Bio1151 Chapter 31 9
  1. Fungi are                : they cannot make their own food, but feed by secreting               and absorbing the digested organic compounds.
    • A mushroom such as this Armillaria ostoyae (honey mushroom) is just the visible above-ground structure.

      Most of the organism is underground; this particular one extends over 890 hectares (2200 acres) in the Malheur National Forest of eastern Oregon, and is estimated to be 2,400 years old.


    • Fungus structure.

      The above-ground mushrooms are sexual structures, which are often temporary and wither after releasing spores.

      The underground bodies consist of mycelia, networks made of hyphae which secrete exoenzymes and absorb nutrients in a heterotrophic life style.

      The cell walls are made of the polysaccharide chitin.

      Review:

     
  2. Most fungi are               , but they can also be              and even             .
    • Shelf fungi are important decomposers of wood, recycling nutrients to the soil.

      Their reproductive structures persist: new layers of spore-bearing tissue is added every growing season.

      The woody, perennial shelves may be several years old.


    • Tremella are jelly-like fungi that are parasitic on other decay fungi. Tremella fuciformis is cultivated in China for food and is know as "silver ear".

    • In Arthrobotrys, a soil fungus, the hyphae are modified as hoops that can constrict around a nematode (roundworm) in less than a second.

      The fungus then penetrates its prey with hyphae and digests the animal.

     
  3. The asexual cycle of many fungi can be seen as          with visible mycelia or as unicellular          .
    • Penicillium often grows as a "mold", forming dense mycelia on citrus fruits. The clusters of conidia produce asexual spores, which form the mycelia by mitosis. Penicillin extracted from this fungus is a well-known antibiotic.

    • Saccharomyces cerevisiae often grows as unicellular "yeast".

      Its asexual reproduction involves mitosis or pinching off "bud" cells from a parent cell.

     
  4. Fungi produce haploid           through           or            life cycles.
    • Fungus life cycle. Asexual reproduction involves germination of haploid spores, which divide by mitosis to produce hyphae. Sexual reproduction (perfect stage) begins when hyphae of different mating types fuse their cytoplasm in a process called plasmogamy, producing heterokaryotic cells with haploid nuclei. This is followed by karyogamy, or fusion of nuclei. The diploid zygote is short-lived and undergoes meiosis, producing haploid spores. Review and exercise:
     
  5. There are five major fungal phyla.
     
    • Chytrids
      • Chytrids produce flagellated spores called zoospores that can swim. Most are multicellular, sprouting branched hyphae. Zoospores video:
       
    • Zygomycetes
      • Zygomycetes are named after heterokaryotic, resistant bodies called zygosporangia during the sexual cycle.

        During the asexual cycle, sporangia rupture to release haploid spores, which germinate and grow into new mycelia.

        Some species produce spores in oriented sporangia, which can be "aimed".


      • The zygomycete Pilobolus decomposes animal dung. The mycelium bends its spore-bearing hyphae toward bright light, where grass is likely to be growing. The fungus then shoots its sporangia like cannonballs as far as 2 m. Grazing animals such as cows ingest the fungi with the grass and then scatter the spores in feces.
       
    • Glomeromycetes
      • Glomeromycetes form symbiotic endomycorrhizae with plant roots: supplying minerals to the roots and obtaining carbohydrates in return.

        Specialized hyphae (arbuscules) perform this exchange by pushing in the plasma membrane.


      • Mutualistic fungi grow specialized hyphae called haustoria that can penetrate the cell wall of plants.

        Haustoria remain separated from a plant cell's cytoplasm by the plasma membrane.

       
    • Ascomycetes
      • Ascomycetes form sexual ascospores in sac-like asci contained in bodies called ascocarps.

        They include edible mushrooms such as morels and truffles, as well as others not as tasty.


      • Ascomycetes can reproduce asexually via conidia.

        A sexual cycle begins when conidia of different mating types fuse (plasmogamy) to form dikaryotic, multicellular hyphae.

        Fusion of the nuclei (karyogamy) and meiosis followed by mitosis forms 8 ascospores contained in asci of ascocarps.


      • Aleuria aurantia (orange peel fungus) is an Ascomycete; the spore-producing structures are found on the upper surface of a cup.
       
    • Basidiomycetes

    • Basidiomycetes include gill mushrooms, shelf fungi, puffballs, as well as some parasites.

      They sometimes form fairy rings.

      The phylum is named after dikaryotic, transient basidiocarps.


    • Fly agaric (Amanita muscaria) is a poisonous Basidiomycete with hallucinogenic properties.

      The "gills" on the underside release spores.



    • The giant puffball Calvatia gigantea can release several trillion spores.

    • Smuts, tar spots, and ergots are Basidiomycete plant parasites.

    • Fairy ring of a Basidiomycete mushroom. Often a mycelium expands outward from an original individual, with older mycelia dying and newer ones obtaining food at the periphery, forming a fairy ring.

    • Basidiomycota Life Cycle.

      Sexual reproduction (perfect stage) occurs when mycelia of different mating types fuse by plasmogamy to produce dikaryotic fruiting bodies called basidiocarps (mushrooms).

      Cells within basidia undergo karyogamy and meiosis to release haploid spores, which can germinate into mycelia.

     
  6. Fungi form symbiotic relationships with many organisms.
     
    • Mycorrhizae are                symbionts between a fungus and plant roots.
       
    • Lichens are a              association between photosynthetic microorganisms and a fungus.
      • Lichens are a symbiotic association of photosynthetic microorganisms held in a mass of fungal hyphae.

        The photosynthetic partners are typically green algae or cyanobacteria.

        The fungal component is most often an ascomycete, but several basidiomycete lichens are known.


      • Lichens can take on a variety of growth forms: crustose lichen grow close to the substrate, foliose lichen form leaf-like lobes, and fruticose lichen are branched like small shrubs.
       
    • Many species of ants and termites raise fungi in fungus "farms".

    • These leaf-cutter ants bring cut leaves to the nest to cultivate a fungus garden, and consume the fungi. The fungi, in turn, depend on the nutrients from the leaves that the ants bring. Video;
     
    Summary: