Fungi

Lacking photosynthetic ability, living most often in soil but sometimes in water, the fungi are represented by almost half as many species (~105) as are the vascular plants. The distinguishing characteristics of fungi are the lack of chlorophyll and growth as a series of manybranched tubules (usually 6–8 m diameter), the hyphae, which constitute the mycelium. The hyphae are not made up of separate cells but contain a mass of protoplasm with many nuclei. Only occasional septa divide the tubules. Most fungi are saprophytic, living on decaying plants or animal tissues. However, others are parasites that produce serious and difficult-to-treat infections in humans. An important medical problem is the lack of adequate antibiotics for treating fungal infections (mycoses). On the other hand, fungi produce important antibiotics such as penicillin. Still others form some of the most powerful toxins known! The lower fungi or Phycomycetes include simple aquatic molds and mildew organisms. Higher fungi are classified as Ascomycetes or Basidiomycetes according to the manner in which the sexual spores are born. In the Ascomycetes these spores are produced in a small sac called an ascus. Each ascus contains four or eight spores in a row, a set of four representing the results of a single pair of meiotic divisions. A subsequent mitotic division will give eight spores. This is one of the features that has made Neurospora crassa a favorite subject for genetic studies. The ascospores can be dissected out in order from the ascus and cultivated separately to observe the results of crossing-over during meiosis. Neurospora also reproduces via haploid spores called conidia. The haploid mycelia exist as two mating types and conidia or mycelia from one type can fertilize cells in a special body (the protoperithecium) of the other type to form zygotes. The latter immediately undergo meiosis and mitosis to form the eight ascospores. Among other Ascomycetes are the highly prized edible truffles and morels. However, most mushrooms and puffballs are fruiting bodies of Basidiomycetes. Other Basidiomycetes include the rusts, which cause enormous damage to wheat and other grain crops. Yeasts are fungi adapted to life in an environment of high sugar content and which usually remain unicellular and reproduce by budding. Occasionally the haploid cells fuse in pairs to form diploid cells and sexual spores. Some yeasts are related to the Ascomycetes, others to Basidiomycetes. Saccharomyces cerevisiae, the organism of both baker’s and brewer’s yeast, is an Ascomycete. It can grow indefinitely in either the haploid or diploid phase. The genetics and biochemistry of this yeast have been studied extensively. The genome is relatively small with 13.5 x 106 base pairs in 17 chromosomes. The sequence of the 315,000 base pairs of chromosome III was determined in 1992101,102 and the sequence of the entire genome is now known. Fungi often grow in symbiotic association with other organisms. Of special importance are the mycorrhizae (fungus roots) formed by colonization of fine roots by beneficial soil fungi. Almost all plants of economic importance form mycorrihizae.