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Date Created: 10/05/15
Chapter 14 About 70 000 species are known and about 1700 new species are described every year The total number of species is estimated to be about 15 million Fungi differ from plants and animals in their life cycles mode of nutrition developmental pattern and molecular and physiological characteristics Molecular evidence strongly suggests that fungi are more closely related to animals than to plants IMPORTANCE OF FUN GI l Fungi with bacteria are the principal decomposers of the biosphere Indispensable in the recycling of matter Used in toxic waste cleanup programs Attack food paper wooden objects waxes leather paint petroleum wires photographic lm etc 0 Individual species are highly speci c to particular substrates Decomposition breaks down the organic material incorporated into the bodies of organisms releasing carbon dioxide into the atmosphere and returning nitrogenous compounds and other materials to the soil The released products of decomposition can then be reused recycled by plants and eventually by animals Some species of fungi are terrestrial others are marine or freshwater species Fungi are the major decomposers of wood They have enzymes that break down lignin and cellulose They are a big concern for the food industry because they attack fruit vegetables and meat Mycotoxins are very toxic substances produced by some species while decomposing vegetables and meat 2 Important pathogens Over 150 species have been identi ed as serious pathogens of domestic animals and humans Over 5000 species attack economically important crops and garden plants Pneumocystis carinii causes about 80 of the deaths of AIDS patients is probably a chytrid and not a protozoan In humans fungi cause ringworm athlete s foot candidiasis and histoplasmosis 3 Indispensable in the baking and brewing industry 4 Food o Mushrooms truf es and morels are used as food 0 Important in the making of cheeses and other foodstuff like soy sauce U Producers of chemicals and medicines Penicillin cyclosporin ethanol Some fungi are used to make citric acid and other chemicals DNA manipulation of fungi is being used to produce hormones Ergot is used to produce certain drugs eg LSD 6 Symbiotic relationships 0 Mycorrhizae a bene cial association between the roots of plants and fungi 0 About 80 of the vascular plants form mycorrhizal associations 0 Endophytes living inside the leaves of plants and produce protective chemicals against grazers 7 Soil binding CHARACTERISTICS OF THE KINGDOM Multicellular or unicellular eukaryotes The laments forming the fungal body are called hyphae plural hypha ploidy n The network of hyphae is called the mycelium ploidy n Hyphae may or may not be divided by crosswalls called septa Hyphae lacking septa are said to aseptate or coenocytic multinucleate The septa usually have a pore which may or may not be obstructed Hyphae may be monokaryotic dikaryotic or coenocytic Hyphae may be packed together to form mushrooms This lamentous growth means that the fungus is in intimate contact with its surroundings it has a very large surface area compared to its volume While this makes diffusion of nutrients into the hyphae easier it also makes the fungus susceptible to desiccation and ion imbalance But usually this is not a problem since the fungus is growing within a moist substrate Specialized hyphae known as rhizoids anchor the fungus to the substrate Parasitic fungi have specialized hyphae called haustoria that penetrated the cells of the host and absorb nutrients directly from the cells 2 Haploid organisms with the diploid phase of the life cycle represented only by the zygote They have diploid nuclei just prior to meiosis 3 Lack chlorophyll are heterotrophs that absorb nutrients from the environment 0 Extracellular digestion by secreting enzymes exoenzymes on the food source 5 U 9 gt1 0 Released molecules are absorbed mostly near the growing hypha tip 0 Saprophytes parasites and symbionts o Parasitic fungi have specialized hyphae called haustoria sing haustorium which absorb nutrients directly from the cells of their host Cell wall including that of spores is made of chitin a polymer of a nitrogencontaining sugar 0 Chitin is also found in the exoskeleton of arthropods o Chitin is very resistant to microbial degradation 0 The structure of chitin Chemically chitin is a polymer formed primarily of repeating units of beta 14 2acetamido2deoxyDglucose or N acetylglucosamine Its structure resembles that of cellulose except that acetylamino groups have replaced the hydroxyl groups in position 2 Not every unit of naturally occurring chitin is acetylated about 16 are deacetylated Store food in the form of lipids and glycogen o Glycogen is a glucose polysaccharide found in also in animals Unique variations in mitosis and meiosis o In some fungi the nuclear envelope does not disintegrate and reform but it becomes constricted near the midpoint between the two daughter nuclei 0 In other fungi the nuclear envelope