Unit 4: Major divisions of Fungi

1. Myxomycota (Slime Molds)

(Covered as per the syllabus listing)

• General Characteristics: These are "fungus-like" protists, not true fungi. They are classified under the Kingdom Protista.
• Thallus Organization: The vegetative stage is not a mycelium. It is a plasmodium: a multinucleate, "naked" (no cell wall) mass of protoplasm that moves by amoeboid movement.
• Nutrition: Phagotrophic (ingests bacteria, spores) not absorptive.
• Ecology & Significance: Found in cool, moist, shady places (e.g., decaying logs). They are important decomposers of bacteria and micro-organisms.
• Reproduction:
  • Under stress, the plasmodium transforms into stalked fruiting bodies (sporangia).
  • Inside the sporangia, meiosis occurs, forming haploid spores (which *do* have a cell wall).
  • Spores germinate into haploid amoeboid cells or flagellated swarm cells, which fuse to form a new diploid zygote, which grows into a plasmodium.

2. Oomycota (Water Molds)

(Note: Oomycetes are also protists, not true fungi. They are in the Kingdom Stramenopila, related to brown algae. They are included here for historical reasons.)General Characteristics, Ecology, Thallus, Reproduction

• General Characteristics: "Water molds" and "downy mildews."
• Cell Wall: Composed of cellulose and glucans (NOT chitin).
• Thallus: Aseptate (coenocytic) mycelium.
• Ecology & Significance: Major plant pathogens (e.g., *Phytophthora* causes potato late blight, *Albugo* causes white rust).
• Reproduction:
  • Asexual: By zoospores produced in a sporangium. The zoospores are biflagellate and heterokont (one tinsel, one whiplash), just like brown algae.
  • Sexual: Oogamous. Involves a female oogonium (with eggs) and a male antheridium. Fertilization produces a thick-walled, diploid resting spore called an oospore.

Life cycle and classification of *Phytophthora*

• Organism: *Phytophthora infestans* (causes late blight of potato).
• Classification: Kingdom Stramenopila, Division Oomycota.
• Life cycle:
  1. Asexual:
     • The diploid (2n) mycelium grows in the host plant.
     • It sends out sporangiophores through the stomata.
     • Lemon-shaped sporangia (2n) form at the tips.
     • In cool, moist conditions, the sporangium acts as a zoosporangium: its contents divide to form biflagellate zoospores (2n).
     • In warm, dry conditions, the sporangium germinates directly like a spore.
     • Zoospores swim, encyst, and infect a new leaf, starting a new infection.
  2. Sexual:
     • Requires two different mating types (heterothallic).
     • A spherical oogonium (n, after meiosis) and a club-shaped antheridium (n, after meiosis) form.
     • Fertilization occurs, forming a diploid oospore (2n).
     • The oospore is a thick-walled resting spore that survives in the soil. It germinates (mitotically) to produce a sporangium, repeating the cycle.

Life cycle and classification of *Albugo*

• Organism: *Albugo candida* (causes white rust of crucifers, e.g., mustard, radish).
• Classification: Kingdom Stramenopila, Division Oomycota.
• Life cycle: Very similar to *Phytophthora*.
  1. Asexual:
     • The diploid (2n) mycelium grows in the host.
     • It forms sporangiophores *under* the epidermis, causing a white, blister-like "pustule."
     • Sporangia (2n) are formed in chains (this is a key difference from *Phytophthora*).
     • The epidermis ruptures, releasing the sporangia.
     • Sporangia are dispersed by wind and produce zoospores (2n), which infect new hosts.
  2. Sexual:
     • Occurs within the host tissue.
     • An oogonium (n) and antheridium (n) form.
     • Fertilization produces a diploid oospore (2n).
     • The oospore is a resting stage. It germinates by meiosis (this is an exception to the diplontic rule, some Oomycetes do this) to produce haploid zoospores, which then fuse, or it germinates to produce diploid zoospores. (Note: Textbooks vary, but the main point is the formation of the oogonium and antheridium). Let's stick to the common diplontic model: oospore germinates mitotically to produce diploid zoospores.

3. Zygomycota (Conjugation Fungi)

(Covered as per the syllabus listing)

• General Characteristics: "Lower" true fungi.
• Thallus: Aseptate (coenocytic) mycelium (like Oomycota).
• Cell Wall: Composed of chitin.
• Ecology & Significance: Mostly saprophytes (e.g., *Rhizopus* - bread mold, *Mucor*). Some are parasites.
• Reproduction:
  • Asexual: By sporangiospores (non-motile aplanospores) formed inside a sporangium.
  • Sexual: By gametangial conjugation.
    1. Two compatible hyphae (+ and - strains) grow towards each other.
    2. The tips (called progametangia) swell and are cut off by septa, forming gametangia.
    3. The wall between them dissolves (plasmogamy).
    4. The many haploid nuclei from both sides fuse (karyogamy) to form many diploid nuclei.
    5. The entire structure develops into a thick-walled, black, warty resting spore called a zygospore (2n).
    6. After dormancy, the zygospore undergoes meiosis and germinates to produce a sporangium, which releases haploid (n) spores.
• Life cycle: Haplontic (the mycelium is haploid, the zygospore is the only diploid stage).

