Chapter 20.  Kingdom Fungi

• What are fungi?
• Fungi are not plants.  They are one of the three eukaryotic, (mostly) multicellular kingdoms in the 5-kingdom classification.  Plants and Animals are the other two kingdoms.
• Fungal traits:
• Heterotrophic (plants are autotrophic).
• External digestion (animals have internal digestion).
• Chitin cell walls (plants have cellulose cell walls).
• Mycelial growth.  Mycelium = system of branching filaments (hyphae, singular hypha).  These penetrate food source (e.g. dead tree, leaf litter, host), secrete digestive enzymes into food source.  Three-dimensional structures (e.g. mushrooms) formed by aggregations of hyphae in which all cell division is transverse (compare plants).
• Varied ecological roles:
• Decomposers: along with bacteria and some animals, consume dead organic matter.  As this happens, carbon returned to atmosphere as carbon dioxide, mineral nutrients returned to soil.
• Mutualists: many fungi form mycorrhizae (=fungus-root mutualism).  Plant "donates" carbohydrate, fungus increases absorptive capacity of root system.
• Parasites: mostly on plants.  Many crop parasites have significant economic importance.  A few animal are parasites/pathogens, e.g., Histoplasma (lungs), Candida (vagina), Trichophyton (toes).
• Fungal reproduction and life cycles (Figure 20.5).
• Fungi disperse to new food source by means of spores.
• Asexual reproduction: spores produced by mitosis (mitospores).
• Sexual reproduction: spores produced by meiosis (meiospores).
• "Fertilization", "gametes", "male", "female" are plant concepts that do not fit fungal life cycles well.  More appropriate terms (see Figure 20.5):
• Mating types: there may be dozens in a fungal population (not just two sexes).  For sexual reproduction to occur, haploid mycelia of different (but compatible) mating types must get together.
• Plasmogamy: hyphae of compatible mating types meet, produce mycelium in which each cell contains two haploid nuclei (this is the dikaryotic condition).  Mycelium can persist in dikaryotic condition for extended period.
• Karyogamy: nuclei in dikaryotic cell fuse, produce diploid nucleus.
• Meiosis of diploid nucleus produces spores (meiospores) which disperse, germinate to produce haploid mycelia.
• Fungi are classified by mode of sexual reproduction, details of life cycles.
• Zygomycota
• Ascomycota
• Basidiomycota
• Deuteromycota
• Zygomycota:  Structurally simple, filamentous fungi without complex reproductive bodies
• Mycelium is composed of aseptate hyphae, facilitating transport of food molelcules.
• But since nuclei can be "anywhere", control of gene expression cannot result in mycelial differentiation as in Asco- and Basidiomycota.
• Life cycles - sexual and asexual.  See Figure 20.7
• Ascomycota: Most species rich group of fungi.
• Some unicellular forms (yeasts), but most are mycelial.
• Life cycle - See Figure 20.10.  Ascocarp forms from dikaryotic mycelium.  In terminal hyphae of ascocarp, karyogamy occurs, followed by  meiosis.  Ascospores (meiospores) form in asci (singular, ascus).  Spores are released, germinate to form haploid mycelia.  Feeding mycelium is haploid.
• Haploid mycelia can produce mitospores.  These are dispersed, germinate to produce haploid mycelia.
• Haploid mycelia of different but compatible mating types get together.  Plasmogamy occurs to form dikaryotic mycelium.
• Basidiomycota: Most complex fungi.  Include mushroom forming species.
• Life cycle (Figure 20.14) - elaboration of ascomycete life cycle.
• Feeding mycelium is dikaryotic.
• Basidiocarp (mushroom, etc.) is reproductive structure formed by dikaryotic mycelium.  Karyogamy occurs in hyphal ends.  Meiosis follows to produce basidia (singular, basidium) with four basidiospores (meiospores).  Spores are released to form haploid mycelia.
• Haploid mycelia of different but compatible mating types get together.  Plasmogamy occurs to form dikaryotic mycelium.
• Deuteromycota: asexual forms. Includes species which never reproduce sexually and the asexual forms of species (Ascomycota) with sexual stages.  Include familiar bread and cheese molds Penicillium and Aspergillus).