Ascomycetes (AS-koh-MIE-seets) are distinguished by the presence of saclike compartments where sexually produced spores form. Ascomycetes, also called sac fungi, live parasitically and in various habitats, including salt water, fresh water, and land.
Sexual reproduction in the ascomycetes begins when the hyphae of two different haploid mating types form male and female haploid gametangia. The female gametangium is called an ascogonium (AS-koh-GOH-nee-uhm), and the male gametangium is called an antheridium (AN-thuhr-ID-ee-uhm), as shown in Figure 26-6. As the ascogonium and antheridium approach one another, a tube forms between them, and the nuclei from the antheridium cross the tube and enter the ascogonium. Dikaryotic hyphae grow out of the ascogonium and intertwine with the monokaryotic hyphae of the original fungi (parents) to form a visible cuplike structure called the ascocarp (AS-koh-KAHRP). Cells that contain one nucleus are called monokaryotic (mahn-oh-KAR-ee-AHT-ik). An example of an ascocarp is shown in Figure 26-7.
Within the ascocarp, sacs called asci (AS-kie) (singular, ascus) develop at the tips of the dikaryotic hyphae. Within the asci, the haploid nuclei fuse. The resulting zygotes undergo meiosis once and divide again by mitosis to form eight haploid nuclei. The nuclei form walls and become ascospores (AS-koh-SPAWRZ). When an ascospore is released and germinates, a new haploid hypha emerges.
Many yeasts, such as Saccharomyces cerevisiae, are ascomycetes. S. cerevisiae makes bread rise, ferments grapes to make wine and grain to make beer, and is an important research organism.
Fungi that do not have a sexual stage are classified in a group called fungi imperfecti, or deuteromycota. Continuing phylogenetic analyses have led some mycologists to place these fungi in the three established phyla. Most species of fungi that were formerly classified as fungi imperfecti can now be classified in the phylum Ascomycota. However, some biologists disagree with this reclassification.
Word Roots and Origins mycorrhiza from the Greek mykes, meaning "fungus," and rhiza, meaning "root"
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