Fungi: Features, Types and Life Cycle

Fungi constitute a unique kingdom of heterotrophic organisms. They play a pivotal role in the ecosystem as decomposers and have a significant impact on human health, agriculture, and industry.

General Features

• Nutrition: They are achlorophyllous (lack chlorophyll) and heterotrophic. They can be Saprophytes (absorb soluble organic matter from dead substrates), Parasites (live on living plants and animals), or Symbionts.
• Symbiotic Associations: ” Lichens: Association with algae.
  • Mycorrhiza: Association with roots of higher plants (e.g., Pinus).
• Cell Wall: Composed of Chitin and polysaccharides.
• Body Structure: With the exception of unicellular Yeast, fungi are filamentous. Their bodies consist of long, slender thread-like structures called hyphae. A network of hyphae is known as mycelium.
• Types of Hyphae:
  • Coenocytic hyphae: Continuous tubes filled with multinucleated cytoplasm.
  • Septate hyphae: Have septa or cross-walls.

Classification of Fungi

Fungi are classified into four main classes based on the morphology of the mycelium, mode of spore formation, and fruiting bodies.

1. Phycomycetes

• Found in aquatic habitats and on decaying wood in moist and damp places.
• Reproduction: Asexual reproduction by motile zoospores or non-motile aplanospores.
• Examples: Mucor, Rhizopus (bread mould), and Albugo (parasitic fungi on mustard).

2. Ascomycetes

• Mostly multicellular (e.g., Penicillium) or rarely unicellular (e.g., Yeast – Saccharomyces).
• They are saprophytic, decomposers, parasitic, or coprophilous (growing on dung).
• Sexual Spores: Ascospores are produced endogenously in sac-like asci (singular: ascus).
• Examples: Aspergillus, Claviceps, and Neurospora (used extensively in biochemical and genetic work).

3. Basidiomycetes

• Grow in soil, on logs, and in living plant bodies as parasites (e.g., rusts and smuts).
• Characteristics: The asexual spores are generally not found, but vegetative reproduction by fragmentation is common. Sex organs are absent, but plasmogamy is brought about by the fusion of two vegetative or somatic cells.
• Sexual Spores: Basidiospores are exogenously produced on the basidium.
• Examples: Agaricus (mushroom), Ustilago (smut), and Puccinia (rust fungus).

4. Deuteromycetes

• Commonly known as imperfect fungi because only the asexual or vegetative phases are known.
• Once sexual stages are discovered, they are often moved to Ascomycetes or Basidiomycetes.
• They are large-scale decomposers of litter and help in mineral cycling.
• Examples: Alternaria, Colletotrichum, and Trichoderma.

Life Cycle and Reproduction

The sexual cycle involves the following three steps:

1. Plasmogamy: Fusion of protoplasms between two motile or non-motile gametes.
2. Karyogamy: Fusion of two nuclei.
3. Meiosis: Occurs in the zygote resulting in haploid spores.

Dikaryophase

In some fungi (Ascomycetes and Basidiomycetes), the fusion of two haploid cells does not immediately result in diploid cells (2n). Instead, an intervening stage occurs where there are two nuclei per cell (n + n). This condition is called a dikaryon, and the phase is called the dikaryophase. Later, the parental nuclei fuse and the cells become diploid.

UPSC Prelims Facts and Trivia

• Antibiotics: Penicillium notatum was the source of the first antibiotic, Penicillin, discovered by Alexander Fleming.
• Fermentation: Yeast (Saccharomyces cerevisiae) is crucial in the baking and brewing industries (Brewer’s yeast).
• Hallucinogens: Amanita muscaria and certain species of Psilocybe contain hallucinogenic compounds.
• Plant Diseases: ” Late Blight of Potato: Caused by Phytophthora infestans (Responsible for the Irish Famine).
  • Wheat Rust: Caused by Puccinia.
  • Ergotism: Caused by Claviceps purpurea affecting rye.
• Edible Fungi: Mushrooms like Agaricus bisporus and Morels (Morchella) are edible and considered delicacies.
• Mycotoxins: Some fungi produce toxins like Aflatoxin (produced by Aspergillus flavus), which can contaminate stored food grains and are carcinogenic.