4. Ascomycota (Sac Fungi)

General Characteristics, Ecology, Thallus, Reproduction

• General Characteristics: "Higher" true fungi. This is the largest, most diverse group (yeasts, cup fungi, morels, truffles, *Penicillium*).
• Thallus: Unicellular (yeasts) or, more commonly, a septate mycelium (with simple pores).
• Cell Wall: Composed of chitin.
• Ecology & Significance: Saprophytes, parasites, and symbionts (lichens). Huge economic importance (baking, brewing, antibiotics, plant diseases).
• Reproduction:
  • Asexual: By conidia (externally produced spores) on conidiophores.
  • Sexual: The defining feature is the ascus (plural: asci).
    1. Plasmogamy occurs (e.g., between an antheridium and ascogonium) but karyogamy is delayed.
    2. This creates dikaryotic (n+n) hyphae (ascogenous hyphae).
    3. The tips of these hyphae form the asci. Inside the ascus, karyogamy (fusion of n+n nuclei) finally occurs, forming a diploid (2n) zygote nucleus.
    4. This nucleus *immediately* undergoes meiosis (to make 4 haploid nuclei) followed by mitosis (to make 8).
    5. The result is 8 haploid ascospores (meiospores) inside the ascus.
    6. In many species, the asci are grouped into a fruiting body called an ascocarp (e.g., a cup-shaped apothecium).

Life cycle and classification of *Saccharomyces* (Yeast)

• Organism: *Saccharomyces cerevisiae* (Baker's or Brewer's Yeast).
• Classification: Division Ascomycota, Class Hemiascomycetes (no ascocarp).
• Thallus: Unicellular, ovoid. No mycelium.
• Life cycle (Haplodiplontic): Yeast can exist stably as both haploid (n) and diploid (2n) cells.
  1. Asexual (Haploid or Diploid):
     • Both (n) and (2n) cells reproduce by budding.
  2. Sexual:
     • Under stress, two haploid cells of opposite mating types (a and α) fuse (plasmogamy + karyogamy).
     • This forms a stable diploid (2n) zygote cell.
     • This diploid cell can bud asexually, forming a diploid population.
     • Under starvation, the diploid cell undergoes meiosis to form 4 haploid ascospores *inside* the original cell wall (which now acts as an ascus).
     • These ascospores are released and germinate as new haploid cells.

Life cycle and classification of *Aspergillus*

• Organism: *Aspergillus* (a common "blue-green mold"). Some species are used in industry (e.g., *A. oryzae* for soy sauce), others are pathogens (e.g., *A. fumigatus*).
• Classification: Division Ascomycota, Class Plectomycetes. (Sexual stage is *Eurotium*).
• Life cycle:
  1. Asexual (Anamorph): This is the most common form.
     • A septate mycelium (n) grows.
     • It produces a long conidiophore that ends in a swollen vesicle.
     • Bottled-shaped cells (phialides) form on the vesicle, which produce long chains of conidia (n).
     • The entire structure looks like a "holy water sprinkler."
     • Conidia are dispersed by air and germinate into new haploid mycelia.
  2. Sexual (Teleomorph):
     • Plasmogamy occurs between an antheridium and an ascogonium (female).
     • Dikaryotic (n+n) hyphae grow from the ascogonium.
     • These form asci. Karyogamy (2n) and Meiosis (n) occur, producing 8 haploid ascospores.
     • The asci are *not* arranged in an orderly layer. They are scattered within a completely enclosed, spherical ascocarp called a cleistothecium.
     • The cleistothecium breaks down to release the ascospores.

5. Basidiomycota (Club Fungi)

General Characteristics & Classification (Fruiting Bodies)

