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    The Wonderful World of Fungi

    The mushroom you see on the forest floor is only
    the fruiting body of what can be a vast network
    of underground filaments.

    The fungal thallus may be either

    • compacted into a solid structure
    • spread diffusely into the substrate

    Fungi can exist a

    • molds - rapidly growing, asexually reproducing hyphae
    • yeast - unicellular, usually in liquid or moist environments

    Fungi are partners in numerous symbiotic interactions.

    Although relatively inconspicuous, fungi are among
    the most vital components of the biosphere.

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    Fungal Anatomy and Structures

  • mycelium (thallus) - vegetative body of the fungus
  • hyphae - filaments that comprise the mycelium
  • fruiting body - (usually) aboveground reproductive structure
  • sporangium - compartment in which spores are formed
  • spore - haploid propagule may be produced
    • sexually via meiosis
    • asexually via mitosis

  • Fungi lack true tissues.
  • Cell walls contain chitin.
  • Most fungi lack cellulose in their cell walls
    x(Only Chytrid fungi have cellulose in their cell walls.)

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    Hyphae

    Hyphae may be
    • coenocytic
      • one long cell with many nuclei
      • more primitive form
    • septate
      • thin, perforated walls (septa) divide nucleated compartments
      • more derived form

    Parasitic fungi have specialized hyphae called haustoria.
    These create a nutrient pathway between fungus and host cell.

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by Rob Hille

    Fungal Nutrition

    All fungi are absorptive heterotrophs.
    They secrete digestive enzymes onto the nutrient source and absorb the product.

    • A saprobe feeds on dead, organic matter.
    • A parasite feeds on living tissue.

    The main storage carbohydrate is glycogen, as in animals.

    Along with bacteria, fungi are the biosphere's most important decomposers.


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    The Generalized Fungal Life Cycle

    A fungus is haploid for most of its life cycle.

    Fungi can reproduce asexually via

    • hyphal fragmentation
    • production of asexual spores via mitosis

    Fungi can reproduce sexually when conditions trigger sex.

      Individuals are of two complementary mating types, "+" or "-"

      When + and - meet, they undergo plasmogamy (fusion of hyphae).
      The hyphae thus become dikaryotic, containing
      two genetically different haploid nuclei

      • some from the + parent
      • some from the - parent

      Dikaryotic hyphae grow and intertwine to form a fruiting body.

      • In specialized regions of the fruiting body,
        karyogamy (fusion of haploid nuclei) occurs.
        • Many + and - nuclei fuse (fertilization).
        • This forms many diploid zygotes
        • Zygotes undergo meiosis to produce haploid spores.

      When mature, haploid spores are released.
      Each spore grows mitotically into a genetically unique (+ or -) haploid mycelium.

      The most familiar fungi undergo this same life cycle,
      though the stages vary in appearance in different taxa.

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    Phungal Phyla

    There are eight (putatively) monophyletic fungal phyla. We will meet five.

    • Microsporidia
    • Chytridiomycota
    • Glomeromycota
    • Ascomycota
    • Basidiomycota
    • (but don't get too attached)

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    Microsporidia - The Microsporidians

    Long believed to be apicomplexan protists,
    this group was only recently removed from Alveolata.

    Molecular data revealed their fungal origins.
    (Or at least status as an outgroup to Fungi)

    They appear to be primitive in many respects, lacking

    • mitochondria
    • Golgi apparatus
    • peroxisomes

    Microsporidians are are obligate, intracellular parasites
    that infect specific organs and organ systems
    in both vertebrates and invertebrates.

    There may be more than one million species,
    but only 1500 have been described and named.

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    Chytridiomycota - The Chytrids

    These fungi retain primitive characters that help us root fungal phylogenetic trees.

    • Most are aquatic , suggesting an aquatic origin of the Fungi.
    • Some are free-living saprobes
    • Others are parasites of protists, plants and aquatic invertebrates, and amphibians.
    • Like all fungi, they have chitin in the cell walls
    • One aberrant group of chytrids has cellulose in the cell walls
    • chytrids are the only fungi to retain flagellated gametes (zoospores)
    • Flagella have been lost in all other fungal species.

    (Read more about Chytridiomycota.)

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    Ascomycota - The Sac Fungi

    Containing about 64,000 species, Ascomycota is the most diverse fungal phylum.
    About 75% of all described fungal species are ascomycetes.

    Many ascomycetes have secondarily lost the sexual life cycle,
    and reproduce only asexually via mitotically produced spores
    called conidia.

    The group includes many economically important species such as

    • Penicillium (source of penicillin)
    • Saccharomyces cereviseae (Baker's Yeast)
    • Neurospora crassa (widely used as a model organism in genetic research)
    • morels and truffles (widely used in fancy cooking)
    • Xylaria polymorpha (Dead Man's Fingers; used in spalting hardwoods).