breaks down near the midregion o The spindle forms inside the nuclear envelope or outside the nucleus and then moves inside in the Basidiomycota o Fungi lack centrioles except chytrids o Spindle pole bodies which are microtubule organizing centers form outside the nucleus Most reproduce sexually and asexually by means of spores o Spores are produced by mitosis or zygotic meiosis o Spores may be produced sexually or asexually o Spores are nonmotile except in chytrids Asexual spores are produced in sporangia or from conidiogenous cells 0 Conidia may be produced singly or in chains 0 The stalk on which the conidia are produced is called conidiophore Sexual spores are produced in three stages plasmogamy karyogamy and meiosis o Plasmogamy refers to the fusion of two hyphae conjugation o Karyogamy is the fusion of nuclei It may occur immediately after conjugation or may be delayed producing a dikaryon a cell with two nuclei 0 Dikaryotic cells may exist for months and years and multiply producing more dikaryotic cells 0 Dikaryon ploidy 7 nn occurs in the l 39 of 39 of Ascomycetes and hyphae of Basidiomycetes o Meiosis follows karyogamy sooner or later reestablishing the haploid condition o Meiosis results in the formation of specialized spores ploidy n zygospores ascospores basidiospores o Fungi are often classi ed according to the types of sexual spores that are produced The diploid phase in the life of the fungus is represented only by the zygote nucleus Meiosis typically follows the formation of the zygote zygotic meiosis Gametes are produced in structures called gametangia sing gametangium The gametangia may form different seX cells called gametes or may simply contain nuclei that function as gametes httpWWW l I 39 39 39 39uml7 9v lnoi html EVOLUTION OF THE FUNGI The evolutionary history of the fungi is poorly understood The fungi appear to be a monophyletic lineage with greater relationship to animals than to plants Link between the fungi and protists may be the chytrids which are fungi that have retained the agellated condition There is considerable molecular evidence that both animals and fungi diverged from a common ancestor probably a colonial protist resembling a choano agellate Earliest fungi were probably aquatic agellated organisms Fungi fossilize poorly The earliest fungus fossils are aseptate laments from the Lower Cambrian about 544 mya Precambrain before 570 mya By 2000 m y a branching nonseptate laments The evidence is equivocal since these branching laments may be the cases of cyanobacteria also known as bluegreen algae The oldest fossil mgi so far known are chytrid like forms from the Vendian of northern Russia Older fossils ofPrecambrian mgi are now usually considered to be empty sheaths of filamentous cyanobacteria or else are not distinct enough to be placed in any taxon with certainty Amazinglyfine chytrid fossils are known from the Devonian Rhynie Chert where they occur alongside representatives of other major mgal grzmps Some Devonian forms are parasitic on rhyniophytes and closely resemble the modern genusAllomyces Blastocladiales both inform and in reproductive features of the life cycle Other Rhynie chytrids resemble members of the Spizellomycetales suggesting that chytrids had diversified by this early date Chytrids are not merely quotfirstquot because of the age of their fossils however Studies of the evolutionary relationships between chytrids and other indicate that they are the sister group to the remaining mgal grzmps or that they may be a paraphyletic basal assemblage This means that chytrids may give us a goodpieture ofwhat the ancestors of mgi were like Tree of Life httpwww m mn berkelev a 39 L tridshtrnl Cambrian 570505 mya and Ordovician 505435 mya Aseptate laments found It has been suggested that during these periods precursors of the Basidiomycetes and Ascomycetes developed as parasites and saprophytes of algae Silurian 435408 mya Some evidence of Ascomycetes There might be a correlation between the appearance of land plants and the appearance of terrestrial fungi passengers of autotrophs Devonian 408345 mya Chytrids present by the Lower Devonian 395374 m y a Fossil resembling Zygomycota found forming endomycorrhizae which penetrate cells First fossil Basidiomycota found in Upper Devonian 380 mya http universereviewcaR l 023plants htm TAXONOMY Phylum Chytridiomycota chytrids Predominantly an aquatic group fresh water a few in salt water and some terrestrial One class is unicellular other three classes form mycelium Only fungal group that produce motile reproductive cells zoospores asexual and gametes Mostly coenocytic with a few septa at maturity that separates the reproductive organs Hyphae of some chytrids have pseudosepta incomplete partitions made of substances different from cell wall material Contain chitin in their cell wall some contain cellulose as well Store food as glycogen Meiosis and mitosis is intranuclear similar to other fungi The nuclear envelope remains intact until the telophase when it breaks up and