ASEXUAL REPRODUCTION

From Big Medical Encyclopedia

ASEXUAL REPRODUCTION — the reproduction which is carried out without participation of sex cells and characterized by existence of the only parent form. The B. river

is the cornerstone cell division (see). Thus, it represents one of forms of a tsitogoniya, i.e. reproduction by means of separate cells. In the evolutionary plan it is the most ancient form of reproduction, on a basis the cut developed probably and sexual process. The B. river in broad understanding includes as well the vegetative reproduction which is characterized by the fact that the multicellular complexes which are quite often possessing a difficult fabric differentiation give to a new organism rise. Vegetative reproduction (see) always secondary phenomenon which is independently arising in different groups of plants and animals.

Asexual reproduction: 1 — consecutive stages (and — d) binary fissions (monotomiya) of an amoeba; 2 — consecutive stages (about — in) binary fissions (monotomiya) of a flagellate of Menoidium incurvum; 3 — reproduction of an infusorian of Ichthyophthirius multifiliis by a palintomy: and — grown trofont at the time of its escaping of leather of fish — palintomichesky divisions in a cyst, in — separate wearies (at bigger increase), formed as a result of a palintomy; 4 — reproduction of yeast cells: and — not breeding cell, biv — budding; 5 — reproduction by budding at the sucking infusorians of Suctoria: and — external budding of Ephelota gemmipara — internal budding of Trichophrya salparum; 6 — consecutive stages of multiple fission (schizogony) of a koktsidiya of Karyolysus lacertae: and — the beginning of the first nuclear fission — a schizont with several kernels, in — the schizont which broke up to one-nuclear merozoites; 7 — formation of zoospores at a filamentous alga of Ulothrix zonata: and — vegetative cells — the formed zoospores, in — the svobodnopodvizhny zoospore leaving thread; 8 — sporogenesis at a fern of Dryopteris filix mas: and — group of the sporangium covered with the general lid (sorus) — the separate opened sporangium with the getting enough sleep disputes; 9 — vegetative reproduction of a fresh-water hydra of Hydra oligactis by budding; 10 — vegetative reproduction of the ciliary Microstomum lineare worm by transverse fission.

At protozoa different forms B. of river are observed. Binary fission (fig., 1 and 2), or a monotomiya is eurysynusic. At the same time process of cellular division and process of reproduction of an organism match. Such form B. of river is observed at amoebas, flagellates, infusorians. At first the kernel, then cytoplasm shares. Daughter cells recover missing organellas (flagellums, circumoral eyelashes, etc.). At elaborate protozoa, such as infusorians, process of division often is followed by partial dedifferentiation (loss of the oral device, specialized parts of the ciliary device, etc.) and the subsequent their development in the course of ontogenesis in an affiliated individual. Contrary to a monotomiya at a palintomy there are several binary fissions which are quickly following one after another without intermediate stages of food and growth. As a result small, often dedifferentiation cells (tomita) which only as a result of the long growth and a differentiation reach the sizes and a structure of a maternal individual turn out.

The palintomy is usually preceded by quite long stage of hypertrophic growth and suralimentation, edges often proceeds in an in-tsistirovanny state. Looking alike crushing of egg at metazoans has palintomies. This form of reproduction is eurysynusic at flagellates, and also at some free living (Ophryoglena) and parasitic infusorians. Ichthyophthirius multifiliis (fig., 3) — the infusorian parasitizing in cover tissues of fishes can be an example. Vegetative phases as a result of growth reach 0,5 mm. Having completed growth, the infusorian separates from the owner and intsistirutsya. In a cyst as a result of consecutive palintomichesky divisions about 2000 small tomit are formed, on each of which cilia develop. After escaping of a cyst of a tomita a nek-swarm time freely float, and then actively are implemented into leather of fishes where enter a propagation step.

Incomplete monotomichesky or palintomichesky division leads to formation of colonies that is especially often observed at flagellates (Synura, Eudorina, Volvox, etc.).

Other form B. of river is budding, napr, at yeast cells and the sucking infusorians — Suctoria (fig., 4 and 5). It differs from division in the fact that its products are not equal. Budding can be outside and internal. In the latter case kidneys are formed in a body of an infusorian in the so-called brood case. The special form of internal budding called an endodiogeniya is found at toksoplazm.

