CH 31: Fungi

Fungal Phyla

1. Phylum Chytridiomycota - Referred to as chytrids; originally thought to be similar to the P.Oomycota. Simple fungi, mainly aquatic forms.
• Ca. 750 species known, and include saprobic and parasitic forms.
• Only phylum to possess motile cells; sexual gametes and asexual zoospores are flagellated (originally were placed in Protista because of this character).
• Chitin present in cell walls.
• Mainly aquatic forms.
2. Phylum Zygomycota ("zygo" - yoke) - These are referred to as the zygomycetes, or more commonly as molds.
• Ca. 800 species known, mainly terrestrial.
• Hyphae are normally coenocytic. Septae are ONLY produced during sexual reproduction and only to wall off the sexual spore.
• Asexual reproduction - Hyphae form a sporangium, a stalked structure in which new haploid spores are produced by mitosis. Spores are released by wind, water or animals.
• New hyphae germinate (the release of a new individual fungi or plant) when appropriate conditions prevail.
• Sexual reproduction - zgomycete species are heterothallic.
• They possess differing strains, called + and - (NOT maile and female).
• Individual strains can asexually reproduce, but sexual reproduction can ONLY occur between opposite strains.
• When + and - strains meet, special hyphae called gametangia extend between them.
• The gametangia form haploid spores, which are walled off from the mycelium by septae.
• These form a single diploid zygosporangium, which begins with heterokaryon nuclei, then many diploid nuclei.
• The zygosporangium protects the dormant spores until appropriate conditions for germination are present.
• When the zygosporangium germinates, it produces an aerial sporangium.
• New haploid spores are produced within sporangium by meiosis.
• Haploid spores are released and produce new hyphae and mycelia.
• Includes the genus Rhizopus.
3. Phylum Ascomycota - ("ascus" - bag or sack)
• Includes a wide variety of forms known as molds, morels, truffles, sac fungi.
• Parasitic forms include species causing Dutch Elm disease and chestnut blight.
• Highly diverse, estimates range from 30,000 species.
• Multicellular forms are called sac fungi.
• All multicellular ascomycetes are septate.
• Sexual spores are produced in a sac-like structures called an ascus (asci).
• Asexual Reproduction:
• In multicellular ascomycetes, a specialized hyphum called a conidiophore ("conidi" - dust, "phore" - bearer) mitotically produces haploid spores called conidia
• Conidia possess various shapes, sizes and colors and are of taxonomic significance.
• Sexual Reproduction - Most species are heterothallic, others are homothallic.
  • After plasmogamy, the nucleus from each hyphum is held within a common cell, however, they do NOT fuse!
  • These dikaryotic (n + n) cells mitotically divide to form the aerial ascocarp, the secondary mycelium (reproductive mycelium).
  • The ascocarp forms a number of cuplike depressions in which the asci form.
  • Within each ascus, the two nuclei fuse.
  • This forms a 2n zygote, which then undergoes meiosis.
  • This forms 4 n spores, each of which undergoes mitosis.
  • Thus, each mature ascus contains eight haploid ascospores within itself.
  • Each ascocarp may have hundreds or thousands of asci.
4. Phylum Basidiomycota ("small pedestal" = "fungus") - Includes the most commonly recognized forms, the mushrooms, toadstools and puffballs, also rusts and smuts (last two tend to be plant pathogens). All are collectively known as club fungi, in reference to the appearance of the sexual spores.
• Ca. 25,000 known species.
• All possess septate hyphae.
• The mycelia of basidiomycetes have two forms, a primary mycelium (vegetative), which may be enormous, and a secondary mycelium, for sexual reproduction.
• Asexual Reproduction - Very rare in basidiomycetes.
  • When it takes place, conidiophores producing conidia are present.
  • Also, can asexually reproduce by fragmentation.
• Sexual Reproduction - Basidiomycetes are heterothallic.
• Hyphae fuse and form dikaryotic cells much like ascomycetes. The resulting secondary mycelium is called the basidiocarp, what we know as the mushroom, toadstool or puffball.
• The reproductive surface appears as raised ridges or thin sheets (the "gills" underneath the cap of mushrooms).
• On each surface are thousands of tiny raised projections. each is a basidium (basidia).
• the processes of dikaryotic fusion is also the same except that the diploid zygote only produces four cells by meiosis.
• These four haploid basidiospores are born on the OUTSIDE of each basidium.
• Thus, each mature basidium has four haploid basidiospores on its surface.