    Ascomycota is named for the microscopic pouch where zygotes
    undergo meiosis to become (asco)spores, the ascus (Latin for "sac").

    Each ascus produces 4-8 spores in the linear sac,
    allowing determination of mitotic chromosomal migrations.

    (Read more about Ascomycota.)

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    Basidiomycota - The Club Fungi

    These are the most commonly recognized fungi,
    and the most commonly eaten by humans, for good or ill.

    Basidiomycota is named for the microscopic structure
    where zygotes undergo meiosis to become (basidio)spores,
    the basidium (Latin for "club").


    (Read more about Basidiomycota.)

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    Glomeromycota

    This is the least diverse (~ 230 species) but most abundant and widespread fungal group.

    All known species are obligate mutualistic symbionts with plants,
    forming arbuscular mycorrhizae (AM).

    Glomeromycete hyphae and spores appear in association with 450 million year old fossil plant roots, indicating that this glomeromycetes are among the most ancient of fungi.

    Glomeromycetes cannot be grown in culture without their plant hosts.
    For this reason, discovery of their nutritional requirements has been challenging.

    (Read more about Glomeromycota.)

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Fungal Species Interactions and Symbioses

Recall the different types of species interactions and symbioses. (<--- required link)

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Diagram of the two types of mycorrhizae.
<-- Arbuscular (AM) mycorrhizae (left)
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxEctomycorrhizae (right) -->

    Obligate Mutualism: Mycorrhizae

    Mycorrhizae (from Greek myco, "fungus" and rhiz, "root") are symbiotic associations between fungi and plant roots (or rhizoids, in the case of bryophytes).

    Mycorrhizae are arguably the most important symbiotic relationship on earth.

    The fungal partner receives photosynthates.
    The plant partner receives a vastly expanded underground absorptive network.

  • Arbuscular Mycorrhizae (AM) - association between a glomeromycete and a plant
    • Fungal hyphae form arbuscules inside root cortex cells
    • These serve as water/nutrient bridges between host and symbiont.
    • Some AM also form storage vesicles

  • Ectomycorrhizae - association between ascomycete or basidiomycete and a conifer or anthophyte (usually a tree).

Ecological importance of mycorrhizae cannot be overstated. <--- MANDATORY LINK!

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Crustose lichens


Foliose lichen


Fruticose lichen

    Lichen: Obligate Mutualism

    Lichens are a symbiotic association between a fungus and a photoautotroph (algae or cyanobacteria).

    The fungus provides safe habitat for the photoautotroph.
    The photoautotroph provide photosynthates for the fungus.

    Lichens are

    • ubiquitous, but inconspicuous
    • able to survive in very harsh (dry, cold) climates
    • vital primary producers in harsh environments, such as tundra.
    • able to absorb nutrients directly from the atmosphere
    • ...so are very sensitive to air pollution

    Growth form may be

    • crustose (encrusting)
    • foliose ("leafy")
    • fruticose (branching and upright)

Lichens are the original source of the compound used to make litmus paper, the old-fashioned way to identify a solution as acidic or basic.

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    Fungi: Predator and Prey

    <--- Who among us has not been a fungivore? --->

    Many animals eat fungi.

    • Rodents can be important spore dispersers for mycorrhizal fungi.
    • Many fungi have evolved deadly toxins as protection from fungivores.

    Toxins produced by fungi are known as mycotoxins.

    Different mycotoxins have different effects:

    • neurotoxicity
    • nephrotoxicity
    • cytotoxicity
    • carcinogenicity
    • etc.


    Some fungi are predatory! --->

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(Click on pic for a St. Paddy's Day Tribute to Yeast)

Arthrobotrys nabs a nematode

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    Fungi: Parasites and Parasitoids

    Not all fungi wait for you to die before they begin dining.

  • Rainforest ascomycete parasitoids infect arthropods
  • A chytrid fungus, Batrachochytrium dendrobatidis, is responsible for recent worldwide amphibian extinctions.
  • Various dermatophytes invade living tissues
  • Respiratory pathogen Histoplasma capsulatum causes Histoplasma capsulatum
  • Dutch Elm disease threatens native North American elm trees
  • Some fungal grain crop parasites produce carcinogenic mycotoxins.
  • Some Aspergillis produce highly toxic ergots which:
      • are sclerotia (fungal resting stage)
      • replace the ovaries of infected grasses
      • produce highly toxic alkaloids that can cause
        • central nervous system damage
        • severe smooth muscle spasms
        • tissue necrosis
        • hallucinations and temporary insanity

    Fungal toxins are collectively known as mycotoxins.