Kingdom Fungi

What is Kingdom Fungi?

In plain words: fungi are eukaryotes that cannot make their own food and absorb it from their surroundings. They range from yeasts to bread moulds, mushrooms and toadstools, and are cosmopolitan: they occur in air, water and soil and on animals and plants, preferring warm, humid places (page 16). That is why we keep food in a refrigerator: to slow bacterial and fungal growth; refrigeration does not sterilise food or prevent all spoilage.

Fungi matter to us in many ways (page 16): yeasts are used to make bread and beer, Penicillium is a source of antibiotics, and some fungi cause diseases, such as wheat rust, caused by Puccinia. The white spots on mustard leaves are due to a parasitic fungus.

Body: hyphae and mycelium

Their bodies consist of long, slender thread-like structures called hyphae. The network of hyphae is known as mycelium.

NCERT Biology (Class 11), Section 2.3, page 16

Some hyphae are continuous tubes filled with multinucleated cytoplasm, called coenocytic hyphae; others have septae, or cross walls (page 16).

Nutrition

• Saprophytes: most fungi absorb soluble organic matter from dead substrates.
• Parasites: they depend on living plants and animals.
• Symbionts: in lichens (with algae) and mycorrhiza (with roots of higher plants).

They can also live as symbionts – in association with algae as lichens and with roots of higher plants as mycorrhiza.

NCERT Biology (Class 11), Section 2.3, page 16

Reproduction

Fungi reproduce in three ways (pages 16–17):

• vegetative: fragmentation, fission and budding;
• asexual: spores called conidia, sporangiospores or zoospores;
• sexual: oospores, ascospores and basidiospores.

Spores form in specialised structures; fruiting bodies bear spore-producing structures in many fungi. NCERT describes sporangia, conidiophores, asci and basidia below—these terms are not interchangeable. In NCERT’s overview, the sexual cycle has three steps:

1. Plasmogamy: fusion of cytoplasm from compatible cells; these may be gametes or, as NCERT describes for basidiomycetes, vegetative cells.
2. Karyogamy: fusion of two nuclei.
3. Meiosis in the zygote, giving haploid spores.

Two haploid hyphae of compatible mating types come together and fuse. In some fungi the fused cell is diploid (2n) at once. In others, the ascomycetes and basidiomycetes, a stage with two nuclei per cell (n + n) comes in between:

such a condition is called a dikaryon and the phase is called dikaryophase of fungus.

NCERT Biology (Class 11), Section 2.3, page 17

Later the parental nuclei fuse, the cells become diploid, and reduction division in the fruiting bodies forms haploid spores (page 17).

The four classes

This section follows NCERT’s traditional four-group account for study. Beyond NCERT: it is not a complete modern classification. For example, NCBI Taxonomy places Albugo among oomycetes outside true Fungi; do not apply the chitin-wall rule uncritically to every organism in the historical “phycomycetes” grouping.

The morphology of the mycelium, mode of spore formation and fruiting bodies form the basis for the division of the kingdom into various classes.

NCERT Biology (Class 11), Section 2.3, page 17

Phycomycetes (page 17)

• Found in aquatic habitats, on decaying wood in moist and damp places, or as obligate parasites on plants.
• Mycelium aseptate and coenocytic.
• Asexual spores: zoospores (motile) or aplanospores (non-motile), produced endogenously in a sporangium.
• Sexual: a zygospore forms by fusion of two gametes, which are similar (isogamous) or dissimilar (anisogamous or oogamous).
• Examples: Mucor (NCERT Figure 2.5a), Rhizopus (the bread mould), Albugo (the parasitic fungus on mustard).

Ascomycetes, the sac fungi (pages 17–18)

• Mostly multicellular, e.g. Penicillium, or rarely unicellular, e.g. yeast (Saccharomyces); saprophytic, decomposers, parasitic or coprophilous (growing on dung).
• Mycelium branched and septate.
• Asexual spores: conidia, produced exogenously on the special mycelium called conidiophores.
• Conidia on germination produce mycelium.
• Sexual spores: ascospores, produced endogenously in sac-like asci (singular ascus); the asci are arranged in different types of fruiting bodies called ascocarps.
• Examples: Aspergillus (NCERT Figure 2.5b), Claviceps and Neurospora, which is used extensively in biochemical and genetic work (page 18); morels and truffles are edible delicacies.