It is eurysynusic among protozoa (at Rhizopoda — foraminifera and at the majority of Sporozoa — koktsidiya, blood Sporozoa, including at the causative agent of malaria) multiple fission, or a schizogony. At a schizogony there are several consecutive nuclear fissions; after that cytoplasm breaks up to one-celled educations — merozoites. Their number corresponds to number of kernels (fig., c).

At many protozoa natural connection (alternation) between a sexless schizogony and sexual process is established. Especially clearly it is expressed at Sporozoa, including at koktsidiya and a malarial plasmodium.

Forms B. of river at sporous plants are various. Different groups of seaweed form the mobile, supplied with flagellums zoospores. They usually result from palintomichesky consecutive divisions of vegetative cells (fig., 7). At some seaweed, and also at the majority of mushrooms disputes are not mobile. At liverworts, mosses, ferns, club mosses, horsetails formation the dispute happens in special bodies — a sporangium (fig., 8). The last are located or on a top of a stalk (at mosses, horsetails), or on a lower surface of leaves. At liverworts, mosses, pteridophytes at development the dispute takes place meiosis (see) and reduction of chromosome number. The B. river of the majority of sporous plants is characterized by alternation of generations. Disputes possess a haploid set of chromosomes (see. Chromosomal complement ), and the generation (gametogophyte) developing from them also gaploidno. On a gametogophyte in special bodies haploid sex cells form (at the same time meiosis does not happen). From the diploid zygote which is formed as a result of fertilization the diploid generation — a sporophyte develops, on Krom further there will be a development a dispute.

At flowering plants educations, homologous to disputes, are the pollen developing in boots of stamens (microdisputes), and mother cells of a germinal bag in ovaries (macrospores). Evolution of geophytes went in the direction of gradual dominance of a sporophyte over a gametogophyte.

At metazoans of a sexless tsitogoniya does not exist. Vegetative reproduction of metazoans is observed preferential at the lowest types — sponges, coelenterates, flat worms, the few annlides, pearlworts. At coelenterates, e.g., vegetative reproduction is carried out by budding (fig., 9). Both cellular layers — an ectoderm and an entoderm participate in formation of a kidney. At some types, napr, at a fresh-water hydra, a kidney, separating from a maternal organism, give rise to a new individual. At the majority of sea gidroidny and coral polyps of a kidney do not separate from a maternal individual and thus there are colonies. On colony there are special kidneys from which not polyps, but freely floating generation — gidroidny jellyfishes develop. At the last reproductive products are formed, and in sea water there is a fertilization; from an oosperm the polyp breeding in the vegetative way develops. Thus there is an alternation of vegetatively breeding and sexual generations called metagenesis (see). Unlike alternation of generations and forms of reproduction, at plants at a metagenesis all cycle is carried out in a diploid phase and the reduction occurs only at formation of sex cells at jellyfishes; only gametes are haploid here, and the diploid phase is recovered at fertilization. At sponges and pearlworts the special form of internal budding leading to formation of nights (at sponges — gemmules, at pearlworts — statoblasts) with a complex system of the covers protecting them from unfavorable conditions is observed.

At some ciliary and annlides vegetative reproduction is carried out by transverse fission of the whole organisms (fig., 10).

Among chordates vegetative reproduction is observed only at some for the second time the simplified forms of a subtype of tunicates (at astsidiya and salp) at which it can take irregular shapes. E.g., bochonochnik (Cyclomyaria) belonging to the class salp on a belly hundred-bosom of sexless individuals have a formation of kidneys. Then these kidneys are taken special amoeboid cells (phorocytes) and transferred on back hundred-bosoms where there is their development. A part them turns further into puberal individuals.

Vegetative B. of river at plants, especially at floral is eurysynusic. Formation of affiliated individuals can come at them from different bodies — roots, stalks, leaves. Many plants form escapes from roots (raspberry, an acacia, etc.). At a lily family reproduction is carried out by means of bulbs. Some plants breed leaves on which kidneys are formed (a fern of Asplenium, a begonia).

See also Reproduction .


Bibliography: Asexual reproduction, a somatic embryogenesis and regeneration, under the editorship of B. P. Tokin, L., 1972; Gartman M. The general biology, the lane with it., M. — L., 1936; Dogel V. A., Polyansky Yu. I. both X e y with and N of E. M. The general protozoology, M. — L., 1962, bibliogr.; Za-hvatkin A. A. Comparative embryology of the lowest invertebrates, M., 1949; T about to and B. I N. General embryology, M., 1970, bibliogr.

Yu. I. Polyansky.

Яндекс.Метрика