Zygomycetes are common saprophytes in soil, and some are parasites on plants and animals. There are about 600 species. Their hyphae do not have cross walls, and so their cytoplasm is multinucleate.. xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />xml:namespace prefix = o />xml:namespace prefix = o />xml:namespace prefix = o />xml:namespace prefix = o />
Two common zygomycetes are Rhizopus, the black mold, and Pilobolus, the shotgun fungus. The following discussion of the life cycle of Rhizopus.
life cycle of Rhizopus
Sexual reproduction in Rhizopus occurs by means of the conjugation of two gametangia. A gametangium is a pecial structure that produces gametes. The fungus ha different mating strains that look alike. When two different mating strains are close together, they form short, club-shaped hyphae that grow toward each other until they touch. Each hypha then produces a multinucleated gametangium at its tip. The gametangia merge, their nuclei fuse in pairs, and a zygote that contains many diploid nuclei is formed. The zygote develops a thick, warty wall and is called a zygospore. A zygospore may remain dormant for several months. When conditions are favorable for growth, the diploid nuclei undergo meiosis, and the zygospore breaks open. One or more sporangiophores grow out of the zygospore and produce sporangia at their tips. The spores from these sporangia are haploid, and they start the asexual cycle over again.
Rhizopus is a common mold on moist, stale bread. Its hyphae form a white, cottony mycelium. Some hyphae penetrate the bread with their tips and formrhizoids and sporangiophores. Rhizoids are short hyphaethat anchor the fungus and produce enzymes that digest thebread. Sporangiophores are upright stalks that producepore cases at their tips. Spore cases are known as sporangia. Spores are produced in the spore cases by division of the protoplasm. Each unit of protoplasm contains a nucleus and becomes surrounded by a wall to form a spore.
One - orangium can produce about 50,000 black spores. Whenthe spores mature the sporangium breaks open and releasesthe spores. If a spore lands on a suitable surface such asanother piece of bread, it germinates and starts the cycleover again.