The generalized life cycle described and illustrated on the Basidiomycota page applies to Agaricomycetes. These fungi are almost always filamentous, without yeast phases. However, yeasts are produced by some Agaricomycetes that are cultivated in underground fungus gardens by attine "leaf-cutter" ants in the neotropics (Mueller et al. 1998). In filamentous forms the hyphae are divided by septa, which are penetrated by a barrel-shaped dolipore that is flanked by membrane-bound parenthesomes (see the Agaricomycotina page for an illustration of septal structure). The dominant stage in the life cycle is typically a dikaryotic mycelium (binucleate, with two haploid nuclei derived from mating), but stable diploids have been reported in Armillaria (Anderson 1983). Dikaryotic and monokaryotic mycelia have been shown to produce asexual spores in some species (Miller 1971; Nakasone 1990), but asexual forms are apparently not as widespread as in the Ascomycota.
Mushrooms are multicellular fruiting bodies produced under favorable conditions by dikaryotic mycelia. Sometimes, mushrooms are produced at the periphery of a circular mycelium, resulting in a fairy ring (Fig. 2). Fairy rings indicate the spatial extent of the mycelium, which is otherwise difficult to establish.
Figure 2. Fairy ring. © Darrell D. Hensley. From the University of Tennessee Turfgrass Diseases page; used with permission.
Nuclear fusion (karyogamy) and meiosis occur in basidia, which are formed in a layer of cells called the hymenium. In addition to basidia, the hymenium often includes specialized non-reproductive cells called cystidia. Cystidia and other anatomical features of mushrooms provide many characters used in Agaricomycete taxonomy (Clémençon 1997; Donk 1964; Singer 1986).
Most groups of Agaricomycetes produce undivided basidia, called homobasidia (Fig. 3), but other members of the Agaricomycetes produce basidia that are divided by septa, including the Auriculariales, Sebacinales, and certain members of the Cantharellales (Tulasnella). Most of the fungi now classified in the Agaricomycetes used to be placed in the "Homobasidiomycetes" (Hibbett and Thorn 2001), but that name has been abandoned in recognition of the fact that not all members of the group have homobasidia.
Two to eight basidiospores (meiospores) are formed on each basidium—the most common number of spores is four. Most textbook illustrations of spore production in Agaricomycetes show four meiotic products migrating into four spores, leaving behind an empty basidium. The actual events of basidiosporogenesis are more complicated, however (Fig. 3). Basidiosporogenesis usually involves a post-meiotic mitosis, yielding eight haploid nuclei (as in Ascomycota). The post-meiotic mitosis may occur in the basidia, spores, or sterigmata (the stalks on which the spores are produced). If it occurs in the spores, then there may or may not be a back-migration of the "extra" nuclei into the basidia. Thus, there are diverse patterns of nuclear behavior leading to the production of spores. Variations in these patterns might provide clues to phylogenetic relationships in Agaricomycetes, but they have been studied in too few species to make broad generalizations (Hibbett et al 1994a, Mueller and Ammirati 1993).
Figure 3. Basidiosporogenesis. A: Pre-fusion dikaryotic stage. B-D: Post-fusion diploid nucleus and meiosis. E-G: Spore development and nuclear migration into spores. H: Post-meiotic mitosis. I: Back-migration of nuclei into the basidium. © Mycological Society of America. From Hasebe et al. (1991); used with permission.
Although much attention in Agaricomycetes has been focused on fruiting bodies, the mycelium also forms other specialized multicellular structures. Examples include rootlike rhizomorphs, which enable Agaricomycetes to forage along the forest floor (Fig. 4), and sclerotia, which are resting structures from which fruiting bodies may be formed.
Figure 4. Rhizomorphs of Armillaria in culture (left; © James B. Anderson; from the Mycological Society of America Slide Collection) and in nature (right; © American Phytopathological Society; from the APS Education Center Illustrated Glossary). Images used with permission.