Class Note for HCS 625 at OSU 06
Class Note for HCS 625 at OSU 06
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Date Created: 02/06/15
HampCS 625 Crop Breeding Period 03 April 4 2005 The evolution of crop plants We are starting our e orts at crop improvement with species that have already been altered considerably Process started about 11000 to 13000 years ago The rst physical evidence of domesticates traces back as far as 9000 years before present BP Human management of wild plants and their eventual domestication occurred in various parts of the world Directed selection of crop plants has been associated with genetic change at the allele substitution level and through genomelevel changes e g ploidy The universe of wild plants 95 Human foraging activity observation trial and error manipulation chance Subset of selected Wild plants that are attractive for various reasons Domestication and genetic change of species A few hundred domesticates Definition of a domesticated crop A species that has been substantially altered 7 generally it can no longer survive in the wild gargma mam teosinte in or modern maize ears Examples of chemical changes include Reduced toxins eg cyanogenic glycosides in cassava and lima bean e g protease inhibitors and lectins haemagglutinins in soybean and Phaseolus beans Many regions are thought to have been important in the domestication of modern crop species Plant Breeding is Still Both and Art and a Science Much credit goes to the men and women who brought wild plants into cultivation and transformed them into domesticated species that undergird our present civilizations The changes we make to improve crops are often rather modest in comparison to the changes they achieved without modern technology Selection and transformation of plants is achieved through directed or artificial selection and through natural selection which is always acting to some degree Progress is made by using both types of selection to our best advantage It is impossible to evaluate future crop varieties in the future so past and present performance must be extrapolated as a predictor of future performance We can alter many features of the environment but some are more difficult than others 7 e g it is not so difficult to alter soil fertility but temperature and rainfall patterns can not be easily changed It is necessary to replicate trials of experimental breeding lines in as many seasons years and locations as possible This will maximize the beneficial effects of both natural and controlled selection Mating systems have been very important in plant improvement The major classifications are selfpollinating and outcrossing The majority of perennials are outcrossing and the majority of annuals are selfpollinating Not all plants are strictly selfpollinators and it is not uncommon to see several percent of outcrosses in what are known as selfpollinated species If this occurs with some regularity we might consider referring to them as inoutbreeders as described by Simmonds and Smartt 1999 The transition form wild to cultivated and from perennial to annual life cycle is frequently associated with a change from outcrossing to inbreeding or selfpollination During the transition there seems to be allowance for inoutbreeding We should perhaps also distinguish the asexually propagated outcrossers from those that cannot be vegetatively propagated The main difference as it affects breeding is the degree to which inbreeding occurs In outbreeding species such as corn throughout the rst thousands of generations of domestication they remained highly heterozygous and discemable multilocus population structure groups of favorable loci that have been fixed to homozygosity developed very slowly as did yield Selection can lead within a few generations to significant improvements in heavily selfing populations As we mentioned earlier it is interesting to note that in outcrossing species where dramatic progress has been made selection most often takes places in selfpollinated progenies Inbreeders achieve a different balance Individuals tend toward homozygosity 7 local adaptation is high but at the expense of longterm exibility to respond to change Perhaps small differences in amounts of sel ng versus outcrossing can greatly in uence success Here are some plants that are normally selfpollinated barley ax peanut tobacco jute rice tomato cowpea lentil sesame triticale sorghum oat soybean vetch cotton Here are some plants that are normally outcrossing maize sorghum sugarcane cotton olive onion alfalfa carrot these are inoutbreeders these are insectpollinated Morphological characteristics of owers that favor selfpollination o Anthesis oftem occurs before the ower opens Cleistogamy o Pollen normally fertilizes a ower on the same plant geitonogamy 0 Flowers are not particularly showy cotton is an exception 7 it has showy owers and does sometimes attract insect pollinators o Selfpollinators make good colonizers because only one is needed to start a new community Outcrossing populations will be highly heterogeneous and if they are selfpollinated inbreeding depression or even lethality can occur This is considered to be the result of deleterious recessive alleles Maintaining these