycologists answers Most of the taxa so far sequenced and classified are ascomycetes with only a few basidiomycetes and fewer still zygomycetes Another problem group is the Chytridiomycota chytrids which arguably may belong with the Protoctists not the fungi 30Page Phylogeny No clearcut ancestral lineage for the fungi has been established but on the basis of molecular DNA sequencing and morphological evidence it seems likely that the fungal life style arose many times from different protoctists The fungi and animals are on the same originating branch Within the modern fungi the chytrids are the oldest of the group with the ascomycetes and basidiomyctes closely related and on a different more recent line from the zygomycetes Fossil record Evidence of fungi growing within the cells of 400millionyearold SilurianAge vascular plants suggests an early origin for the fungi The first fungi developing from very early eukaryotes undoubtedly were unicellular coenocytic filamentous forms were a later development An interesting proposal postulates that a symbiosis between early fungi and early plants permitted the plants to establish them selves on land before they had evolved roots with which to absorb vital water and minerals from the soil The fungi could do this for them and already were associated with some plants hence the start of the mycorrhizal association Ecology of Fungi Wherever there is moisture moderate temperatures and a supply of organic food there are fungi Since they digest their food outside of their bodies they literally live within their food supplies When the area around them is depleted they grow into a new supply They occur worldwide although there may be more taxa in the tropicsian assertion that is difficult to support because while there are an estimated 15 million species of fungi less than 10 percent of them have been described About 500 species are marine39 the rest are terrestrial with several thousand described symbionts and plant and animal pathogens Fungi usually are the primary decomposers in their natural habitats and are capable of digesting a wide array of organic materialsi including unfortunately some substances of economic importance to humans Most are saprobes but some like their animal relatives attack living prey a notorious example being the fungus that sets hyphal traps ensnares and then digests nematodes Many fungi are parasitic and the major pathogens of many crop plants such as corn and wheat Symbiotic Relationships Two important symbioses involve fungi the mycorrhizae that occur on the roots of almost all vascular plants and the lichens that have evolved entirely different body forms from those of their symbionts Mycorrhizae Fungi and the roots of almost all vascular plants form mutualistic associations called mycorrhizae singular mycorrhiza The fungus gets its energy from the plant and the plant acquires an efficient nutrient absorbing mechanismithe actively growing hyphae that penetrate regions of the soil untapped by root hairs Phosphate uptake especially is increased when mycorrhizae are present Two general types of mycorrhizae occur differentiated by whether the hyphae live within the cortical cells of the roots or remain outside the cells endomycorrhizae endo within myco fungus rhizae roots and ectomycorrhizae ecto outside taxa are 1 of most 39 while 39 quot and a few ascomycetes form ectomycorrhizae Lichens The symbiotic relationship of fungi with either algae or cyanobacteria produces a bodyia licheniso distinctly different from either of its symbionts that it is treated as a separate organism The fungal hyphae give the lichen thallus its characteristic shape and the cells of its photosynthetic partner are dispersed among them While the algal or in member can live 39 J the fungus cannot so the fungus in essence is a parasite on the photosynthesizer in the lichen thallus The fungus however provides a home for the photosynthetic cells as well as absorbing water and nutrients that the photobiont uses This makes the symbiosis mutualistic as much as parasitic in the view of some biologists Life is becoming precarious for lichens in many urban environments today Many lichens are intolerant of air pollutants They have no means of getting rid of the elements toxic or useful which they absorb Sulfur is particularly toxic to many and sulfur dioxide 31Page released from burning coal has eliminated many susceptible species from cities Lichens can be used as biomonitorsiand warningsi of the quality of the air we breathe Plant Pathogens Many of the fungi are pathogens that infect plants and animals causing diseases of many kinds The life cycles of many of these are complex and involve two or more host plants Rusts The rusts are specialized basidiomycetes that are parasites on plants They have complex life cycles and some produce five different kinds of spores in addition to basidiospores Many rusts are heteroecious and complete their life cycles on two different kinds of host plants whereas autoecious parasites produce all of their different kinds of spores on a single host species Wellknown examples of heteroecious rusts are wheat rust white pine blister rust and cedarapple rust Smuts Smuts are parasitic basidiomycetes that produce powdery masses of black spores enclosed in a membrane This membrane is often found in the ovaries of species of grasses or on their leaves The smut life cycle is less complex than that of the rusts and only one other kind of spore in addition to basidiospores is produced All smuts complete their life cycle on only one kind of plant Smuts live as saprobes in the soil however and readily attack developing seedlings planted in infected soil Corn smut is common in the Midwest Despite the unappetizing appearance of the spore masses and their dustlike texture membraneenclosed corn smut spore masses are considered delicacies in some cultures and are eaten either boiled or fried Yeasts Yeasts are unicellular fungi that reproduce asexually by budding a process by which a new cell is form ed from a bulge or bud that enlarges and pinches off from the parent cell The nuclear material is divided by mitosis and the new cell receives a nucleus and cellular organelles before severance from the parent Yeasts are found in all three of the fungal phyla but most are ascomycetes Many are filamentous most of the time and change to the yeast growth form only occas1onally Yeasts are of great importance to the baking and brewing industries with particular strains guarded and nurtured closely because the products of the yeast metabolism give the distinctive avors to the brews and cause the bread dough to rise in a predictable fashion 32Page
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