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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 largest single organism on earth is a parasitic fungus.

    Fungi are partners in numerous symbiotic interactions.

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

    Along with bacteria, they are the only organisms
    that can dcompose organic matter back into its
    inorganic components.

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(click on pic for source)

    Hyphae

    Fungi lack true tissues.

    The fungal body, or thallus, is composed only of
    branching, threadlike structures known as hyphae.

    A hypha is a multinucleate strand of cytoplasm
    surrounded by a tubular cell wall fortified by chitin.

    Hyphae branch into the fungal substrate.

    During reproduction, hyphae clump together
    to form fruiting bodies.

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    Hyphae: Coenocytic or Septate

    Hyphae can be either coenocytic (primitive form)
    or septate (derived form).

    In most species, hyphae are septate:

    • Internal septa divide hypha into compartments.
    • Septa are ~perpendicular to the cell walls.
    • Septa are perforated with pores.
    • Small organelles (ribosomes, mitochondria) can move through pores.
    • Each compartment may contain multiple nuclei.
    • Nuclei can sometimes move between compartments.

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(photo by Emmanuel Boutet, Wikimedia Commons)

    Haustoria: Parasitic Straws

    Parasitic fungi have specialized hyphal extensions
    called haustoria.
    (Latin haust, "to draw up or suck")

    These penetrate the host's cells, creating
    a nutrient pathway between fungus and host.

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(click on pic for source)

    Fungal Forms

    A fungal thallus may be either
    • compacted into a solid structure
    • spread diffusely into the substrate

    Fungal form can be either

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

    Some species can alternate between the two forms.

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    Fungal Terminology

    • 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

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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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Fungal Reproduction

Budding Candida by Tom Volk, U Wisconsin
Hyphal fragments from Inspectapedia
Aspergillus fumigatus by David Gregory & Debbie Marshall

    Asexual Reproduction

    A fungus is haploid for most of its life cycle.

    Fungi can reproduce asexually via

    • mitosis of single cells
    • budding of single cells
    • hyphal fragmentation
    • asexual spores via mitosis (different among species)
      • conidiospores (conidia)
      • arthrospores

      • chlamyhdospores
      • blastospores
      • sporangiospores

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    The Sexual Fungal Life Cycle, Part 1

    Fungi reproduce sexually when conditions are conducive.

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

    • When + and - meet, plasmogamy (fusion of hyphae) occurs.
    • Resulting hyphae are dikaryotic:
      • two genetically different haploid nuclei
        • some from the + parent
        • some from the - parent

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(click on pic for source)

    The Sexual Fungal Life Cycle, Part 2

    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.

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    The Sexual Fungal Life Cycle, Part 3

    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

    Fungal phylogeny is in constant flux as new molecular data
    reveal artificial taxa in need of repair.

    Some currently accepted groups are...

    • 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 1800 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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(by Sava Krstic, Wikimedia Commons)

    Zygospore Fungi

    Two phyla of fungi, Mucoromycota and Zoopagomycota,
    were once grouped in "Zygomycota", now defunct.

    Their hyphae are coenocytic and aseptate.

    The ~1060 species are mostly terrestrial, found in soil,
    organic detritus, or in plant or animal tissues.

    Some are parasites of plants or small animals (e.g., insects).
    Others form mutualistic symbioses with plants.

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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 via asexual conidiospores.

    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).

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    Ascomycota - The Ascus

    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.

    (Learn 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.

    (Learn more about Basidiomycota.)

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(by Debivort, Wikimedia Commons)

    Basidiomycota - The Basidium

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

    Each basdium undergoes meiosis to produce four basidiospores
    that are shed from the gills or pores on the mushroom cap
    at maturity.

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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
    the glomeromycetes are among the most ancient fungi.

    Because glomeromycetes cannot be grown in culture
    without their plant hosts, study of their nutritional requirements
    has been challenging.

    (Learn more about Glomeromycota.)

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Fungi: Ecology and Symbioses

    Lignin Decomposition

    Decomposition of organic matter is arguably the most vital
    ecological service provided by fungi.

    But not all decomposers are created equal.

    A few species of bacteria are able decompose lignin,
    but bacterial degradation of lignin appears to be uncommon.

    A few species of fungi can break down lignins, and they serve
    as the primary decomposers of lignin.

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    Lignin, Cellulose and Wood Rot

    The color of a rotting log can reveal the type of fungi decomposing it.

    Brown Rot Fungi

    • are more common in cool, wet climates (e.g., Pacific NW rainforest)
    • feed mainly upon cellulose,
    • cannot fully break down ligin
    • produce rotting wood that
      • is reddish-brown in color (the color of lignin)
      • breaks down in large chunks (cellulose network is destroyed)

    White Rot Fungi

    • are more common in warmer climates
    • feed upon both cellulose and hemicellulose
    • can hydrolyze ligin
    • cellulose residues last longer (more abundant)
    • produce rotting wood that
      • is whitish-grey in color (the color of celluloses)
      • becomes uniformly spongy as it decays ("dry rot")

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(click on pic for source)

    Decay Can Be Beautiful: Spalting

    Wood discoloration caused by fungal decay is called spalting.
    It can occur in both dead and live wood.

    Even though spalted wood is lighter and weaker than unspalted wood, its unique colors and patterns make it a rare commodity sought after by

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(click on pic for source)

    Root Symbiosis: Mycorrhizae

    Recall the various species interactions and symbioses.

    Mycorrhizae (from Greek myco, "fungus" and rhiz, "root") are a symbiotic relationship between plant roots and fungi.

    Most water and nutrients are taken up from the soil by mycorrhizae, not by the root hairs.

    Mycorrhizae are arguably the most important symbiotic relationship on earth.
    Without them, the plant partner cannot thrive.

    The fungal partner receives photosynthetically produced organic nutrients.
    The plant partner receives a vastly expanded underground absorptive network.

    The ecological importance of mycorrhizae cannot be overstated.

Without their mycorrhizal fungal partners, plants do not thrive, and have greatly lowered Darwinian fitness.

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    Two Major Types of Mycorrhizae

    • Arbuscular Mycorrhizae (AM)
      • Fungal hyphae form arbuscules inside root cortex cells
      • These serve as water/nutrient bridges between host and symbiont.
      • Some AM also form storage vesicles
      • more common in herbaceous and smaller plants
      • formed by fungi in Phylum Glomeromycota

    • Ectomycorrhizae
      • Fungal hyphae wrap around cortex cells to form a Hartig Net.
      • This net serves as a water/nutrient bridge between plant and fungus.
      • most common in large trees
      • formed by fungi in Phyla Ascomycota and Basidiomycota

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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 histoplasmosis
    • 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.

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