Bài giảng Biology - Chapter 31: Fungi

Chapter 31FungiOverview: Mighty MushroomsFungiAre diverse and widespreadAre essential for the well-being of most terrestrial ecosystems because they break down organic material and recycle vital nutrientsFigure 31.1Concept 31.1: Fungi are heterotrophs that feed by absorptionDespite their diversityFungi share some key traitsNutrition and Fungal LifestylesFungi are heterotrophsBut do not ingest their foodFungi secrete into their surroundings exoenzymes that break down complex moleculesAnd then absorb the remaining smaller compoundsFungi exhibit diverse lifestylesDecomposersParasitesMutualistic symbiontsBody StructureThe morphology of multicellular fungiEnhances their ability to absorb nutrients from their surroundingsHyphae. The mushroom and its subterranean mycelium are a continuous network of hyphae.Reproductive structure.The mushroom produces tiny cells called spores.Spore-producing structures20 mMyceliumFigure 31.2Fungi consist ofMycelia, networks of branched hyphae adapted for absorptionMost fungiHave cell walls made of chitinSome fungiHave hyphae divided into cells by septa, with pores allowing cell-to-cell movement of materialsCoenocytic fungiLack septaNucleiCell wallSeptumPore(a) Septate hypha(b) Coenocytic hyphaCell wallNucleiFigure 31.3a, bSome unique fungiHave specialized hyphae that allow them to penetrate the tissues of their hostNematodeHyphae25 m(a) Hyphae adapted for trapping and killing prey(b) HaustoriaFungal hyphaPlant cell wallHaustoriumPlant cell plasma membranePlant cellFigure 31.4a, bMycorrhizaeAre mutually beneficial relationships between fungi and plant rootsConcept 31.2: Fungi produce spores through sexual or asexual life cyclesFungi propagate themselvesBy producing vast numbers of spores, either sexually or asexuallyThe generalized life cycle of fungiKeyHaploid (n)Heterokaryotic(unfused nuclei fromdifferent parents)Diploid (2n)PLASMOGAMY(fusion of cytoplasm)HeterokaryoticstageKARYOGAMY(fusion of nuclei)SEXUALREPRODUCTIONSpore-producingstructuresSporesASEXUALREPRODUCTIONZygoteMyceliumGERMINATIONGERMINATIONMEIOSISSpore-producingstructuresSporesFigure 31.5Sexual ReproductionThe sexual life cycle involvesCell fusion, plasmogamyNuclear fusion, karyogamyAn intervening heterokaryotic stageOccurs between plasmogamy and karyogamy in which cells have haploid nuclei from two parentsThe diploid phase following karyogamyIs short-lived and undergoes meiosis, producing haploid sporesAsexual ReproductionMany fungi can reproduce asexuallyMany fungi that can reproduce asexuallyGrow as mold, sometimes on fruit, bread, and other foods2.5 mFigure 31.6Other asexual fungi are yeastsThat inhabit moist environmentsWhich produce by simple cell division10 mParent cellBudFigure 31.7Many molds and yeasts have no known sexual stageMycologists have traditionally called these deuteromycetes, or imperfect fungiConcept 31.3: Fungi descended from an aquatic, single-celled, flagellated protistSystematists now recognize Fungi and Animalia as sister kingdomsBecause fungi and animals are more closely related to each other than they are to plants or other eukaryotesThe Origin of FungiMolecular evidenceSupports the hypothesis that fungi and animals diverged from a common ancestor that was unicellular and bore flagellaFungi probably evolvedBefore the colonization of land by multicellular organismsThe oldest undisputed fossils of fungiAre only about 460 million years old50 mFigure 31.8 The Move to LandFungi were among the earliest colonizers of landProbably as symbionts with early land plantsConcept 31.4: Fungi have radiated into a diverse set of lineagesThe phylogeny of fungiIs currently the subject of much researchMolecular analysisHas helped clarify the evolutionary relationships between fungal groups, although there are still areas of uncertaintyThe phylogeny of fungiChytridsZygote fungiArbuscular mycorrhizal fungiSac fungiClub fungiChytridiomycotaZygomycotaGlomeromycotaAscomycotaBasidiomycotaFigure 31.9A review of fungal phylaTable 31.1ChytridsFungi classified in the phylum Chytridiomycota, or chytridsAre found in freshwater and terrestrial habitatsCan be saprobic or parasiticChytrids are unique among fungiIn having flagellated spores, called zoospores25 m4 mHyphaeFlagellumFigure 31.10Until recently, systematists thought thatFungi lost