Ascomycetes are the largest group of fungi. The group consists of about 50,000 species, including the ascomycetes that form lichens with algae. Ascomycetes grow in soil, on rotted wood, in marine and fresh waters, and on dung. Powdery mildews are ascomycetes that cause many plant diseases such as apple mildew. xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" o />xml:namespace prefix = o />xml:namespace prefix = o />xml:namespace prefix = o />
Apothecia of some ascomycetes are brightly colored and hold many saclike asci that contain spores.
Characteristics of Ascomycetes
Many ascomycetes produce asexual spores called conidia at the ends of their hyphae. Conidia are formed either singly or in long chains, and they are produced in large numbers.
Ascomycetes also reproduce sexually. They form specialized structures that contain male and female nuclei. Male nuclei are carried to female nuclei in various ways. After the male and female structures fuse, the nuclei move into specialized hyphae that produce asci (singular, ascus) at their tips. Asci are sacs in which the sexual spores are produced. In the ascus the male and female nuclei fuse and then undergo meiosis. The result is the formation of four haploid nuclei. Each of these divides once, forming eight ascospores. Each ascospore contains one haploid nucleus.
In many ascomycetes the asci are arranged in rows. Spores of many ascomycetes are discharged explosively from the asci to a distance of xml:namespace prefix = st1 />2 centimeters (.4-.8 inch).
Yeasts are the only unicellular ascomycetes. Other ascomycetes produce hyphae that have cross walls. Each cross wall has a pore that is wide enough to let nuclei and mitochondria move from one cell to another. There are two large groups of ascomycetes: pyrenomycetes and discomycetes. Pyrenomycetes have flasklike fruiting bodies, while discomycetes have cuplike fruits.
Yeasts are ascomycetes that do not have hyphae or fruiting bodies, but they do have asci, which contain spores. Yeasts reproduce asexually by budding. Yeasts reproduce sexually by the fusion of two cells to form a zygote. The zygote either forms a diploid generation by budding or undergoes meiosis and produces haploid spores in an ascus.
There are about 350 species of yeasts. Some species cause infections in humans and animals. The best known yeast is Saccharomyces cerevisiae. Different strains of this yeast are used in baking and in making wine, beer, and cider. In bread making dried yeast is added to a dough containing flour, water, and sugar. The yeast cells carryon alcoholic fermentation and produce carbon dioxide and alcohol. The carbon dioxide becomes trapped in the dough and causes the dough to rise. The alcohol vaporizes and the yeast cells die when the dough is baked.
The morel, or sponge mushroom, is a highly prized edible fungus.
The flasklike fruiting bodies of pyrenomycetes are called perithecia (singular, perithecium). Each perithecium has a narrow opening, the ostiole, through which spores are released. One of the best-known pyrenomycetes is Neurospora crassa, the red bread mold. Neurospora is used in biochemical and genetic research because it grows quickly and undergoes mutations easily. In the laboratory it can be grown on a simple medium that includes a sugar, a few minerals, and the vitamin biotin.
Pyrenomycetes cause destructive plant diseases such as black rot of sugarcane, wilt diseases of rubber and coffee plants, apple canker, and ergot of cereals.
Most discomycetes have cupshaped or saucer-shaped fruiting bodies called apothecia (singular, apothecium). Many disease-causing fungi and lichen-forming fungi are discomycetes. There are several unusual discomycetes. One, the morel, looks like a mushroom and is prized by collectors for its flavor.Another, the truffle, produces fruiting bodies underground