Other Fungal Types

• The following are fungal types that either have representatives among several of the phyla, or which currently fit into none of them:

1. Molds - Molds that belong to the Zygomycota, Ascomycota or Basidiomycota are so named because they represent the rapidly growing, asexually reproducing stage of the fungus.
• When the sporocarp is sexually produced, we refer to the fungi by their other common names, e.g. mushroom, truffle, etc.
• Some molds have NEVER been determined to have a sexual stage.
• These are generally referred to as "deuteromycetes" or "Fungi Imperfecti" (some taxonomists classify them as the phylum Deuteromycota).
2. Yeasts - refers to unicellular fungi of the phyla Ascomycota and Basidiomycota, along with some deuteromycetes.
• These typically asexually reproduce by budding, although the ascomycetes and basidiomycetes can form sporocarps.
3. Lichens - These organisms result from the symbiotic association of a fungus (usually ascomycete, some are basidiomycetes) and a photosynthetic protistian (P. chlorophyta) or cyanobacterium.
• Ca. 20,000 identified species.
• The autotrophs are facultative symbionts, but the fungi are obligate. As a result, there is a controversy as to whether the relationship is mutualistic or parasitic.
• Three lichen body forms:
1. Crustose - Flat, sheet-like, encrusting forms.
2. Foliose - Flat, but possessing raised "leaf like" edges.
3. Fruticose - Erect, branched forms, look like small shrubs.
• Lichens are highly tolerant of extreme conditions and are "pioneer species".
• Usually are the first inhabitants of barren, rocky areas.
• Substances secreted by the lichens helps to weather the rocks to form soil (inorganic sedimentary material & organic matter). Eventually other organisms can colonize the area.
• Reproduction is usually asexual via fragmentation, special pieces break off and establish new individuals.
• Alternatively, can produce soredia, small hyphal clusters encasing algae.
4. Mycorrhizae - ("myco" - fungus; "rhiz" - root) A mutualistic fungus living on the roots of species of Plantae.
• Occurs on about 90% of plant families.
• The fungus provides essential minerals and assists in water absorption, the plant provides food.
• Plants with mycorrhizae removed are typically stunted, and may die or fail to reproduce.

Fungal Ecology

• Fungi are extremely important in ecosystems as decomposers, reducing organic matter to molecular components and recyling nutrients.
• Various important products for humans:
1. Food - Yeasts can undergo glycolysis in anaerobic conditions to produce alchohol by fermentation.
• Such products include breads and other "leavened" baked goods, beer and wine.
• Most cheese develop their flavors due to actions of mold. In blue and Roquefort cheese, the colored bits in the cheese are conidia!
• Soy sauce is produced by a fungus converting the amino acids in soybeans.
• Mushrooms, morels and truffles are eaten directly, many are "delicacies".
2. Drugs - Some antibiotics are naturally secreted by fungal species, e.g. penicillin from Penicillium
• Numerous hallucinogens and psychotropic drugs (e.g. lysergic acid, psilocybin). Ergotism - a convulsive disorder produced by ingestion of alkaloids from molds growing on grains.
3. Diseases - In humans, athlete's foot, "ringworm", thrush, "yeast infection".
• Histoplasmosis - Serious disease in humans that results from fungal spores that occur in bird/bat guano (feces).
• Spores can be inhaled in the dried guano dust and germinate in the lungs.
• Numerous serious plant pathogens, e.g. Dutch Elm disease, Chestnut Blight, corn smut, leaf rust.
• Local fungus on oak leaves. Causes blistering of leaves.

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