PARTHENOGENESIS

From Big Medical Encyclopedia

PARTHENOGENESIS (parthenogenesis; grech, parthenos the girl + genesis origin) — a form of a syngenesis, at Krom development of an organism comes from a female sex cell without fertilization by its men's sex cell. The item should distinguish from asexual reproduction when development comes not from sex cells, and from somatic cells or bodies (see. Reproduction ).

The item — a natural normal way of reproduction of nek-ry animal species and plants. Full natural P. occurs at backboneless animal all types, but most often — arthropods. From vertebrate animals species of fish, amphibians, reptiles, separate bird species (turkeys) breeding parthenogenetic are known. At mammals natural P. is not known. Ways of reproduction, close to P., at plants are the apogamy and an apomixis.

The item can be obligate when eggs are capable only to parthenogenetic development, and optional, at Krom of egg can develop or parthenogenetic, or as a result of fertilization. So, at a number of hymenopterous insects (e.g., at bees) from unfertilized eggs, i.e. parthenogenetic, males (drones), and from impregnated — females develop (a uterus and working bees).

At cyclic P. bisexual generations alternate with parthenogenetic. So, at nek-ry species of plant louses bisexual generations (winged forms) are replaced parthenogenetic (wingless females), at the same time different generations use different types of forage plants.

At seasonal P. bisexual and parthenogenetic generations are dated for different seasons of year. E.g., at fresh-water crustaceans of water fleas summer generations entirely consist from parthenogenetic the breeding females that provides higher reproductive rate due to increase in populations of number of the individuals capable to leave posterity. There are generations of males and females in the fall, to-rye leave for the winter oosperms.

Parthenogenetic can develop or an ovum, last meiosis (see) and containing a haploid set of chromosomes (meiotic, or haploid, P.), or an ovum of one of premeyotichesky stages oogenesis (see) with preservation of chromosomal complement inherent to this look — diploid or polyploid (ameyotichesky P.). At nek-ry forms P. merge of a haploid kernel of an ovum to a haploid kernel of a napravitelny little body leads to recovery of diploidy (automiktichesky P.). The genotype, floor of parthenogenetic posterity, and also preservation or loss of heterozygosity, acquisition of homozygosity and so forth depend on these features of P.

For the first time artificial P. A. A. Tikhomirov in 1886 by impact on unfertilized eggs of a silkworm various reproduced physical. - chemical irritants. The circle of the objects and irritants causing P. was considerably expanded by Zh. Loeb. The theory and artificial P.'s practice is developed by the Soviet geneticist academician B. L. Astaurov and his pupils on the basis of what artificial receiving purely samtsovy parthenogenetic clones of a silkworm which are giving vent raised quality silk fiber widely practices.

At nek-ry breeds of turkeys natural tendency to P. is brought by means of artificial selection to regular receiving same-sex parthenogenetic posterity (males with big, than at females, live weight). The item is used also in selection of plants.

In experimental embryology artificial P. is widely used for a research of processes gametogenesis (see) and fertilisations (see) at the most different types of animals.

Carry to a parthenogenesis also an androgenesis and a gynogenesis, at to-rykh there is an activation of an ovum a spermatozoon of the or close look (kernels of an ovum and a spermiya do not merge). Fertilization in this case is false, and the germ develops only with men's (androgenesis) or only with a female (gynogenesis) kernel.



Bibliography: Astaurov B. L. Parthenogenesis, androgenesis and polyploidy, M., 1977.


V. I. Ivanov.

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