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FUNGI Unique Character of Fungi Studied by mycologists Traditionally classified as plants Unlike plants due to lack of chlorophyll Like plants due to immobility and linear growth form Fungi are filamentous, plants are three dimensional Composition of cell walls differs Plants photosynthesize, fungi absorb food after secretion of enzymes and extracellular digestion Some plants have flagellated sperm while fungi do not Economic Value of Fungi Many are harmful Cause decay, rot and spoilage Cause serious plant and animal diseases Many are beneficial Yeasts are used in the manufacture of bread and beer Used to flavor cheese, wine and other foods Industrial production of acids, antibiotics, etc Detoxify environment NUTRITION AND ECOLOGY Fungi and Bacteria Are the Principle Decomposers in Every Ecosystem Break down lignin, a major part of wood Provide building blocks for growth of other organisms Some fungi attack still living organic matter Cause agricultural damage Destroy food stores making them unpalatable or poisonous Important Mutualistic Associations Lichens = fungi + green algae or cyanobacteria Mycorrhizae = fungi + plant roots fig 32.2 Photosynthetic partner fixes carbon dioxide and provides food Fungal portion enhances existence within a particular habitat Mycorrhizae facilitate absorption of essential nutrients by plant roots STRUCTURE Filamentous Growth Form Slender filaments called hyphae May be divided into cells by septa Barrier incomplete except when separating reproductive cells Cytoplasm flows freely through pores in septa fig 32.3 Results in rapid growth with optimum food, water and temperature Mass of hyphae called mycelium If strung end-to-end would be many meters long Surrounds and penetrates substrate All parts are metabolically active and interact with environment Rapid growth conspicuous if visible reproductive structures formed Evidence of Unique Evolution that Is Not Related to Plants or Animals Composition of cell wall Polysaccharide plus chitin (not cellulose) Same component of arthropod exoskeleton More resistant to bacterial degradation Distinctive form of mitosis Nuclear envelope remains intact Spindle apparatus forms within nuclear envelope Lack centrioles Microtubule formation regulated by spindle plaques REPRODUCTION Fungal Genetic Status All nuclei haploid except for zygote nuclei Sexual reproduction in two of the three phyla Hyphae of two different mating strains meet and fuse Two types of nuclei coexist without fusion for most of the life of the fungus Important terminology Heterokaryotic hyphae have two kinds of genetically different nuclei Homokaryotic hyphae have genetically similar nuclei Dikaryotic compartment has two genetically distinct nuclei Monokaryotic compartment has a single nucleus Dikaryotic hypha is always heterokaryotic Monokaryotic hypha is always homokaryotic Dikaryotic hyphae have some properties of diploids, both genomes transcribed Types of Reproductive Structures Cytoplasm flows freely if no septa, or through perforations in septa Sporangia: involved in spore formation Gametangia: structures in which gametes are formed Conidia: multinucleate asexual spores not produced in sporangia Spores are always non-motile Distribution via wind, over great distances Dispersed by insects, small animals FUNGAL PHYLA Historical Aspects of Classification Plant-like, thus grouping was called division, now phylum is preferred Presently differentiated from protist slime molds and water molds Characteristics of oomycetes (water molds) Motile spores Cellulose-rich cell walls Regular patterns of mitosis Diploid hyphae Differentiation of phyla by sexual reproductive structures Zygomycetes Hyphal fusion results in formation of zygote Zygote undergoes meiosis at germination Ascomycetes and basidiomycetes Distinctive reproductive cells formed from dikaryotic hyphae Meiosis immediately follows syngamy, nuclear fusion Produces haploid spores Phylum Zygomycota Nonreproductive hyphae lack septa Include common bread molds Produce characteristic zygospores, temporarily dormant structures Typical life cycle and sexual reproduction Sexual reproduction via fusion of multinucleate gametangia Gametangium cut off from hypha by complete septum May occur between same or different mating types Nuclei of different mating types fuse immediately Massive, haploid zygospore forms around diploid zygote nuclei Zygospore may contain one or more zygotes Meiosis occurs during germination All nuclei are haploid except for zygote nuclei Asexual reproduction is common Haploid spores produced within sporangia Sporangium forms at tip of erect hypha, with separating septum Spores shed above substrate, dispersed by wind Phylum Ascomycota Beneficial forms include yeast, molds, morels and truffles Harmful forms include chestnut blight and Dutch elm disease Characteristic reproductive structure called ascus Diploid zygote formed within haploid ascus Asci form on ascocarp of densely interwoven hyphae Asexual reproduction is common Conidia are produced at ends of conidiophores Spores separated from hyphae by septa Hyphae are septate, but septae are perforated and cytoplasm flows through them Septae at reproductive structures are initially perforated but are sealed later Multinucleate hyphae may be homokaryotic or heterokaryotic Multinucleate gametangia are specialized hyphae Ascogonia are female, have trichogyne outgrowth Antheridia are male and fuse with trichogyne Male nuclei travel to ascogonium to pair with opposite nuclei Heterokaryotic, dikaryotic hyphae arise from area of fusion An ascus containing two nuclei forms at hyphal tip Nuclei fuse forming diploid zygote, immediately undergoes meiosis Four haploid