reforms around the daughter nuclei 0 Gametes could be similar isogametes or different one at least motile 0 Some species are parasitic on algae protozoa and other aquatic fungi and spores pollen and terrestrial plants Other species are saprophytic About 790 species 0 NOTE Some taxonornists consider the chytrids to be protists due to the production of motile cells These scientists consider members of the Fungi those species that do not produce motile spores Phylum Zygomycota zygomycetes o Multicellular land fungi o Divided into three to seven classes according to different schemes of classi cation Coenocytic mycelium with septa separating the reproductive structures rhizoids are formed by the hyphae at intervals of the growing stolons hyphae Asexual haploid spores formed in sporangia supported by sporangiophores Sexual spores formed the fusion of hypha endings called gametangia The fused tips develop into a zygosporangium which in turn will produce zygospores Mating types or strains required in some for conjugation to occur heterothallic species Chitin in cell wall no cellulose present Saprophytic mostly some parasitic on plants and insects some form endomycorrhizae About 1060 species described kellnoo m mi 39 39 39 39uiu39 ll 1208Wf u mm Phylum Ascomycota sac fungi ascomycetes Multicellular or unicellular fungi Terrestrial fungi Four classes recognized by many mycologists Hyphae are narrower than the Zygomycota with a perforated septum and multinucleated Chitin present in cell wall cellulose absent Asexual spores are called conidia are multinucleate and produced at a hypha tip the conidiophore This phylum is characterized by a sexual state composed of ascospores within saclike asci sing ascus The asci are contained within or on a variety of ascocarps ascomata including perithecia cleistothecia and apothecia The asci develop on an inner surface of the ascoma a layer called the hymenium The formation of multinucleate gametangia antheridia and ascogonia precedes sexual reproduction Male nuclei pass into the ascogonium via the trichogyne which is an outgrowth of the ascogonium The male and female nuclei pair up within the ascogonium but do not fuse ascogenous hyphae begin to grow and septa are formed separating pairs of nuclei This phylum has a dikaryon in the ascogenous hyphae Asci form at the tip of the ascogenous hyphae Homothallic and heterothallic Important parasites and saprophytes Yeasts are unicellular ascomycetes Many edible species About 32300 species described Phylum Basidiomycota club fungi Multicellular fungi Terrestrial fungi Two or three classes are recognized Welldeveloped mycelium Hyphae are narrower than the Zygomycota and typically septate dolipore septum Two phases in the life cycle monokaryotic primary mycelium and a dikaryotic phases secondary mycelium Dikaryotic mycelium is formed by the fusion of monokaryotic hyphae from different mating types They have a prolonged binucleate dikaryotic stage which is maintained by use of clamp connection The mycelium that forms the basidioma pl basidiomata is called tertiary mycelium which becomes differentiated into specialized hyphae that plays different functions within the basidioma Sexual spores are called basidiospores through meiosis Meiotic basidiospores are formed eXtemally on the differentiated hyphal tips basidia which are usually the site of nuclear fusion and meiosis Important saprophytes and parasites major decomposers of wood and plant litter Many edible species Mushrooms puffballs shelf fungi toadstools rusts smuts stinkhorns etc About 22300 species Class Basidiomycetes Basidia are incorporated into compleX fruiting bodies called basidiomata sing basidioma also known as basidiocarps Hymenomycetes Basidioma structure volva or cup stipe or stalk pileus or cap lamellae or gills lined with the hymenium Each basidium develops from a terminal cell of a dikaryotic hypha Karyogamy occurs as the young basidium enlarges which is followed immediately by meiosis resulting in four haploid nuclei that migrate into a sterigma the sterigma enlarges at its tip and forms a basidiospores with a haploid nuclei within The basidiospores are produced on a distinct layer called the hymenium and the spores are discharged forcibly when mature Basidiospores are attached to the basidium by minute stalks called sterigmata sing sterigma Gasteromycetes o Characterized by basidiospores developing inside the basidioma and are not discharged forcibly o Basidioma covered by the peridium which could be thin and papery to thick and leathery o In stinkhoms the fertile portion of the basidioma is called the gleba o Puffballs stinkhoms bird snest fungus earth s star mushrooms are examples of gasteromycetes Class Teliomycetes rusts Commonly known as rusts they do not form basidiomata Their spores are formed in masses called sori Form dikaryotic hyphae and basidia Hyphae and basidia are septate They have a compleX life cycle They are plant pathogens of tremendous importance eg Puccinia graminis parasitize wheat rye oats