THE LIVING WORLD
Unit Four. The Evolution and Diversity of Life
The phylum Basidiomycota contains the most familiar of the fungi among their 22,000 named species—the mushrooms, toadstools, puffballs, and shelf fungi. Many mushrooms are used as food, but others are deadly poisonous. Some species are cultivated as crops—the button mushroom Agaricus bisporus is grown in more than 70 countries, producing a crop in 1998 with a value of over $15 billion. Also among the basidiomycetes are bread yeasts and plant pathogens including rusts and smuts. Rust infections resemble rusting metal, whereas smut infections appear black and powdery due to their spores.
The life cycle of a basidiomycete (figure 18.8a) starts with the production of a hypha from a germinating spore. These hyphae lack septa at first, just as in zygomycetes. Eventually, however, septa are formed between each of the nuclei—but as in ascomycetes, there are holes in these cell separations, allowing cytoplasm to flow freely between cells. These hyphae grow, forming complex mycelia, and when hyphae of two different mating types (+ and -) fuse, they form cells in which the nuclei remain separate—they do not fuse into one nucleus. Recall that if two distinct nuclei occur within each cell of the hypha, it is called dikaryotic, indicated by the n + n tan area of the cycle. The dikaryotic hypha that results goes on to form a dikaryotic mycelium. The mycelium forms a complex structure made of dikaryotic hyphae called the basidiocarp, or mushroom (figure 18.8b).
Figure 18.8. Life cycle of a basidiomycete.
(a) Basidiomycetes usually reproduce sexually, with the fusion of nuclei in the basidia to produce a zygote. Meiosis follows syngamy and produces basidiospores that eventually form a basidiocarp (b).
The two nuclei in each cell of a dikaryotic hypha can coexist together for a very long time without fusing. Unlike the other two fungal phyla, asexual reproduction is infrequent among the basidiomycetes, which typically reproduce sexually.
In sexual reproduction, zygotes (the only diploid cells of the life cycle) form when the two nuclei of dikaryotic cells fuse (on the right-hand side of the cycle). This occurs within a club-shaped reproductive structure called the basidium (plural, basidia). Meiosis occurs in each basidium, forming haploid spores called basidiospores. The basidia occur in a dense layer on the underside of the cap of the mushroom, where the surface is folded like an accordion. It has been estimated that a mushroom with an 8-centimeter cap can produce 40 million spores per hour!
Key Learning Outcome 18.7. Mushrooms are basidiomycetes, which form club-shaped reproductive structures called basidia.