Life cycle
life cycle (noun, pl. life cycles) – the alternation of two generations, one haploid generation reproducing with gametes (or hyphae that function as gametes) and another dikaryotic generation reproducing by formation of ascospores. The ascospore germinates, growing into haploid hyphae forming the dominant, somatic mycelium. These hyphae represent the gametomycelium (or gametothallus), i. e., a generation of hyphae that reproduce sexually by the formation of sexual cells (gametes or entire gametangia that can merge with one another). fertilization in ascomycetes is a complex process that typically involves the fusion of entire gametangia (gametangiogamy). Fertilization in ascomycetes is also characterized by the fusion of the cytoplasm (plasmogamy) long before the fusion of the cell nuclei (karyogamy). Four different ways of sexual reproduction are currently known from ascomycetes: (1) isogamic gametangiogamy; (2) anisogamic gametangiogamy; (3) spermatogamy; and, (3) somatogamy. The gametomycelium can also reproduce asexually by the formation of somatic spores (see conidia). If sexual reproduction takes place, a second, dikaryotic generation can be distinguished. This second generation is sometimes reduced to a single, dikaryotic cell, or, more frequently develops into a dikaryotic mycelium, i. e., the ascogeneous hyphae or sporomycelium. This mycelium is generally inconspicuous and never develops into a distinct thallus. It is usually reduced to a layer of hyphae within an ascocarps that is largely built by haploid hyphae from the surrounding gametomycelium (or gametothallus). In non-lichenized ascomycetes ascocarps and thus the ascogeneous hyphae are generally short-lived. In contrast, most lichenized ascomycetes have persistent ascocarps and with every new season ascogeneous hyphae produce new asci. Within these asci karyogamy takes place, almost immediately followed by meiosis and the production of ascospores by the enveloping membrane system (= EMS).
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