The Wonderful World of Fungi

Part XIII – Division Basidiomycota – Sexual Reproductive Structures

By: H. Wayne Shew, Ph.D.
NAB Certified Pollen Counter
AAAC Collection Station—Birmingham, Alabama

The Division Basidiomycota is the second largest division of fungi, having approximately 30,000 described species. They show great diversity in appearance and habitats where they are found. They range in morphology from single-celled yeast forms, to large multicellular mushrooms and puffballs. They are found in virtually every habitat: in all terrestrial ecosystems and in freshwater and marine ecosystems. Most members produce sexual reproductive structures and sexual spores which are formed following meiosis. In fact, the most common diagnostic feature is the production of basidia, singular basidium, which are the cells that produce the sexual spores called basidiospores. Most members of this group also have a characteristic mycelium in which each cell in the thallus has two haploid nuclei resulting from a mating event, typically through fusion of hyphae having different mating types. A few species are asexual and produce no sexual spores. The general or typical life cycle is shown below (Figure 1).

Basidiomycetes were formerly divided into two major groups: homobasidiomycetes (true mushrooms) and heterobasidiomycetes (jelly, rust, and smut fungi). The division is currently divided into three subdivisions along with some undetermined problematic members that are classified as incertae sedis (Latin, uncertain placement), meaning their exact taxonomic relationship to other members of the Basidiomycetes is not yet determined. The three major groups are:

  • Subdivision Pucciniomycotina – includes the rusts and other taxa including Septobasidium
  • Subdivision Ustilaginomycotina – includes the smuts and Exobasidiales
  • Subdivision Agaricomycotina – includes mushrooms (Agaricomycetes), jelly fungi, and the now obsolete taxon Gasteromycetes (puffballs)

Basidiomycetes produce sexual reproductive spores called basidiospores which are the product of meiosis and are thus haploid. Basidiospores are formed on the outside of a structure called a basidium. (Figures 2 & 3). Basidiospores differ in shape, size, and color. The number of basidiospores produced and how they are attached to the basidium also varies between species. The most typical number of basidiospores per basidium is four. If a basidiospore finds a suitable habitat following its release from the basidium, then it can germinate and produce a haploid mycelium and the life cycle continues. The basidiospores are most commonly discharged forcibly from the sterigmata (singular, sterigma) into the air where they can be transported great distances from the basidiomycete fruiting body as part of the aerospora. These spores that are forcibly discharged are referred to as ballistospores. Other groups of basidiomycetes release their spores using other methods: raindrops, sticky spores that are transported by insects, and physical release of spores following rupture of the fruiting body

Basidia with basidiospores

Figure 3. Basidia with basidiospores—light micrograph (brown basidiospores on sterigmata)

My next blog will consider the thallus (body) of basidiomycete fungi which is composed of thread-like hyphae that forms a mycelium, and its unique structures. These unique structures include the presence of a dikaryotic mycelium and clamp connections. Then in subsequent blogs I will address some of the interesting members of the Basidiomycota.