flagella only once in their historyMolecular dataIndicate that some “chytrids” are actually more closely related to another fungal group, the zygomycetesSome chytridsZygomycetes and other chytridsGlomeromycetes, ascomycetes, andbasidiomycetesCommon ancestorKeyLoss of flagellaFigure 31.11ZygomycetesFungi in the phylum Zygomycota, the zygomycetesExhibit a considerable diversity of life historiesInclude fast-growing molds, parasites, and commensal symbiontsAre named for their sexually produced zygosporangiaRhizopus growing on breadASEXUAL REPRODUCTIONMyceliumDispersal and germinationMEIOSISKARYOGAMYPLASMOGAMYKeyHaploid (n)Heterokaryotic (n + n)DiploidSporangiumDiploid nucleiZygosporangium (heterokaryotic)100 mYoung zygosporangium (heterokaryotic)SEXUAL REPRODUCTIONDispersal and germinationMating type (+)Mating type ()Gametangia with haploid nuclei50 mSporangiaThe life cycle of Rhizopus stoloniferIs fairly typical of zygomycetes Mycelia have various mating types (here designated +, with red nuclei, and , with blue nuclei).1 Neighboring mycelia of different mating types form hyphal extensions called gametangia, each walled off around several haploid nuclei by a septum.2 A heterokaryoticzygosporangiumforms, containingmultiple haploidnuclei from the twoparents.3 The sporangium disperses genetically diverse, haploid spores.74 This cell develops a rough, thick-walled coating that can resist dry environments and other harsh conditions for months. 5 When conditions are favourable, karyogamy occurs, followed by meiosis.6 The zygosporangium then breaks dormancy, germinating into a short sporangium. The spores germinate and grow into new mycelia.89 Mycelia can also reproduce asexually by forming sporangia that produce genetically identical haploid spores.Figure 31.12Some zygomycetes, such as PilobolusCan actually “aim” their sporangia toward conditions associated with good food sources0.5 mmFigure 31.13Zygosporangia, which are resistant to freezing and dryingAre capable of persisting through unfavorable conditionsCan undergo meiosis when conditions improveMicrosporidiaMicrosporidiaAre unicellular parasites of animals and protistsAre now classified as zygomycetes10 mHost cell nucleusDeveloping microsporidianSporeFigure 31.14GlomeromycetesFungi assigned to the phylum GlomeromycotaWere once considered zygomycetesAre now classified in a separate cladeAll glomeromycetesForm a distinct type of endomycorrhizae called arbuscular mycorrhizae2.5 mFigure 31.15AscomycetesFungi in the phylum AscomycotaAre found in a variety of marine, freshwater, and terrestrial habitatsAre defined by the production of sexual spores in saclike asci, which are usually contained in fruiting bodies called ascocarpsAscomycetesVary in size and complexity from unicellular yeasts to elaborate cup fungi and morels(a) The cup-shaped ascocarps (fruiting bodies) of Aleuria aurantia give this species its common name: orange peel fungus.(b) The edible ascocarp of Morchella esculenta, the succulent morel, is often found under trees in orchards.(c) Tuber melanosporum is a truffle, an ascocarp that grows underground and emits strong odors. These ascocarps have been dug up and the middle one sliced open.(d) Neurospora crassa feeds asa mold on bread and other food (SEM).10 mFigure 31.16a–dAscomycetes reproduceAsexually by producing enormous numbers of asexual spores called conidiaThe life cycle of Neurospora crassa, an ascomyceteDispersalASEXUALREPRODUCTIONGerminationMyceliumConidiophoreGerminationDispersalMyceliaAsciEightascosporesAscocarpFourhaploidnucleiMEIOSISKARYOGAMYPLASMOGAMYSEXUALREPRODUCTIONDiploid nucleus(zygote)AscogoniumAscus(dikaryotic)DikaryotichyphaeMatingtype ()Conidia;mating type ()KeyHaploid (n)Dikaryotic (n  n)Diploid (2n) Ascomycete mycelia can also reproduce asexually by producing haploid conidia.7 Neurospora can reproduce sexually by producing specialized hyphae. Conidia of the opposite mating type fuse to these hyphae.1 A dikaryotic ascus develops.2 Karyogamy occurs within theascus, producing adiploid nucleus.3 The diploid nucleusdivides by meiosis, yieldingfour haploid nuclei.4 The developing asci are contained in an ascocarp. The ascospores are discharged forciblyfrom the asci through anopening in the ascocarp. Germinating ascosporesgive rise to new mycelia.65 Each haploid nucleus dividesonce by