loci in the predominantly heterozygous state covers the action of these undesirable alleles Outcrossing populations can carry a load of deleterious recessive alleles without serious impact Some un t individuals are produced but increases in recombination frequency and evolutionary exibility are achieved Inbreeding depression is usually manifested through reduced vigor and fitness Carefully selecting inbred lines and then crossing them as in the development of modern maize hybrids can result in very high vigor often referred to heterosis or hybrid vigor Normally self pollinating species do not lose vigor when most or all loci are in a homozygous state What breeding approaches will be most successful depends not only on the mating system but it also depends on the particular aspects of the crop For example maize hybrids can be developed through intensive selfpollination and selection practically because of the ease with which artificial pollination can be affected On the other hand more elaborate procedures need to be undertaken in the development of a synthetic alfalfa multiinbred derived hybrid because pollination and seed production is much more difficult insect pollination Plants have adapted various mechanisms to control outcrossing One is a physical one wherein the ower parts actually physically cover the ower to assure selffertilization Flowers may also be found on separate parts of the plants or entirely on different plants In other instances the timing of pollen shed and stigma receptivity do not overlap When the pollen sheds first the ower is referred to as protandrous and when the stigma is receptive first the ower is referred to as protogynous Pollination control can also arise from genetic mechanisms The two main systems are refereed to gametophytic and sporophytic selfincompatibility systems In addition to morphological chemical and molecular level changes changes at the genome or cytogenetic level occurred during crop evolution A quick review of cvtogenetics ploidv 2n was once widely used to refer to a diploid organism in the manner 2x has been used in the table So where did all the Ns go De nition the haploid number N or n is equal to the gametic chromosome number King 1968 For a normal haploid gamete n or 1n the ploidy level of the whole organism would be 2n ln is what you would find in the single nucleus of a pollen grain from eg a diploid 2n plant It gets a little confusing when you have to deal with polyploids or eg unreduced gametes For example what is the ploidy level of a strawberry gamete Cultivated strawberry is an octoploid F ragarz39a species it has 8 genome complements or is 2n 8X so the gamete has 4 genome complements quite unlike the haploid genome of a diploid that has one and is n or In Thus the gamete of a strawberry is 1n which equals 4X The term n is perhaps a bit ambiguous because it was commonly used both for the reduced gametic number of chromosomes of an organism and for the chromosome number of the lowest or base genomic complement monoploid of a polyploid series In practice confusion can be avoided by using the symbols n and X to connote haploidy in the two senses it conveys If n the number of the haplophase reduced gametic number and 2n the chromosome number of the diplohase unreduced zygotic number then X the haploid monoploid number or the basic number of a polyploid series Hence 2X 3X etc would refer to diploid triploid numbers etc actual chromosome numbers and n would be reserved for description of the gamete chromosome number Tsuchiya and Gupta and 1991 provide an example for a ploidy series in barley H ordeum vulgare wherein the chromosome numbers of haploid diploid triploid and tetraploid barley are as follows mm somatic cell 2n 2X 14 Gamete normal n X 7 Tetraploid 4X 28 The somatic cell is double the gamete so it is 2n 4X 28 n 2X 14 mm Gamete unreduced n X 7 so the somatic cell 2n X 7 mm Somatic 2n 3X 21 Because there may be some pairing across genomes meiosis will be abnormal and the gametes will be n X to 2X X X 1 X 2 etc 7 to 14 As you can see from the eXamples in Table 11 ploidy has played an important role during the domestication of many important crops How do you wind up with odd levels of ploidy If a diploid species has two similar genomes designated AA 7 then an autotriploid becomes AAA and an autotetraploid AAAA The latter would have its origin directly from the diploid by doubling of its chromosomal number either by somatic doubling or by the union of two diploid unreduced gametes This may occur in nature infrequently due to temperature stress e g cold shock or may be induced by using colchicine The triploid could arise as an offspring of a tetraploid and a diploid parent or from diploid parents by the union of an unreduced and a reduced gamete in an individual Autopolyploids The number of autopolyploids existing in nature is meager Compared to allopolyploids autopolyploidy has had much less evident evolutionary impact It is particularly important when the desired form when once induced can be vegetatively propagated For example polyploid fruits such as seedless grapes and bananas triploids may be vegetatively propagated as is Solanum potato 4x autopolyploid Clonal propagation of crop plants