Sac fungi

About 30,000 species belong to the class Ascomycetes. Members of this class include the gourmet delicacies morels and truffles as well as the single-celled yeast used in making bread. Ascomycetes are called sac fungi because sexually produced spores form in an ascus, or "little sac". An ascus begins to develop when two gametes or two mating strains fuse. Nuclei divide as the hyphae grow, resulting in a row of haploid ascospores within the ascus. In sac fungi and club fungi, hyphae are divided by cross walls. Nuclei and cytoplasm flow through pores in these cross walls as the hyphae grow.

Yeasts

Yeasts are unusual sac fungi. They contain chitin and reproduce sexually by forming ascospores, but they are unicellular and do not form hyphae. Yeasts also reproduce asexually by budding. Many types of yeasts grow most rapidly in environments with high sugar content. In bread dough, yeast cells feed on carbohydrates. As yeast cells grow, they produce carbon dioxide gas by fermentation. The process of fermentation causes bread dough to rise and creates the bubbles in beer.

Parasitic Sac Fungi

The powdery mildews are among the most destructive of the parasitic sac fungi. The mycelia of these fungi form a white powder on the leaves of apples, roses, grapes, and other economically important plants. The growth of the powdery mildew destroys the tissues of the host plant. This process hinders the plant’s ability to carry out photosynthesis, and further damage results.

Dutch elm disease is caused by another sac fungus. The hyphae of the fungus grow into the wood of an elm tree and clog the tissues that carry water and nutrients from the soil up to the leaves. Dutch elm disease threatens to wipe out all American elms. Chestnut blight, caused by a related fungus, poses a similar threat to chestnut trees.

Date: 2014-12-22; view: 1002