Unit Four. The Evolution and Diversity of Life
The zygomycetes, members of the phylum Zygomycota, are unique among the fungi in that the fusion of hyphae does not produce a heterokaryon (a cell with two haploid nuclei). Instead, the two nuclei fuse and form a single diploid nucleus. Just as the fusion of sperm and egg produces a zygote in plants and animals, so this fusion produces a zygote. The name zygomycetes means “fungi that make zygotes.”
Zygomycetes are the exception to the rule among fungi, and there are not many different kinds of them—only about 1,050 named species (about 1% of the named fungi). Included among them are some of the most frequent bread molds (the so-called black molds) and many microscopic fungi found on decaying organic material including strawberries and other fruits. Another important group of zygomycetes is called the Glomales. These soil-borne fungi form symbiotic relationships with roots of terrestrial plants and may have aided in the evolution of terrestrial plants, enhancing the uptake of minerals and water from the soil.
Reproduction among the zygomycetes is typically asexual. A cell at the tip of a hypha becomes walled off by a complete septum, forming an erect stalk tipped by a sporangium within which haploid spores are produced. These are the lollipop-shaped structures you see in the life cycle illustrated in figure 18.6. Their spores are shed into the wind and blown to new locations, where the spores germinate and attempt to start new mycelia. Sexual reproduction is unusual but may occur in times of stress. It is shown in the lower part of the figure, where hyphae from two different mating strains (+ is green and - is red) fuse and their nuclei also fuse, forming a diploid zygote. At the point where the two hyphae fuse, a sturdy and resistant structure called a zygosporangium forms. The zygosporangium is a very effective survival mechanism, a resting structure that allows the organism to remain dormant for long periods of time when conditions are not favorable. When conditions improve, the zygosporangium forms a stalked structure topped with a sporangium. Meiosis occurs within the sporangia and haploid spores are released, just as in the asexual portion of the life cycle.
Figure 18.6. Life cycle of a zygomycete.
(a) In the life cycle of Rhizopus, a zygomycete that grows on moist bread and other similar substrates, the hyphae grow over the surface of the bread or other material on which the fungus feeds, producing erect, sporangium-bearing stalks in clumps, also shown in (b). If two hyphae grow together, their nuclei may fuse, producing a zygote. This zygote, which is the only diploid cell of the life cycle, acquires a thick, black coat (colored purple in the diagram above) and is then called a zygosporangium. Meiosis occurs during its germination, and normal, haploid hyphae grow from the haploid spores that result from this process.
Key Learning Outcome 18.5. Zygomycetes are unusual fungi that typically reproduce asexually; when hyphae do fuse, a zygote (one 2n nucleus), rather than a heterokaryon (two haploid nuclei), is produced.