7.4 Diversity Among Fungi

Fungi are a diverse kingdom of over 100,000 known terrestrial species. Their ancestry dates back approximately 570 million years, and they are thought to have evolved from red algae, as both groups lack flagella at any stage of their life cycles.

Classification of Fungi

Fungi are classified into four main groups, primarily based on their methods of reproduction and the types of sexual reproductive structures they form. These groups also differ in their hyphal structure.

Zygomycota (Conjugating Fungi)

• Habitat: Found in soil and on decaying organic matter (saprotrophs on bakery goods, fruits, etc.). They are important decomposers, but some are pathogenic.
• Structure: Hyphae are non-septate (coenocytic), forming a well-developed, branching mycelium.
• Asexual Reproduction: Spores are produced in a structure called a sporangium. Example: Rhizopus nigricans.
• Sexual Reproduction: Occurs via conjugation between two different mating strains (+ and −).
  1. Hyphae of opposite strains meet and hormones cause the tips to form gametangia.
  2. Fusion of haploid nuclei (karyogamy) forms a diploid zygote.
  3. The zygote develops into a thick-walled, resistant zygospore.
  4. Under favorable conditions, the zygospore germinates, undergoes meiosis, and produces a sporangium that releases haploid spores.

Ascomycota (Sac Fungi)

• Habitat and Diversity: A large, diverse group found in various ecosystems. Includes economically important fungi like morels, truffles, and yeast.
• Structure: Hyphae are typically septate, but with pores that allow cytoplasm to move between compartments.
• Asexual Reproduction:
  • Conidia: Spores that form on the tips of modified hyphae called conidiophores. The color of conidia gives many molds their characteristic tint (blue, green, pink).
  • Budding: Asexual reproduction in unicellular yeasts, where a small bud separates from the parent cell.
• Sexual Reproduction:
  1. Two hyphae fuse their cytoplasm (plasmogamy), but not their nuclei — forming a dikaryotic ($n+n$) hypha.
  2. These hyphae form a fruiting body called an ascocarp (e.g., cup-shaped, flask-shaped).
  3. Inside the ascocarp, specialized sac-like structures called asci (singular: ascus) develop.
  4. Within each ascus, the two nuclei fuse (karyogamy) to form a diploid zygote ($2n$).
  5. The zygote undergoes meiosis (forming 4 haploid nuclei) and then mitosis (forming 8 haploid nuclei total).
  6. Each nucleus develops into an ascospore. The mature ascus bursts, releasing the 8 ascospores.

Basidiomycota (Club Fungi)

• Habitat and Diversity: Includes mushrooms, puffballs, shelf fungi, rusts, and smuts. They can be decomposers or form mycorrhizal relationships with plants.
• Structure: Hyphae are septate with perforations. The mycelium often grows extensively underground.
• Asexual Reproduction: Occurs occasionally via conidiospores.
• Sexual Reproduction:
  1. A primary mycelium (monokaryotic, $n$) fuses with another of a different mating type (plasmogamy).
  2. A secondary mycelium forms, which is dikaryotic ($n+n$).
  3. This secondary mycelium grows into a compact mass (button) which develops into the fruiting body, the basidiocarp (e.g., a mushroom).
  4. The basidiocarp consists of a stalk and a cap. Under the cap are gills.
  5. On the gills, club-shaped structures called basidia (singular: basidium) form.
  6. Within each basidium, the two haploid nuclei fuse (karyogamy) to form a diploid zygote ($2n$).
  7. The zygote undergoes meiosis to produce four haploid nuclei, which develop into basidiospores on the tip of the basidium.
  8. Basidiospores are released and can germinate into new primary mycelia.

Deuteromycota (Imperfect Fungi)

• Defining Feature: These fungi are called "imperfect" because they have no known sexual stage in their life cycle.
• Reproduction: They reproduce asexually by forming conidiospores on conidiophores.
• Lifestyle: Many are saprophytic or parasitic on plants.
• Genetic Recombination: Despite the lack of sexual reproduction, they can achieve genetic diversity through parasexuality — the exchange of chromosomal portions between two nuclei within the same hypha.
• Examples: Penicillium, Aspergillus, Alternaria, Fusarium.

Key Terminology

Summary

Fungi are a diverse kingdom classified into four major phyla:

• Zygomycota (e.g., Rhizopus — black bread mold): non-septate hyphae; sexual reproduction by conjugation forming a resistant zygospore.
• Ascomycota (e.g., yeasts, morels, truffles): septate hyphae; sexual reproduction produces 8 ascospores inside a sac-like ascus within an ascocarp.
• Basidiomycota (e.g., mushrooms, rusts, smuts): septate hyphae; sexual reproduction produces 4 basidiospores on club-shaped basidia within a basidiocarp.
• Deuteromycota (e.g., Penicillium, Aspergillus): no known sexual stage; reproduce asexually by conidia; genetic diversity via parasexuality.

All groups except Deuteromycota pass through a dikaryotic ($n+n$) phase between plasmogamy and karyogamy during sexual reproduction.