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Moving through the phylogenetic tree, we next encounter several groups traditionally united as the Zygomycota, or zygomycetes (see Fig. 34.15). These groups share a number of traits.

Collectively, zygomycetes make up less than 1% of the known fungal species. Some are decomposers, specializing on dead leaves, animal feces, and food. There are probably zygomycetes in your kitchen. Others live on and in plants, animals, and even other fungi. Zygomycetes have many typical fungal traits, including the growth of a mycelium and the production of aerial spores. These traits may have evolved as adaptations for finding food and dispersing on land. The loss of flagellated spores may also reflect adaptation for life on land. Unlike more complex fungi, zygomycetes do not form regular septa along their hyphae, nor do they produce multicellular fruiting bodies.

Earlier, we introduced the black bread mold Rhizopus, which is a zygomycete (see Fig. 34.10b). Rhizopus and its relatives are specialists on substrates containing abundant, easy-to-digest carbon compounds, such as bread, ripe fruits, and the dung of herbivorous animals. These fungi consume their substrates rapidly, and once their meal is finished, they release large numbers of aerial spores to locate another food source.

Sexual reproduction occurs when two compatible hyphal tips fuse to form a thick-walled structure containing many nuclei of each mating type. Karyogamy and meiosis are followed by germination of the haploid cell to form an elevated stalk. Each stalk develops a sporangium that contains spores produced asexually by mitotic cell division. If you look closely at a bread mold, you can see the white filamentous hyphae and small black sporangia on slender stalks. Each sporangium can produce as many as 100,000 spores that are dispersed by the wind, explaining how these organisms seem to get everywhere.

Although zygomycete fungi do not form multicellular fruiting bodies, some have evolved truly spectacular means of dispersal. Pilobilus, which consumes the dung of herbivorous animals, has a life cycle similar to Rhizopus except that, instead of releasing individual spores, it forcibly ejects the entire sporangium (Fig. 34.17). Turgor pressure generated in the supporting stalk propels the sporangia as far as 2 m. Light-sensitive pigments in the stalk’s hyphae control the orientation of this water cannon, ensuring that the spores have the best chance of escaping the dung pile and landing on vegetation that is attractive to grazing herbivores. Feeding herbivores disperse the spores further, while supplying them with fresh dung for food. The remarkable dispersal of Pilobilus sporangia benefits other species, as well: Parasitic nematode worms hitch a ride on the sporangia to find a new host.