spore(redirected from Asexual spore)
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spore,term applied both to a resistant or resting stage occurring among various unicellular organisms (especially bacteria) and to an asexual reproductive cellcell,
in biology, the unit of structure and function of which all plants and animals are composed. The cell is the smallest unit in the living organism that is capable of integrating the essential life processes. There are many unicellular organisms, e.g.
..... Click the link for more information. produced by many unicellular plants and animals and by all plants that undergo an alternation of generations. A spore is typically a cell surrounded by a cell wall; in resistant spores and in the resting stage of reproductive spores this wall becomes tough and waterproof, permitting the cell to survive unfavorable circumstances such as extremes of temperature and moisture. Upon germination, spores that were generated asexually may produce cells or multicellular forms that can engage in sexual reproductionreproduction,
capacity of all living systems to give rise to new systems similar to themselves. The term reproduction may refer to this power of self-duplication of a single cell or a multicellular animal or plant organism.
..... Click the link for more information. . Many unicellular plants and animals reproduce both by the formation of spores and by simple cell division (mitosismitosis
, process of nuclear division in a living cell by which the carriers of hereditary information, or the chromosomes, are exactly replicated and the two copies distributed to identical daughter nuclei.
..... Click the link for more information. ). Yeasts, for instance, reproduce by forming spores as well as by budding. Among the fungi some spores are thin-walled and germinate quickly; others are thick-walled resistant types. In multicellular plants the sporophyte generation produces (by meiosismeiosis
, process of nuclear division in a living cell by which the number of chromosomes is reduced to half the original number. Meiosis occurs only in the process of gametogenesis, i.e., when the gametes, or sex cells (ovum and sperm), are being formed.
..... Click the link for more information. ) spores with half the normal number of chromosomeschromosome
, structural carrier of hereditary characteristics, found in the nucleus of every cell and so named for its readiness to absorb dyes. The term chromosome
..... Click the link for more information. for the species; these grow directly into the gametophyte generation, which produces (by mitosis) male and female reproductive cells that when united give rise to a sporophyte.
(1) In lower and higher plants, a microscopic rudiment of varying origin that facilitates reproduction and/or survival under unfavorable conditions. Spores are unicellular or, less commonly, bicellular or multicellular structures. They are usually somewhat spherical or ellipsoidal in shape; cylindrical or other forms occur occasionally. The spores of many plants have tough, frequently sculptured, sheaths of complex structure; such spores usually retain their germinating power a long time, because their protoplasts contain reserve nutrients.
The spores of lower plants are often given names that reflect their structural characteristics (actively motile flagellated zoospores and nonflagellated aplanospores), their shape (stylo-spores), or their capacity to grow (auxospores). Their names may denote the presence of a thick sheath (chlamydospores), the type of fruiting body in which the spores are produced (sporangio-spores, carpospores, ascospores, basidiospores, and aecio-spores), origin in a spore case or on a sporogenous organ (endo-spores and exospores), the number of spores in a single sporangium (monospores and tetraspores), the type of sexual process resulting in sporogenesis (oospores and zygospores), or the category of plants to which the spores belong (smut spores and urediospores).
Spores can be divided into three groups according to mode of origin and place in the developmental cycle of the plant: (1) diploid zygotes, (2) mitospores, and (3) haploid meiospores. Diploid zygotes result from the fusion of gametes (oospores of many green algae and oomycetes, auxospores of diatoms), whereas synzygotes, containing many diploid nuclei, arise from the fusion of multinuclear gametangia (zygospores of zygomycetes, oospores of some oomycetes). Mitospores, whose formation is not directly preceded by meiosis, usually have haploid nuclei or contain dikaryons. Haploid meiospores form during meiosis or shortly after meiosis, when the cells or nuclei resulting from meiosis divide. Examples of haploid meiospores are the zoospores and aplanospores of green algae having a haplontic cycle of development and of some archimycetes and oomycetes arising during the growth of zygotes. Other examples are the carpospores of bangiaceous and some floridean algae; the zoospores and nonflagellated tetraspores of green, brown, and red algae with isomorphic and heteromorphic cycles of development; the nonflagellated spores of myxomycetes; the spores in the germinal sporangia of mucoraceous fungi; and the ascospores and basidiospores of ascomycetous and basidiomycetous fungi.
Higher plants form only meiospores that develop in tetrads from archesporial cells in the sporangia. The spores develop into gametophytes, which occupy a dominant place in the developmental cycle of bryophytes but a subordinate position (as prothallia) in the developmental cycle of all other higher plants. The spores of bryophytes and pteridophytes separate from the sporangia and are dispersed by air or water currents. Isosporous pteridophytes produce morphologically and physiologically identical spores, and the gametophytes formed from them are bisexual. Heterosporous plants, which include some pteridophytes and all seed plants, produce small microspores and large mega spores. The microspores give rise to male prothallia, and the megaspores to female prothallia. Reduction of the prothallia, especially the male ones, is characteristic of all heterosporous plants, and the development of gametophytes in sporangia (the spores do not disperse) is typical of all seed plants. In seed plants male prothallia—pollen grains—develop from microspores in the microsporangia, and female prothallia develop from microspores in the megasporangia. Male prothallia then leave microsporangia, and female prothallia—the primary endosperm in gymnosperms and the embryo sac in angiosperms—do not leave the megasporangia.
A. N. SLADKOV
(2) In parasitic protozoans (sporozoans other than haemospori-dians and cnidosporidians), a unicellular or multicellular structure surrounded by a thick wall that aids in dissemination and survival under unfavorable conditions. The structure and development of spores in protozoans vary from class to class. Spores develop in sporozoans as a result of sporogony, that is, zygote development. The zygote secretes a cyst and is converted into an oocyst. Double division of the nucleus and cytoplasm of the oocyst results in the formation of four sporoblasts that form the wall and become spores. Two sporozoites (embryos) are formed in each spore. A mature oocyst with four spores is capable of infecting animals. In myxospordians (a class of cnidosporidians) the spores are multicellular and arise in the endoplasm; two spores are formed in cavitary species and several hundreds and thousands of spores in tissue species. In most species the spores have a tough bivalve capsule; sometimes the capsule has three, four, or six valves. Inside the capsule is a binucleate amoeboid embryo and two to six stinging capsules with long strands that hold the spores in place after penetrating the host’s intestine. Six cells usually participate in the formation of each spore. The spores of mi-crosporidians are unicellular and they possess a nonvalvate coat, a stinging capsule, and an amoeboid embryo.
REFERENCEZhizn’ zhivotnykh, vol. 1. Moscow, 1968.
IU. I. POLIANSKII