in a sense frees the plant of the need to set viable seed Polyploid plant characteristics 0 Larger than their related diploids as a result of an increase in cell size just as haploids are smaller 0 Increase in size of various plant parts particularly in such organs as owers 0 Delay in growth and in owering 0 Increase in the darkness and thickness of the foliage as one goes from haploid to diploid to tetraploid plants of the same genetic stock may result 0 Beyond the tetraploid level increases in chromosome number often result in abnormalities such as dwarfing wrinkled foliage and weak plants Allopolyploidy The genomic constitution can be represented as AABB with the individual having arisen by a doubling of the chromosome number of an F1 hybrid between species A and species B If the genomes are sufficiently dissimilar structurally no synapsis pairing will occur in the diploid hybrid and high sterility will result due to random segregation of unpaired chromosomes Doubling of the chromosome number to give the allotetraploid AABB will provide for regular synapsis and segregation Genome A will pair with genome A genome B with genome B The gametes will be AB and if sterility in the F1 hybrid is due only to irregular chromosome distribution then the allotetraploid can be expected to have a high degree of fertility However when hybrid sterility is due to genic as well as to chromosomal imbalance doubling of the chromosome number will not restore fertility A number of allotetraploids are known to have occurred in nature Nicotiana tabacum 2n 48 and Gossypium hirsutum 2n 52 tobacco upland cotton respectively are examples whose diploid ancestors are known and from which the allotetraploids have been resynthesized Tetraploid tobacco is the result of the doubling of the chromosome number in the hybrid between N otophora 2n 24 and N silvestris 2n 24 Cotton is the result of crossing Old World cotton G arborium 2n 26 with American cotton G thurberi 2n 26 Allopolyploids of a more complex nature can arise Partial quadrivalent formation may occur at meiosis This occurs when the two genomes are not totally dissimilar and partial synapsis and crossing over can take place between the chromosomes of different genomes Ledyard Stebbins has referred to such individuals as segmental allopolyploids The segmental allopolyploids fall in between the autoploids and the alloploids and it is believed that they are more common in nature than either of the two extreme types Auto and allopolyploids are the extremes of a spectrum of forms in which there is a range from complete homology between all of the sets of chromosomes auto to a complete lack of homology allo between the two kinds of genomes Inheritance and a high degree of fertility in polyploids is possible only when pairing between homologues leads to bivalent formation and anaphase segregation is highly regular Triticum aestivum bread wheat indicates that the distinction between these two kinds of polyploids may be a matter of genetic control rather than of structural or genic dissimilarities T aestivum is a hexaploid 2n 6x 42 and its three genomes labelled A B and D have been traced to specific wild relatives only bivalent formation occurs with no homoeologous pairing evident thus gametes with 21 chromosomes are regularly produced This is due to the overall suppression of homology so that only the most homologous chromosomes e g within the respective genomes A B and D pair at all This type of genetic control of meiosis is not unique to wheat It has also been documented in barley oats and F estuca At first the single gene Ph for pairing homoeologus suppressor on chromosome 5B was thought to control meiotic pairing Later it was found that several other chromosome pairing genes also have lesser effects The significance of polyploidy in the evolution of plants is suggested by its prevalence and distribution Among other living organisms only amphibians seem to have undergone considerable changes in ploidy level during evolution Grasses have about 75 of their species exhibiting polyploidy to one degree or another Genetic segregation among such forms will be curtailed The disadvantage is apparently outweighed by the relative vigor of a select number of polyploids and their ability to permit the species to exploit new and diverse ecological habitats the latter activity arising out of the new gene combinations brought into being by the mixture of two previously separated species Polyploidy enables escape from outbreeding o Preserved hybridity o Enables tolerance to inbreeding It is important in the founder effect of small populations References Allard R W 1999 Principles of Plant Breeding 2quotd Edition Academic Press Tsuchiya T and PK Gupta eds 1991 Volume 2A Chromosome engineering in plants genetics Breeding and Evolution Part A 1991 Elsevier Science Publishers Amsterdam King RC 1968 A Dictionary of Genetics Oxford University Press Simmonds NW and J Smartt 1999 Principles of Crop Improvement 2nd Edition Blackwell Science Swanson CP T Mertz and WJ Young ed Cytogenetics the Chromosome in Division Inheritance and Evolution Prentice Hall 1981 Singh RJ Plant Cytogenetics CRC Press 1993
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