Basidiomycetes are among the most advanced fungi in Kingdom Fungi. Familiar examples include mushrooms, bracket fungi and puffballs. This lesson covers their general characters and walks through their sexual life cycle step by step.
General characters
Moist, damp and shady places: soil, logs and tree stumps. Some are parasites on plants.
Multicellular, with a loose tissue organisation.
Branched, septate hyphae with a barrel-shaped dolipore septum. The cell wall is made of chitin and polysaccharides.
Vegetative by fragmentation. Asexual spores are generally absent (exception: Puccinia). Sexual reproduction occurs without gametes or sex organs.
Sexual life cycle
Spore germination
The sexual cycle starts with basidiospores, the sexual spores of basidiomycetes. They are small, non-motile and dispersed by wind. Basidiospores are haploid (with a single set of chromosomes) because they are produced by meiosis.
On a suitable surface, such as a decaying log, a basidiospore germinates. Thin, branched, thread-like outgrowths called hyphae develop, and a criss-cross network of hyphae forms the mycelium. This mycelium has only one nucleus per cell (monokaryotic) and is called the primary mycelium.
1. Plasmogamy
The primary mycelium can reproduce vegetatively by fragmentation, producing identical copies. When two primary mycelia of different strains or genotypes come into contact, their cytoplasm fuses. This fusion of two somatic (vegetative) cells is called plasmogamy or somatogamy.
The two nuclei do not fuse immediately, so the resulting cell has two haploid nuclei, one from each parent. This marks the start of the dikaryotic phase (n + n).
2. Dikaryotic phase and basidiocarp
The dikaryotic cell grows into new hyphae called the secondary mycelium. Every cell of the secondary mycelium keeps both nuclei, with the help of special structures called clamp connections. The secondary mycelium then organises into the fruiting body, the basidiocarp.
The shape, colour and size of the basidiocarp vary between species. In Agaricus, the umbrella-shaped cap is the pileus, held up on a long stalk. Under the pileus are thin radiating plates called gills, and each gill bears many small club-shaped basidia.
3. Karyogamy and meiosis
In each basidium the two nuclei finally fuse. This is karyogamy, and it forms a diploid (2n) nucleus. The diploid nucleus immediately divides by meiosis I and meiosis II, producing four haploid nuclei of different genotypes.
Each nucleus moves out of the basidium, gathers a little cytoplasm and a cell wall, and develops into a basidiospore. So every basidium produces four haploid basidiospores, formed outside it (exogenously). Each basidiospore can germinate into a new primary mycelium, completing the cycle.
Remember the sequence: basidiospore (n) → primary mycelium (n) → plasmogamy → secondary mycelium (n + n) → basidiocarp → karyogamy in basidium (2n) → meiosis → 4 exogenous basidiospores (n).
Karyogamy and meiosis both take place in the basidium. Basidiospores are exogenous (formed outside the basidium), whereas ascospores are endogenous (formed inside the ascus). This contrast is a common NEET question.
Examples
- Agaricus: edible mushroom
- Puccinia: rust fungus, causes rust disease of wheat
- Ustilago: smut fungus, causes smut disease of cereals such as wheat and maize
- Bracket fungi
- Puffballs