daughter nuclei formed Daughter nuclei divide by mitosis forming eight ascospores Ascocarps contain monokaryotic and dikaryotic hyphae May be open as in cup fungi and morels May be closed or have only small opening Yeasts Unicellular, mostly ascomycetes Most reproduction is asexual cell fission or budding Fusion of two cells results in ascus, undergoes sexual reproduction Diverse degenerate fungi derived from filamentous forms Yeasts related only by being single-celled Most are ascomycetes, but other two groups are represented Even ascomycetes are not clearly related to one another Ferment carbohydrates, produce carbon dioxide and ethanol Important future in genetic engineering Other ascomycetes Most frequent kind of fungus found in lichens Judged by asexual structures, may include many Fungi Imperfecti Phylum Basidiomycota Include mushrooms, jelly fungi, puffballs, rusts and smuts Include edible and poisonous varieties Characteristic reproductive structure called a basidium Syngamy occurs within basidium; forms diploid zygote fig 32.12 Meiosis occurs immediately, forming four haploid basidiospores Four basidiospores borne on one sterigma Basidiomycetes life cycle Spore germinates forming homokaryotic hyphae Hyphae initially lack septae Eventually, septa form between nuclei of primary mycelium Dikaryotic, heterokaryotic secondary mycelium forms when hyphae of different mating types fuse Unique pattern of cell division Simultaneous division of nuclei in cell at tip of hypha and formation of new septa Other cells that form lateral branches grow in same way Basidiocarps form of completely dikaryotic mycelium Basidia line gills of typical mushrooms Asexual reproduction rare in basidiomycetes Common cultivated mushrooms have high protein content Rusts are important plant pathogens Basidiocarps are not formed Basidia arise from hyphae at surface of host plant A COMPARISON OF FUNGAL PHYLA Phyla Differ in Many Characteristics Formation of hyphal septae Zygomycetes: nonseptate hyphae, except of septae at reproductive structures Ascomycetes: perforated septae, septae at reproductive structures later blocked Basidiomycetes: same as Ascomycetes Presence of unique sexual reproductive structures Zygomycetes: unspecialized gametangia Ascomycetes: distinctive ascogonia and antheridia Basidiomycetes: no distinct reproductive organs Characteristics of hyphae during sexual reproduction Zygomycetes: fusion of gametangia produces zygote, surrounded by zygospore Ascomycetes: sexual fusion followed by formation of heterokaryotic hyphae, individual cells are multinucleate Basidiomycetes: sexual fusion followed by formation of dikaryotic hyphae, each cell has two genetically distinct nuclei Location of sexual spores Ascomycetes: produced in ascocarps of homo- and heterokaryotic mycelia Basidiomycetes: produced in basidiocarp of dikaryotic, secondary mycelia Asci or basidia within or on surface of ascocarp or basidiocarp Syngamy, meiosis and production of haploid spores occur in both phyla Asexual reproduction Zygomycetes: common, spores produced in sporangia Ascomycetes: form multinucleate spores called conidia Basidiomycetes: rare, few rusts and smuts produce conidia Evolutionary Development Nonseptate zygomycetes most primitive Probable evolution of ascomycetes and basidiomycetes from common zygomycetes ancestor PHYLUM DEUTEROMYCOTA: FUNGI IMPERFECTI Fungi that Have Lost the Ability to Reproduce Sexually Sexual reproductive stages not observed Mostly ascomycetes, few zygomycetes and basidiomycetes Have great economic importance Many are human and plant pathogens Others produce important chemicals including penicillin May Exhibit Parasexuality Provides a certain amount of genetic recombination Exchange portions of chromosomes between genetically distinct nuclei within a common hyphae May be responsible for production of new strains of rusts Examples: Penicillium, Aspergillus, Fusarium FUNGAL ASSOCIATIONS Lichens Mostly ascomycetes with green alga and/or cyanobacterium Specialized hyphae penetrate or envelop photosynthetic cells Fungal chemical signals direct photosynthetic metabolism Could be considered a form of controlled parasitism Reproduction Via normal fungal sexual processes Photosynthetic cell reproduction generally asexual Asexual reproduction by fragmentation Ecology Inhabit cold, dry, generally harsh environments Help break rock surfaces and prepare habitat for other organisms Lichens with cyanobacteria can fix atmospheric nitrogen Survive adverse conditions by nearly halting metabolism Coloration of lichen protects photosynthetic partner Lichens and Pollution Extremely sensitive to atmospheric pollutants Absorb substances dissolved in rain or dew Sensitive to sulfur dioxide an automobile pollutant Destroys chlorophyll and alters membrane permeability Indicates radioactive pollution Mycorrhizae Most plant roots associated with certain fungi Fungus aids in transfer of soil nutrients into roots Plant provides organic carbon to fungus fig 32.18 Two types: endomycorrhizae and ectomycorrhizae Endomycorrhizae Hyphae penetrate outer cells of root More common mycorrhizae, generally a zygomycetes May increase yield of crops with less energy input Ectomycorrhizae fig 32.19 Hyphae surround, but do not penetrate roots Less common, mostly basidiomycetes, some ascomycetes Characteristic symbiont of shrubs and trees Advantages of mycorrhizial associations Plants more resistant to drought, cold and harsh conditions May provide better protection against acid precipitation Prevent accumulation of toxic metals Speed germination of orchid seeds Provide better growth in poor soils Present in early fossil plants Early soil lacking organic matter Adapted from McGraw- Hill Textbook 2011
