Basidium

A basidium (plural: basidia) is the spore-producing cell of fungi in the phylum Basidiomycota — the club fungi. It is a microscopic, club-shaped structure that forms on the surface of a fruiting body’s fertile tissue. Each basidium typically produces four spores (basidiospores) on slender projections called sterigmata. When the spores mature, they are discharged — often by a surface-tension catapult mechanism called Buller’s drop — and dispersed by air currents.

In a gilled mushroom, basidia line the surfaces of the gills in a dense layer called the hymenium. A single mushroom may contain millions of basidia, each producing four spores, yielding billions of spores from one fruiting event. In bracket fungi, basidia line the inner surfaces of pores. In puffballs, basidia fill the interior, and spores are released when the outer wall ruptures. In each case, the basidium is the cell where sexual reproduction culminates: two compatible haploid nuclei fuse (karyogamy), undergo meiosis to produce four genetically distinct haploid nuclei, and each nucleus migrates into a developing basidiospore.

The Basidiomycota — defined by the basidium — include most of the fungi people recognize: the mushrooms, brackets, puffballs, earthstars, stinkhorns, jelly fungi, rusts, and smuts. Many ectomycorrhizal fungi are Basidiomycota — boletes, chanterelles, fly agarics — forming the mycorrhizal associations that connect forest trees. The basidium is the diagnostic feature that unites this diverse phylum. Compare with the ascus, the equivalent structure in the Ascomycota.

Related terms

• Ascus — the spore-producing structure of Ascomycota, the parallel to the basidium
• Spore — the reproductive units that basidia produce
• Fruiting Body — the macroscopic structure whose fertile surfaces bear basidia
• Fungal Reproduction — sexual reproduction and mating systems in Basidiomycota
• Fungal Taxonomy — the basidium defines the phylum Basidiomycota
• Mycorrhiza — many ectomycorrhizal fungi are Basidiomycota