barley and several species of wild grasses 0 Study the life cycle of Puccinia graminis A heteroecious parasite that requires two hosts barberry and a grass Autoecious parasites require one host Barberry phase Spermogonia produce spermatia n Spermatia and receptive hyphae fuse and form dikaryotic hyphae n n Dikaryotic hyphae produce aescia with chains of aesciospores n n Grass phase Aesciospores infect the grass and germinate Red streaks of uredinia sing uredinium are formed Uredinia form urediniospores n 11 throughout the summer In late summer the uredinia darken and become telia Each telia forms teliospores n n The nuclei in the dikaryotic teliospores fuse Teliospores 2n overwinter in the diploid stage In the spring the teliospores undergo meiosis and germinate to form basidia Each basidia produces four haploid basidiospores n Basidiospores infect the barberry Basidiospores germinate on the barberry bush and the cycle begins anew Class Ustomycetes smuts Commonly known as smuts and most form septate basidia They are parasites of economically important crop and ornamental plants About 1070 species have been described Ustilago maydis infects corn and is known as the corn smut Oats and wheat are also infected by other species of Ustilago YEASTS Ustilago is an autoecious parasite requires only one host Infected corn ears form tumors caused by the mycelium growing inside the kernels The dikaryotic mycelium gives rise to thickwalled teliospores Karyogamy and meiosis occurs within the teliospores Teliospores give rise to a fourcelled basidium Basidia produce basidiospores which infect other corn plants Basidiospores also produce sporidia uninucleate spore produce by budding basidiospores Upon germination basidiospores or sporidia produce a or strain mycelium Mycelia of opposite strains undergo plasmogamy and produce dikaryotic hyphae Dikaryotic hyphae produce teliospores Yeasts are unicellular fungi They could belong to the Z or 39 quot they are not a taxon Most are ascomycetes about 14 are basidiomycetes There are about 80 genera with about 600 species They usually reproduce by budding an asexual method of reproduction Some species form short laments and can exist in both lamentous and unicellular forms Yeasts are important in the baking Saccharomyces cerevisiae and brewing Saccharomyces carlsbergensis industries Some are important human pathogens Candida albicans causes thrush Cryptococcus neoformans causes cryptococcosis Some species are used DNA and chromosome studies molecular genetics and physiology DEUTEROMYCETES or CONIDIAL FUNGI anamorphs This is an arti cial grouping of about 15000 species of fungi for which the sexual stage is not known Only their conidial or asexual reproducing state is known They are also known as the Fungi Imperfecti the quotimperfectly known fungiquot What fungi are classi ed as conidial fungi l The sexual stage may have been lost in the course of evolution 2 The sexual stage still has to be discovered 3 The sexual stage is not used as the basis of classi cation because of the close resemblance to conidial fungi and their features eg Aspergillus and Penicillium On the basis of their overall characteristics most conidial fungi are clearly ascomycetes a small percentage are basidiomycetes presence of clamp connection or zygomycetes Many exhibit heterokaryosis the presence of different nuclei in the same cytoplasm due to mutation or plasmogamy with different hyphae Heterokaryosis may exist in different portions of the mycelium These portions have different properties Haploid nuclei may fuse and form a new diploid nucleus This diploid nucleus loses chromosomes until the haploid condition is reestablished A phenomenon called haploidization o Haploidization does not involve meiosis o It was discovered in Aspergillus The occurrence of plasmogamy karyogamy and haploidization in sequence is called parasexuality Parasexuality increases the genetic variability and evolutionary potential of the fungus Deuteromycetes are of great economic importance Penicillium species are important in the making of a variety of cheeses and antibiotics Aspergillus oryzae andA soyae produce soy sauce Some species of Aspergillus cause a atoxins a liver carcinogen Dermatophytes are conidial fungi that cause skin diseases ringworm athlete s foot etc SYMBIOTIC RELATIONSHIPS Symbiosis is the close and long term association of two different species There are different forms of symbiosis eg parasitic mutualistic Lichens The association of a photobiont an alga or cyanobacteria and a mycobiont a fungus forms lichens Lichens are polyphyletic DNA studies evidence indicates that they have evolved independently at least ve times About 13250 species of lichen forming fungi are known About 40 genera of photobionts are found in combination with the mycobionts Some lichens incorporate two photobionts green algae and a cyanobacterium About 90 of the lichens have one of four genera of photobionts T rebouxia Pseudotrebouxia and T rentepohlia and the cyanobacterium Nostoc The