• General Characteristics: "Higher" fungi (mushrooms, puffballs, rusts, smuts).
• Thallus: Septate mycelium. The septa are complex dolipore septa. The mycelium exists in two states:
  • Primary Mycelium: Haploid (n), from a basidiospore.
  • Secondary Mycelium: Dikaryotic (n+n), formed by plasmogamy. This is the dominant, long-lived, vegetative part of the fungus. Often shows clamp connections during cell division.
• Reproduction:
  • Asexual: Less common than in Ascomycota. Can be by conidia or fragmentation.
  • Sexual: The defining feature is the basidium (plural: basidia).
    1. The dikaryotic (n+n) mycelium forms a fruiting body called a basidiocarp (e.g., a mushroom).
    2. On the gills, terminal cells of the dikaryotic hyphae form basidia.
    3. Inside the basidium, karyogamy occurs (n+n -> 2n).
    4. The diploid nucleus *immediately* undergoes meiosis to form 4 haploid (n) nuclei.
    5. These 4 nuclei migrate to the outside of the basidium, into four small "horns" (sterigmata), and become 4 exogenous (external) basidiospores (n).
• Classification (by Fruiting Bodies):
  • Teliomycetes: No basidiocarp. Form thick-walled resting spores (teliospores) where karyogamy occurs. (e.g., *Puccinia*, *Ustilago*).
  • Hymenomycetes: Basidiocarp present, with an exposed hymenium (spore-bearing layer) at maturity. (e.g., *Agaricus* - mushrooms).
  • Gasteromycetes: Basidiocarp present, but the hymenium is enclosed until the spores are mature. (e.g., Puffballs).

Heterokaryosis and Parasexuality

• Heterokaryosis:
  • The condition where a single mycelial cell contains genetically different nuclei (e.g., 'A' and 'B').
  • This is the foundation of the dikaryon (n+n) state, which is a stable, long-term heterokaryon.
  • It allows the fungus to have the genetic advantages of diploidy (masking bad mutations) while remaining functionally haploid.
• Parasexuality:
  • A mechanism for genetic recombination *without* the standard sexual cycle (plasmogamy, karyogamy, meiosis).
  • It occurs in some fungi (especially Deuteromycota) that lack a sexual stage.
  • Process: 1. Heterokaryosis: Two different hyphae fuse (plasmogamy). 2. Karyogamy (Rare): Two different nuclei (A+B) in the heterokaryon *accidentally* fuse to form a diploid (2n) nucleus. 3. Mitotic Recombination: During *mitosis* (not meiosis) of this diploid nucleus, crossing-over can occur, shuffling the genes. 4. Haploidization: The diploid nucleus gradually (and randomly) loses chromosomes one by one until it returns to a stable haploid (n) state.
  • Significance: It provides a way for asexual fungi to create new gene combinations and adapt.

Teliomycetes: *Puccinia* and *Ustilago*

These are parasitic fungi that do not form a basidiocarp.

*Puccinia* (Black Stem Rust of Wheat)

• Life cycle: Heteroecious (requires two different hosts to complete its life cycle: Wheat and Barberry) and Macrocyclic (produces all 5 spore types).
• Spores on Wheat (Primary Host): 1. Urediniospores (n+n): The "repeating stage." Red-orange spores that re-infect wheat, causing the "red rust" stage. 2. Teliospores (n+n -> 2n): Formed later in the season. Thick-walled, black resting spores ("black rust"). Karyogamy (n+n -> 2n) occurs here.
• Spores on Barberry (Alternate Host): 3. Basidiospores (n): Teliospore germinates, undergoes meiosis, and forms a basidium with 4 haploid basidiospores. These infect the *barberry* leaf. 4. Pycniospores/Spermatia (n): Form in "pycnia" on the barberry leaf. They are gametes. They cross-fertilize to form a dikaryon. 5. Aeciospores (n+n): Form in "aecia" on the *underside* of the barberry leaf. These dikaryotic spores are released and *infect the wheat*, starting the cycle over.

*Ustilago* (Symptoms)

• Organism: *Ustilago* (causes smut diseases, e.g., loose smut of wheat, corn smut).
• Symptoms: The fungus infects the grain (e.g., corn kernel, wheat head) and replaces it with a black, powdery mass of teliospores (n+n -> 2n).
• Life cycle: Autoecious (completes its life cycle on one host). It is much simpler than *Puccinia*. 1. Teliospores (2n) are released from the "smut." 2. They undergo meiosis to form haploid (n) basidiospores. 3. Haploid basidiospores fuse (plasmogamy) to form a dikaryotic (n+n) mycelium, which is the only stage that can infect the host plant.

Agaricales: *Agaricus*

• Organism: *Agaricus* (the common field mushroom, *A. campestris*).
• Classification: Division Basidiomycota, Order Agaricales.
• General Characteristics: This order includes the gilled mushrooms.
• Life cycle: 1. A haploid (n) basidiospore germinates to form a haploid primary mycelium. 2. Two compatible primary mycelia fuse (plasmogamy) to form the secondary mycelium, which is dikaryotic (n+n) and long-lived in the soil. 3. When conditions are right, this secondary mycelium forms a complex fruiting body: the basidiocarp (the mushroom). 4. The mushroom consists of a stipe (stalk) and a pileus (cap). 5. Under the cap are gills, which are lined with a spore-bearing layer called the hymenium. 6. The hymenium is packed with basidia. 7. Inside each basidium, karyogamy (2n) followed by meiosis (n) occurs. 8. Four haploid basidiospores (n) are produced externally on each basidium, are forcefully discharged, and are dispersed by the wind.