Sporogenesis(redirected from Spore formation)
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(also spore formation), the formation of spores. In prokaryotes, that is, plant organisms whose cells do not have typical nuclei, spores may arise from the entire cell, which has accumulated nutrient matter and thickened its capsule (for example, the exospores of many blue-green algae), or from a protoplast that has divided into a large number of spores (for example, the endospores of certain blue-green algae). Sporogenesis in prokaryotes may also result from the thickening and contraction of the protoplast inside the cell capsule and the formation of a new layered capsule on top of the protoplast (for example, in bacteria) or from the decomposition of special areas of mycelium into segments (for example, in actinomycetes).
Eukaryotes, that is, plants having typical nuclei, are characterized by three principal types of spores (oospores, mitospores, and meiospores) that occupy different places in the developmental cycle. Hence, there may be three variants of sporogenesis—oosporogenesis, mitosporogenesis, and meiosporogen-esis, respectively. The term “sporogenesis” is usually used in reference to meiosporogenesis. Oosporogenesis is associated with fertilization and, consequently, with changes of nuclear phases in developmental cycles; it ends with the formation of oospores (in many green algae and oomycetes), auxospores (in diatoms), and zygospores (in zygomycetes), which consist of mononuclear or multinuclear zygotes. Mitosporogenesis leads to the development of mitospores, small or large numbers of which form as a result of mitotic divisions of haploid cells (for example, the zoospores of a number of algae and fungi) or, less frequently, diploid cells (for example, the carpospores of most Florideae). Mitospores sometimes form without cell division, for example, the monospores of Oedogonium, Bangiaceae, and Helminthocladiaceae.
A change in the nuclear phase does not occur in mitospores. It does occur, however, in unicellular mitosporangia (for example, in the zoosporangia of Ulothrix, the monosporangia of Oedogonium, and the cystocarps of Florideae). Unicellular algae seem to form sporangia themselves. Mitosporogenesis may accompany the decomposition of mycelium consisting of cells with dikaryons, for example, the mycelium of smut and rust fungi.
Meiosporogenesis is due to the replacement of diplophases by a haplophase in the developmental cycles of lower and higher plants. In lower plants meiospores arise during meiosis or shortly afterward from mitotically divided haploid cells formed during meiosis. In algae and fungi having a haploid cycle of development, sporogenesis involves the sprouting of the zygote (oospore), whose diploid nucleus forms four haploid nuclei by dividing meiotically. Four meiospores develop (for example, the zoospores of Chlamydomonas and the aplanospores of Ulothrix), or three out of the four haploid nuclei atrophy and only one mei-ospore is formed (for example, in Spirogyra).
Meiosis may be followed by one to three mitotic divisions, resulting in the formation of eight to 32 spores (for example, in Bangiaceae). In algae that have isomorphous or heteromorphous developmental cycles, meiosporogenesis occurs in unicellular meiosporangia and is characterized by the formation of only four meiospores (for example, the tetraspores of brown algae and most Florideae) or of 16 to 128 meiospores (for example, the zoospores of Laminariaceae) as a result of two to five mitotic divisions after meiosis. In the sporangia of ascomycetous fungi (sacs, or asci) the four haploid nuclei resulting from meiosis divide mitotically, and eight endogenous meiospores (ascospores) are formed. In basidiomycetous fungi, four haploid nuclei arise in each of the sporiferous organs, or basidia. The four nuclei move to special outgrowths on the surfaces of the basidia, and the outgrowths, known as basidiospores, subsequently separate from the basidia.
Higher plants form only meiospores; meiosporogenesis occurs in multicellular sporangia. Sporocytes (meiotically dividing cells) usually develop as a result of mitotic divisions of the diploid cells of the archespore; they each form four spores (tetrads of spores). Isosporous pteridophytes produce morphologically and physiologically identical spores, from which bisexual prothallia develop. In heterosporous pteridophytes and seed plants two types of spores develop as a result of microsporogenesis, megasporogen-esis, and meiosporogenesis.
Microsporogenesis occurs in the microsporangia and is completed by the formation of a large number of microspores, which subsequently develop into male prothallia. Megasporogenesis occurs in the megasporangia, where a small number of mega-spores—often only four or one—mature and develop into female prothallia. The developing sporocytes and spores (in most higher plants) feed on substances obtained from cells of the tapetum, that is, the layer that lines the interior of the cavity of the sporangium. In many plants the tapetal cells dissolve and form a peri-plasmodium (a protoplasmic mass with degenerating nuclei), which contains sporocytes and, later, spores. In a number of plants some of the sporocytes participate in formation of the peri-plasmodium. In the megasporangia, or ovules, of some angiosperms, meiosis results in formation of cells having two or four haploid nuclei, which correspond to two or four megaspores. Female gametophytes, in the form of bisporous and tetrasporous embryo sacs, develop from these cells.
For a discussion of sporogenesis in protozoans, see.
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A. N. SLADKOV