Chapter 31: FungiOverview: 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 absorptionAbsorptive 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 mycelia can be huge, but they usually escape notice because they are subterranean.• Fungal hyphae have cell walls.◦ These are built mainly of chitin, a strong but flexible nitrogen-containing polysaccharide identicalto 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.• Fungi that lack septa, coenocytic fungi, consist of a continuous cytoplasmic mass with hundreds or thousands of nuclei.◦ This results from repeated nuclear division without cytoplasmic division.• 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 absorptivenutrition 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 many fungi, the haploid nuclei do not fuse right away.• 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.◦ Such a mycelium is called dikaryotic, meaning “two nuclei.”• 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.◦ 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 reproductivestructures.◦ 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.• The processes of asexual reproduction in fungi vary widely.◦ Some species reproduce only asexually.• 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