Division (Phylum): Basidiomycota

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            The most familiar of all fungi are members of this large sub-division.  It includes some 25,000 described species, not only the mushrooms, toadstools, stinkhorns, puffballs, and shelf fungi but also two important plant pathogens: the rusts and smuts.  The Basidiomycota are distinguished from all other fungi by the production of basidiospores, which are borne outside a club-shaped, spore-producing structure called the basidium (plural, basidia).The basidia are produced by basidiocarps, which are the fruiting bodies of the so-called higher fungi, such as mushrooms and puffballs. Basidiocarps, like the ascocarps, are the large fruiting structures, which are the most visible stage of the fungus. A typical mushroom is a familiar example of a basidiocarp.   

            1) Life Cycle of Basidiomycota

             The mycelium of the Basidiomycota is always septate and in most species passes through three distinct phases -primary, secondary, and tertiary- during the life cycle of the fungus.  When it germinates, a basidiospore produces the primary mycelium.  Initially the mycelium may be multinucleate, but septa soon form and the mycelium is divided into monokaryotic (uninucleate) cells. This septate mycelium grows by division of the terminal cell.  Branches do occur, and the mycelial mass can become very complex.  Commonly the secondary mycelium is produced by the fusion of primary mycelium from two different mating types (plasmogamy) [Figure 19].

           

The tertiary mycelium, which is also dikaryotic, arises directly from the secondary mycelium, and forms the basidiocarp. The spore forming basidia are produced by the terminal cell on millions of dikaryotic hyphae. In a typical mushroom, basidia are found on gills, under the cup (Figure 20A). Karyogamy occurs between the two haploid nuclei within a developing basidium. Then, the diploid nucleus undergoes meiosis to produce four haploid nuclei.  These nuclei then migrate into four small extensions at the apical end of the basidium, and are walled off to form the four basidiospores.

Figure 19: Life Cycle of Typical Basidiomycota.

2) Examples of Basidiomycota

            a) Gilled Mushrooms.

            The basidiocarps of this group are large and conspicuous.  They are the familiar mushrooms and toadstools.  The vegetative portion of the fungus exists as a mycelial network, which grows saprobically beneath the substrate, often as mycorrhizae with trees.  The basidia are borne in a layer on the surface of "gills" which, in turn, are produced on the underside of fleshy umbrella-like basidiocarps.  The basidiospores are forcibly ejected form the basidium. The basidiocarp consists of a stout stalk (stipe) bearing a circular cap (pileus) from which the lamellae (gills) hang down (Figure 20).  Most members of this order are saprobic but some are tree parasites.  It should be recognized that in the Agricales, the fruiting body (basidiocarp) is an ephemeral structure usually lasting only a few days, whereas the mycelium, living on organic matter in the soil, may last for years.