mitosis, yielding eightnuclei. Cell walls develop aroundthe nuclei, forming ascospores (LM).Figure 31.17BasidiomycetesFungi in the phylum BasidiomycotaInclude mushrooms and shelf fungiAre defined by a clublike structure called a basidium, a transient diploid stage in the life cycleBasidiomycetes(a) Fly agaric (Amanita muscaria), acommon species in conifer forests in the northern hemisphere(b) Maiden veil fungus (Dictyphora), a fungus with an odor like rotting meat(c) Shelf fungi, important decomposers of wood(d) Puffballs emitting sporesFigure 31.18a–dThe life cycle of a basidiomyceteUsually includes a long-lived dikaryotic mycelium, which can erect its fruiting structure, a mushroom, in just a few hoursFigure 31.19PLASMOGAMYDikaryoticmyceliumBasidiocarp(dikaryotic)KARYOGAMYKeyMEIOSISGills linedwith basidiaSEXUALREPRODUCTIONMatingtype ()Matingtype ()HaploidmyceliaDispersalandgerminationBasidiosporesBasidium withfour appendagesBasidium containingfour haploid nucleiBasidia(dikaryotic)DiploidnucleiBasidiospore1 mBasidiumHaploid (n)Dikaryotic (n  n)Diploid (2n)The life cycle of a mushroom-forming basidiomycete Each diploid nucleus yields four haploid nuclei. Each basidiumgrows four appendages, and one haploid nucleusenters each appendage and develops into a basidiospore (SEM).6 Two haploid mycelia of different mating typesundergo plasmogamy.1 A dikaryotic mycelium forms, growing faster then, and ultimately crowding out, the haploid parental mycelia.23 Environmental cues such as rain ortemperature changes induce the dikaryoticmycelium to formcompact masses thatdevelop intobasidiocarps(mushrooms, in thiscase). The basidiocarpgills are lined withterminal dikaryoticcells called basidia.4 Karyogamy in the basidia produces diploidnuclei, which thenundergo meiosis.5When mature,the basidiosporesare ejected, fallfrom the cap, andare dispersed bythe wind.7In a suitableenvironment, thebasidiospores germinate andgrow intoshort-livedhaploid mycelia.8Figure 31.20Concept 31.5: Fungi have a powerful impact on ecosystems and human welfareDecomposersFungi are well adapted as decomposers of organic materialPerforming essential recycling of chemical elements between the living and nonliving worldSymbiontsFungi form symbiotic relationships withPlants, algae, and animalsMycorrhizaeMycorrhizaeAre enormously important in natural ecosystems and agricultureIncrease plant productivityRESULTS Researchers grew soybean plants in soil treated with fungicide (poison that kills fungi) to prevent the formation of mycorrhizae in the experimental group. A control group was exposed to fungi that formed mycorrhizae in the soybean plants’ roots.EXPERIMENT The soybean plant on the left is typical of the experimental group. Its stunted growth is probably due to a phosphorus deficiency. The taller, healthier plant on the right is typical of the control group and has mycorrhizae.CONCLUSION These results indicate that the presence of mycorrhizae benefits a soybean plant and support the hypothesis that mycorrhizae enhance the plant’s ability to take up phosphate and other needed minerals.Figure 31.21 RESULTSFungus-Animal SymbiosisSome fungi share their digestive services with animalsHelping break down plant material in the guts of cows and other grazing mammalsMany species of ants and termitesTake advantage of the digestive power of fungi by raising them in “farms”Figure 31.22LichensLichensAre a symbiotic association of millions of photosynthetic microorganisms held in a mass of fungal hyphae(a) A fruticose (shrub-like) lichen(b) A foliose (leaf-like) lichen(c) Crustose (crust-like) lichensFigure 31.23a–cThe fungal component of a lichenIs most often an ascomyceteAlgae or cyanobacteriaOccupy an inner layer below the lichen surfaceAscocarp of fungusFungalhyphaeAlgallayerSorediaAlgal cellFungal hyphae10 mFigure 31.24PathogensAbout 30% of known fungal speciesAre parasites, mostly on or in plants(a) Corn smut on corn(b) Tar spot fungus on maple leaves(c) Ergots on ryeFigure 31.25a–cSome of the fungi that attack food cropsAre toxic to humansPractical Uses of FungiHumans eat many fungiAnd use others to make cheeses, alcoholic beverages, and breadAntibiotics produced by fungiTreat bacterial infectionsStaphylococcusPenicilliumZone of inhibited growthFigure 31.26Genetic research on fungiIs leading to applications in biotechnology