![]() For example, the metabolic products of some species of Rhizopus are intermediates in the synthesis of semi-synthetic steroid hormones. Zygomycetes play a considerable commercial role. Most species are saprobes, living off decaying organic material a few are parasites, particularly of insects. They include the familiar bread mold, Rhizopus stolonifer, which rapidly propagates on the surfaces of breads, fruits, and vegetables. The zygomycetes are a relatively small group of fungi belonging to the Phylum Zygomycota. Allomyces produces diploid or haploid flagellated zoospores in a sporangium. Its reproductive cycle includes both asexual and sexual phases. ![]() The chytrid species Allomyces is well characterized as an experimental organism. Some species thrive as parasites on plants, insects, or amphibians (Figure 2), while others are saprobes. Chytrids usually live in aquatic environments, although some species live on land. The ecological habitat and cell structure of chytrids have much in common with protists. An unusual feature of the chytrids is that both male and female gametes are flagellated. They produce both gametes and diploid zoospores that swim with the help of a single flagellum. The Chytrids are the only fungi that have retained flagella. Most chytrids are unicellular however, a few form multicellular organisms and hyphae, which have no septa between cells (coenocytic). Like all fungi, chytrids have chitin in their cell walls, but one group of chytrids has both cellulose and chitin in the cell wall. The evolutionary record shows that the first recognizable chytrids appeared during the late pre-Cambrian period, more than 500 million years ago. The chytrids are the simplest and most primitive Eumycota, or true fungi. The only class in the Phylum Chytridiomycota is the Chytridiomycetes. The five true phyla of fungi are the Chytridiomycota (Chytrids), the Zygomycota (conjugated fungi), the Ascomycota (sac fungi), the Basidiomycota (club fungi) and the recently described Phylum Glomeromycota (Figure 1). Rapid advances in molecular biology and the sequencing of 18S rRNA (ribosomal RNA) continue to show new and different relationships among the various categories of fungi. ![]() However, most mycologists have discontinued this practice. Polyphyletic, unrelated fungi that reproduce without a sexual cycle, were once placed for convenience in a sixth group, the Deuteromycota, called a “form phylum,” because superficially they appeared to be similar. The kingdom Fungi contains five major phyla that were established according to their mode of sexual reproduction or using molecular data.
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