Chapter 31 Fungi Overview Mighty Mushrooms The honey mushroom Armillaria ostoyae in Malheur National Park in eastern Oregon is enormous Its subterranean mycelium covers 890 hectares weighs hundreds of tons and has been growing for 2 600 years Ten thousand species of fungi have been described but it is estimated that there are actually up to 1 5 million species of fungi Fungal spores have been found 160 km above the ground Fungi play an important role in ecosystems decomposing dead organisms fallen leaves feces and other organic materials This decomposition recycles vital chemical elements back to the environment in forms other organisms can assimilate Most plants depend on mutualistic fungi to help their roots absorb minerals and water from the soil Humans have cultivated fungi for centuries for food to produce antibiotics and other drugs to make bread rise and to ferment beer and wine Concept 31 1 Fungi are heterotrophs that feed by absorption Absorptive nutrition enables fungi to live as decomposers and symbionts Fungi are heterotrophs that acquire their nutrients by absorption They absorb small organic molecules from the surrounding medium Exoenzymes powerful hydrolytic enzymes secreted by the fungus break down food outside its body into simpler compounds that the fungus can absorb and use The absorptive mode of nutrition is associated with the ecological roles of fungi as decomposers saprobes parasites and mutualistic symbionts Saprobic fungi absorb nutrients from nonliving organisms Parasitic fungi absorb nutrients from the cells of living hosts Some parasitic fungi including some that infect humans and plants are pathogenic Fungi cause 80 of plant diseases Mutualistic fungi also absorb nutrients from a host organism but they reciprocate with functions that benefit their partner in some way Extensive surface area and rapid growth adapt fungi for absorptive nutrition Yeasts are single celled fungi Most other species of fungi are multicellular The vegetative bodies of most fungi are constructed of tiny filaments called hyphae that form an interwoven mat called a mycelium Fungal hyphae have cell walls Fungal mycelia can be huge but they usually escape notice because they are subterranean These are built mainly of chitin a strong but flexible nitrogen containing polysaccharide identical to that found in arthropods Most fungi are multicellular with hyphae divided into cells by cross walls or septa These generally have pores large enough for ribosomes mitochondria and even nuclei to flow from cell to cell This results from repeated nuclear division without cytoplasmic division Fungi that lack septa coenocytic fungi consist of a continuous cytoplasmic mass with hundreds or thousands of nuclei Parasitic fungi usually have some hyphae modified as haustoria nutrient absorbing hyphal tips that penetrate the tissues of their host Some fungi even have hyphae adapted for preying on animals The filamentous structure of the mycelium provides an extensive surface area that suits the absorptive nutrition of fungi One cubic centimeter of rich organic soil may contain 1 km of fungal hyphae with a surface area of more than 300 cm2 A fungal mycelium grows rapidly Proteins and other materials synthesized by the entire mycelium are channeled by cytoplasmic streaming to the tips of the extending hyphae The fungus concentrates its energy and resources on adding hyphal length and absorptive surface area While fungal mycelia are nonmotile by swiftly extending the tips of its hyphae it can extend into new territory Concept 31 2 Fungi produce spores through sexual or asexual life cycles Fungi reproduce by producing vast numbers of spores either sexually or asexually The output of spores from one reproductive structure can be enormous Puffballs may release trillions of spores Dispersed widely by wind or water spores germinate to produce mycelia if they land in a moist place where there is food Many fungi have a heterokaryotic stage The nuclei of fungal hyphae and spores of most species are haploid except for transient diploid stages that form during sexual life cycles Sexual reproduction in fungi begins when hyphae from two genetically distinct mycelia release sexual signaling molecules called pheromones Pheromones from each partner bind to receptors on the surface of the other The union of the cytoplasm of the two parent mycelia is known as plasmogamy In some species heterokaryotic mycelia become mosaics with different nuclei remaining in separate parts of the same mycelium or mingling and even exchanging chromosomes and genes In some fungi the haploid nuclei pair off two to a cell one from each parent In many fungi with sexual life cycles karyogamy fusion of haploid nuclei contributed by two parents occurs well after plasmogamy cytoplasmic fusion of cells from the two parents Such a mycelium is called dikaryotic meaning two nuclei In many fungi the haploid nuclei do not fuse right away The delay may be hours days or even centuries During karyogamy the haploid nuclei contributed by the two parents fuse producing diploid cells In most fungi the zygotes of transient structures formed by karyogamy are the only diploid stage in the life cycle These undergo meiosis to produce haploid cells that develop as spores in specialized reproductive structures These spores disperse to form new haploid mycelia The sexual processes of karyogamy and meiosis generate genetic variation The heterokaryotic condition also offers some of the advantages of diploidy in that one haploid genome may be able to compensate for harmful mutations in the other Many fungi reproduce asexually Some species reproduce only asexually The processes of asexual reproduction in fungi vary widely Some fungi that can reproduce asexually grow as mold Molds grow rapidly as mycelia and produce spores Yeasts live in liquid or moist habitats Instead of producing spores yeasts reproduce asexually by simple cell division or by budding of small cells Most molds and yeasts have no known sexual stage Such fungi are called deuteromycetes or imperfect fungi Whenever a sexual stage of a deuteromycete is discovered the species is classified in a particular phylum depending on its sexual structures Fungi can be identified from their sexual stages and by new genetic techniques Concept 31 3 Fungi descended from an aquatic single celled flagellated protist Data from paleontology and molecular systematics offer insights into the early evolution of fungi Systematists