Basidiomycetes (page 18)

• Mushrooms, bracket fungi and puffballs; in soil, on logs and tree stumps, and in living plants as parasites (rusts and smuts).
• Mycelium branched and septate.
• Asexual spores generally absent; fragmentation is common. Sex organs are absent: plasmogamy is by fusion of two vegetative or somatic cells of different strains or genotypes.
• The dikaryotic structure gives rise to the basidium, where karyogamy and meiosis produce four basidiospores, formed exogenously on the basidium; the basidia are arranged in fruiting bodies called basidiocarps.
• Examples: Agaricus (mushroom, NCERT Figure 2.5c), Ustilago (smut), Puccinia (rust fungus).

Deuteromycetes, the imperfect fungi (page 18)

Commonly known as imperfect fungi because only the asexual or vegetative phases of these fungi are known.

NCERT Biology (Class 11), Section 2.3.4, page 18

• When their sexual stages are discovered, they are moved to the class they belong to, often ascomycetes or basidiomycetes. Sometimes the asexual and the sexual stage of one fungus had been given two names and placed in different classes until the link was found (page 18).
• They reproduce only by asexual spores, conidia; mycelium septate and branched.
• Saprophytes or parasites, many being decomposers of litter that help in mineral cycling.
• Examples: Alternaria, Colletotrichum, Trichoderma.

Solved examples

Common mistakes to avoid

Quick revision summary

• Fungi: heterotrophic eukaryotes with walls of chitin and polysaccharides; filamentous (hyphae, mycelium) except the unicellular yeasts.
• Hyphae are coenocytic (multinucleate, no cross walls) or septate.
• Nutrition: mostly saprophytic; also parasites, and symbionts in lichens and mycorrhiza.
• Reproduction: vegetative (fragmentation, fission, budding), asexual spores (conidia, sporangiospores, zoospores), sexual spores (oospores, ascospores, basidiospores); sexual cycle: plasmogamy, karyogamy, meiosis; dikaryon (n + n) in ascomycetes and basidiomycetes.
• NCERT’s traditional classes: phycomycetes (aseptate, zygospores: Mucor, Rhizopus, Albugo), ascomycetes (sac fungi: Penicillium, yeast, Aspergillus, Claviceps, Neurospora), basidiomycetes (Agaricus, Ustilago, Puccinia), deuteromycetes (imperfect: Alternaria, Colletotrichum, Trichoderma).

Questions students ask

What is the difference between ascomycetes and basidiomycetes?

Ascomycetes and basidiomycetes both have a branched, septate mycelium and a dikaryotic stage, but ascomycetes, the sac fungi, form their sexual ascospores endogenously, inside sac-like asci grouped in ascocarps, while basidiomycetes form four basidiospores exogenously, on a basidium, in basidiocarps. Ascomycetes also reproduce asexually by conidia on conidiophores, whereas in basidiomycetes asexual spores are generally not found, fragmentation is common and, with no sex organs, plasmogamy is by fusion of two somatic cells of different strains. Examples are Penicillium, Aspergillus and Neurospora among ascomycetes, and the mushrooms, bracket fungi and puffballs among basidiomycetes.

Why are ascomycetes called sac fungi and phycomycetes algal fungi?

Ascomycetes are called sac fungi, NCERT’s common name for them, because their sexual spores, the ascospores, are produced inside sac-like asci (singular ascus). Phycomycetes (from the Greek ‘phykos’, seaweed) are called algal fungi, a name NCERT does not use, because many of them resemble algae: they live in water or in moist, damp places and have an aseptate, coenocytic mycelium and motile zoospores. On the same pattern, basidiomycetes are often called club fungi, after their club-shaped basidium.

How is Kingdom Fungi different from Monera and Protista?

Kingdom Fungi differs from Monera in being eukaryotic, with a nuclear membrane, and from both Monera and Protista in being wholly heterotrophic (saprophytic or parasitic), with no photosynthetic or chemosynthetic members (NCERT Table 2.1). Fungal cell walls contain chitin, while typical eubacterial walls contain peptidoglycan (archaebacteria differ and mycoplasmas have no wall), and only some protists have a wall at all. Most fungi are filamentous, with a mycelium of hyphae (multicellular or loose tissue in Table 2.1, against cellular for Monera and Protista), and the yeasts are the unicellular exception.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. What is the difference between coenocytic and septate hyphae?

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   Coenocytic hyphae are continuous tubes filled with multinucleated cytoplasm; septate hyphae have septae, or cross walls (NCERT page 16).

2. Q2. Name two symbiotic associations of fungi.

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   Lichens (with algae) and mycorrhiza (with roots of higher plants) (NCERT page 16).

3. Q3. Which class are Agaricus, Ustilago and Puccinia in, and what are they commonly called?

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   Basidiomycetes; mushroom, smut and rust fungus (NCERT page 18).

4. Q4. What are ascocarps?

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   The fruiting bodies of ascomycetes, in which the asci bearing ascospores are arranged (NCERT page 18).