About Basidiomycota
Basidiomycota is one of two large divisions that, together with Ascomycota, make up the subkingdom Dikarya (often called "higher fungi") within the kingdom Fungi. Members of this division are called basidiomycetes. This group includes agarics, puffballs, stinkhorns, bracket fungi, other polypores, jelly fungi, boletes, chanterelles, earth stars, smuts, bunts, rusts, mirror yeasts, and Cryptococcus, the human pathogenic yeast. Most Basidiomycota are filamentous fungi made up of hyphae; only yeasts within Basidiomycota are an exception. They reproduce sexually through the formation of specialized club-shaped end cells called basidia, which normally carry external meiospores, usually four in number. These specialized spores are called basidiospores. However, some Basidiomycota are obligate asexual reproducers. Asexual Basidiomycota can usually be identified as part of this division by their overall similarity to other basidiomycetes, by their formation of a distinctive anatomical feature called the clamp connection, and by their cell wall components. Definitive identification comes from phylogenetic molecular analysis of DNA sequence data. Unlike animals and plants, which have easily recognizable male and female counterparts, Basidiomycota (except for rusts in Pucciniales) tend to have mutually indistinguishable, compatible haploids that are usually mycelia made of filamentous hyphae. Typically, haploid Basidiomycota mycelia fuse through plasmogamy. After fusion, compatible nuclei migrate into each other's mycelia and pair up with the resident nuclei. Karyogamy is delayed, so the compatible nuclei remain in pairs, a structure called a dikaryon. Hyphae in this state are called dikaryotic, while haploid mycelia are called monokaryons. Often, the dikaryotic mycelium grows more vigorously than individual monokaryotic mycelia, and it grows to take over the substrate it is growing in. Dikaryons can be very long-lived, surviving for years, decades, or centuries. Monokaryons are not male or female. They have either a bipolar (unifactorial) or a tetrapolar (bifactorial) mating system. This means that after meiosis, the resulting haploid basidiospores and their resulting monokaryons have nuclei that are compatible with 50% of their sister basidiospores and their monokaryons when the mating system is bipolar, or 25% when it is tetrapolar. Compatibility requires that the mating genes are different. In some species, there can be more than two alleles for a given locus. In these species, over 90% of monokaryons can be compatible with each other, depending on the specific genetics. For many Basidiomycota, the maintenance of dikaryotic status is aided by the formation of clamp connections. These structures help coordinate and re-establish pairs of compatible nuclei after synchronous mitotic nuclear divisions. Multiple frequent variations to this pattern exist. In a typical Basidiomycota life cycle, long-lived dikaryons periodically (either seasonally or occasionally) produce basidia. In these specialized, usually club-shaped end cells, a pair of compatible nuclei fuse (karyogamy) to form a diploid cell. Meiosis follows quickly, producing 4 haploid nuclei that migrate into 4 external, usually apical basidiospores. Variations on this step also occur. Typically, basidiospores are ballistic, so they are sometimes also called ballistospores. In most species, basidiospores disperse, and each can grow into a new haploid mycelium to continue the life cycle. Basidia are microscopic, but they are often produced on or in large multi-celled fruiting structures called basidiocarps, basidiomes, or fruitbodies, which are commonly known as mushrooms, puffballs, etc. Ballistic basidiospores form on sterigmata, which are tapered spine-like projections on basidia that are typically curved like bull horns. In some Basidiomycota, spores are not ballistic. In these species, sterigmata may be straight, reduced to stubs, or absent entirely. The basidiospores of these non-ballistosporic basidia may bud off, or be released when the basidia dissolve or disintegrate. To summarize the life cycle: meiosis takes place inside a diploid basidium. Each of the four haploid nuclei migrates into its own basidiospore. The basidiospores are ballistically discharged and grow into new haploid mycelia called monokaryons. There are no distinct males or females; instead there are compatible thalli with multiple compatibility factors. Plasmogamy between compatible individuals leads to delayed karyogamy that forms a dikaryon. The dikaryon is long-lived, but eventually grows into fruitbodies that hold basidia, or directly produces basidia without a fruitbody. The paired nuclei of the dikaryon in the basidium fuse (karyogamy occurs). The diploid basidium then starts the cycle over again.
