Free cell formation
free-cell formation (noun, ascospore term, spore morphology term, no plural) – ascospore formation, ascosporogenesis; a unique process of new cell formation found only in ascomycetes; characterized by the separation of portions of ascoplasm and nuclei by an enveloping membrane system (= EMS). First karyogamy of two haploid nuclei takes place in the cytoplasm of the immature ascus. A diploid nucleus is thus formed. This nucleus is short-lived and almost immediately undergoes meiosis, forming four haploid nuclei. These four haploid nuclei are not immediately separated into new cells, but instead divide again by mitosis. New cells are only formed at the last stage of the formation of new nuclei. Unlike in most other organisms, both meiosis and mitosis therefore do not immediately result in the segregations of nuclei into new cells. Instead, the new cells form at a much later stage by a cleaving process from the enveloping membrane system. During this process portions of the cytoplasm containing nuclei are separated from ascoplasm that remains outside the newly formed cells. The typically eight nuclei in the ascoplasm are thus segregated into eight vesicles enclosed by two membranes. With the accumulation of new wall material in the inter-membrane space of these vesicles typically eight ascospores are formed. In some ascomycetes these nuclei undergo several additional mitoses. If several mitotic divisions take place before the formation of separate ascus vesicles, more than eight ascospores will be formed, typically 16, 32 or any other multiple of eight. If additional mitoses take place within the ascus vesicle the resulting ascospores will become multi-nucleate. Multi nucleate spores typically form septa that segregate several spore cells. Some ascomycetes have asci with less than eight ascospores, sometimes only four, two or even only one single spore. It can be assumed that in these species free cell formation follows an unusual pattern by which not all of the nuclei are separated into individual ascospores.
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