The classes of fungi

Four classes

Fungi are placed in four classes, based on the mycelium, the spores and the fruiting body.

Phycomycetes

• They live in water, on decaying wood in damp places, or as obligate parasites on plants.
• The mycelium is aseptate and coenocytic.
• Asexual spores are made inside a sporangium: zoospores (they swim) or aplanospores (they do not).
• Zygospores form by the fusion of two gametes.
• Examples: Mucor, Rhizopus (the bread mould) and Albugo (a parasite on mustard).

Ascomycetes: the sac fungi

• Some are single-celled, such as yeast (Saccharomyces); others are many-celled, such as Penicillium.
• They are saprophytes and decomposers. Some are parasites, and some are coprophilous: they grow on dung.
• The mycelium is branched and septate.
• Asexual spores are conidia, made on the outside of special hyphae, the conidiophores.
• Sexual spores are ascospores, made inside sac-like asci. The asci lie in fruiting bodies called ascocarps.
• Examples: Aspergillus, Claviceps and Neurospora, which is much used in work on biochemistry and genetics. Morels and truffles are edible and costly.

Basidiomycetes

• They are the mushrooms, bracket fungi and puffballs. They live in soil, on logs and stumps, and as parasites (rusts and smuts) in living plants.
• The mycelium is branched and septate.
• Asexual spores are generally not found. Fragmentation is the usual vegetative way.
• There are no sex organs. Plasmogamy happens by the fusion of two vegetative cells of different strains.
• The basidium bears four basidiospores on the outside. The basidia lie in basidiocarps.
• Examples: Agaricus (the mushroom), Ustilago (a smut) and Puccinia (a rust).

Deuteromycetes: the imperfect fungi

• They are called imperfect because only the asexual (vegetative) stage is known. If the sexual stage is found, the fungus moves to another class.
• Asexual spores are conidia, and the mycelium is septate and branched.
• Some are saprophytes or parasites. Many are decomposers of litter and help in the cycling of minerals.
• Examples: Alternaria, Colletotrichum and Trichoderma.

You should find that taking away the cross walls always gives Phycomycetes, whatever the spores. With cross walls but no known sexual stage, the fungus is one of the imperfect fungi.

Try it in Python

This program keeps the table of the four classes, and finds the classes with a septate mycelium.

Practise with new numbers

Common mistakes

• Putting yeast in Basidiomycetes. Yeast is a sac fungus, in Ascomycetes.
• Mixing up asci and basidia. Ascospores form inside a sac; basidiospores form outside a club.
• Calling Deuteromycetes spore-less. They make conidia; only their sexual stage is unknown.
• Giving Phycomycetes cross walls. Their mycelium is aseptate and coenocytic.

Remember these

• Phycomycetes: aseptate coenocytic mycelium, zygospores. Mucor, Rhizopus, Albugo.
• Ascomycetes: ascospores in asci. Yeast, Penicillium, Aspergillus, Neurospora.
• Basidiomycetes: basidiospores on basidia. Mushrooms, rusts, smuts.
• Deuteromycetes: only the asexual stage is known. Alternaria, Trichoderma.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which class are the mushrooms in?

2. Which class has an aseptate and coenocytic mycelium?

3. Why are the Deuteromycetes called imperfect?

4. Name two costly edible sac fungi.