photobiont may be scattered evenly throughout the thallus or form a distinct layer within the thallus The majority of the mycobionts 98 are Ascomycota The rest are Basidiomycota Lichen species are usually specialized to a particular substrate Growth habit o Crustose rmly and atly attached to the substrate 0 Foliose forms are leaflike o Fruticose lichens are upright and branched bushylike Lichens reproduce by fragmentation soridia clusters of hyphae and alga cells and isidia small outgrowths The mycobiont may form ascospores basidiospores or conidia o The ascomycete may form ascomata Independent dispersal and reassociation occurs commonly in nature SURVIVAL OF LICHEN S Lichens are well adapted to survive under adverse conditions specially desiccation and cold they remain dormant when dry 0 Lichen dry out very quickly 0 Their water content in nature is between 2 and 10 0 When a lichen dries out photosynthesis stops Lichens are found from deserts to the Attic and Antarctic regions 0 One species is a marine species 0 Seven species live within 40 ofthe South Pole Lichens grow very slowly adding to their thallus 01 to 10 mm a year The oldest known fossil lichen is from the Early Devonian 400 million years ago A lVIUTUALISTIC RELATIONSHIP BETWEEN THE MYCOBIONT AND PHOTOBIONT The nature of this relationship is under scrutiny is this a mutualistic or a parasitic relationship 0 The mycobiont produces haustoria or appressoria that penetrate the cells of the photobiont and absorb carbohydrates and nitrogen compound from the photobiont o The mycobiont controls the reproductive rate of the photobiont o The mycobiont provides the photobiont with moisture minerals attachment and protection against elements On the whole the lichen association is mutualistic at the cellular level the mycobiont parasitizes the photobiont Neither partner can ourish in the niches in which they are commonly found without the partner ECOLOGICAL IMPORTANCE Mycobionts produce a large number of acids that weather rocks and promote soil formation Cyanobacteria X nitrogen Lichens trap soil particles and play a role in ecological succession Some have antibiotic properties provide food for animals and nesting materials to birds Lichens can be used as ecological indicators o Are sensitive to S02 0 Accumulate heavy metals outside their cells 0 Used in monitoring radioactive fallout Mycorrhizae Mycorrhizae are mutualistic associations between fungi and roots It has been found in most cultivated and wild plants The mustard family Brassicaceae and the sedge family Cyperaceae do not form mycorrhizae Mutualism The host plant bene ts by their increase ability to absorb water and minerals especially phosphorous zinc copper and manganese Mycorrhizal fungi protect the root of plant against attack by parasitic fungi and nematodes The fungus receives carbohydrates and other organic molecules synthesized by the plant Endomycorrhizae penetrate root cells It occurs in about 80 of the plants The fungal component is usually a zygomycete About 200 species are involved worldwide The hypha forms an invagination of the cortical cells of the root and forms arbuscules with vesicles The other portion of the hypha eXtends into the surrounding soil and absorbs water and nutrients Ectomycorrhizae surrounds but do not penetrate the root cells Ectomycorrhizae is commonly found in trees growing in temperate regions Some have been found in the tropics also Willow family Salicaceae birch family Betulaceae beech family Fagaceae and pine family Pinaceae have ectomycorrhizal associations It possibly makes the trees more resistant to cold dry conditions The hyphae grow in between the cortical and epidermal cells of the root forming a network called Hartig net A mantle of hyphae covers the root surface and mycelium eXtends from the mantle into the soil Most 39 39 39 fungi are 39 39 quot but are also involved In the Ericaceae heather family the ectomycorrhizal hyphae form a web surrounding the roots o The hyphae do not extend into the soil 0 The hyphae release enzymes that break down organic matter in the soil which becomes available to the plant 0 It enhances nitrogen rather than phosphorus uptake 0 It allows members of this family to colonize infertile acidic soils 0 Basidiomycetes and ascomycetes are involved in this association Orchid seed germinate only in the presence of suitable fungus o The fungus supplies the seedling with carbon compounds 0 Basidiomycetes and ascomycetes form these associations There is evidence that mycorrhizal associations eXisted very early and it might have been crucial in the colonization of land by plants Biological terms hum F 39 39 nsn quot 39 J 39 39 39 htm Other fungi related sites lmn F quotnsn 4 39 J quotquot1 htm httpWwwmycologcom fthtochtml httpwwwmycologcomchapterl lahtm fungal ecology httn39 hno hinnsvd edn All 39 Glossarv gloqqaw a h quot 39 J glossary Lichens http wwwlichencomindeXhtml http wwwlichencomlinks html http wwwlichencomp ortraits html
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