Gynogenesis


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gynogenesis

[¦gīn·ō′jen·ə·səs]
(embryology)
Development of a fertilized egg through the action of the egg nucleus, without participation of the sperm nucleus.

Gynogenesis

 

a special form of reproduction and development in which, after penetration of the egg cell by the sperm, their nuclei do not unite, and only the nucleus of the egg cell participates in subsequent development. Inasmuch as an important part of the sexual process—the union of genetic material of the parents by the merging of the nuclei of their sex cells—is absent in gynogenesis, fertilization here is false. The role of the spermatozoa is limited to activating development of the inseminated ovum. Therefore, gynogenesis is properly regarded as a peculiar form of virgin reproduction, or parthenogenesis. An analogue, but also the antithesis of gynogenesis, is androgenesis. In old biological literature gynogenesis is called merospermia, stressing the partial participation of the sperm in the process of pseudofertilization and development.

As a normal means of reproduction gynogenesis was until recently known only in a few roundworms (the nematode Rhabditis aberrans) and in two or three cases in fish—Mollienesia formosa, a viviparous fish of the family Cyprinodontidae; the silver crucian (Carassius auratus var. gibelio); and, without complete verification, in the Baikal escolar (Comephorus baicalensis). In 1964 gynogenesis was discovered in amphibians; it proved common in two species of North American Ambystoma. Gynogenesis was also discovered in one species of higher plants of the family Amaryllidaceae (Atamosco mexicana).

With gynogenesis in fishes the offspring consists only of females, and the roe is inseminated by sperm of closely related species. Thus, roe of the silver crucian may be inseminated by the sperm of the carp, the roach, the crucian carp, the goldfish, tench, loach, or other cyprinoid fish. However, the offspring does not bear the slightest resemblance to the inseminating species. Rather, it resembles precisely the maternal form of the silver crucian (matroclinous inheritance). Two gynogenetic, purely female races of AmbystomaA. tremblayi and A. platinerum—inhabit the areas of distribution of two bisexual species (A. laterale and A. jeffersonianum), from which they are descended. The two gynogenetic species produce by means of pseudofertilization by the males of the bisexual species. The biological advantage of polyploidy and the ease of preserving it in parthenogenetic forms, including those of gynogenesis, explain why gynogenetic races of the silver crucian and Ambystoma have proved to be triploid. They have three sets of chromosomes instead of the usual two.

Gynogenesis may be produced experimentally by disturbance of the process of fertilization with thermoshock (heat or cold), centrifugation, or suppression of sperm with radium rays, X rays, and chemical agents. It may also be obtained by artificial crossbreeding of species so distant from each other that genuine fertilization and hybridization are impossible. One of the most reliable and simple methods of obtaining gynogenesis experimentally is radiation gynogenesis—that is, insemination of the egg cell with sperm that has been severely damaged by penetrating radiation. In studying the development of frog eggs inseminated by irradiated sperm, a paradoxical phenomenon is observed: at first, with increasing dosage of radiation, the development of the ova is retarded; with further increases in radiation dosage, its harmful effect suddenly decreases sharply. This phenomenon (Hertwig’s effect) is explained by the fact that development of the embryo now progresses by means of gynogenesis, since severely irradiated spermatozoa prove to be incapable of genuine fertilization, although they activate the egg cell. Experimentally produced gynogenesis ends for the most part with early death of the embryo. Inasmuch as the nuclei of the egg cells and sperm that contain haploid (single) sets of chromosomes do not unite, the embryo does not receive a diploid (double) set of chromosomes, but only half the normal number, which does not ensure normal development. In order to obtain adult gynogenetic organisms experimentally, it is necessary to find a means of restoring the diploid number of chromosomes or producing a polyploid number of them. Diploid gynogenesis has thus far been produced only in the mulberry silkworm and in a few fish. Triploid gynogenesis has been produced in the axolotl and other caudate amphibians.

Gynogenesis has great general biological interest from the point of view of elucidating the role of the nuclei and the cytoplasm of sex cells in fertilization, development, and heredity. Experimental gynogenesis is also interesting as a means of artificial regulation of development and sex, since in a number of instances it guarantees production of offspring of a single, definite—usually female—sex.

REFERENCES

Wilson, E. B. Kletka i ee rol’ v razvitii i nasledstvennosti, vol. 1. Moscow-Leningrad, 1936. (Translated from English.)
Astaurov, B. L. “Problema reguliatsii pola.” In Nauka i chelovechestvo, vol. 2. Moscow, 1963.
Astaurov, B. L. “Genetika pola.” In Aktual’nye voprosy sovremennoi genetiki. Moscow, 1966.
Romashov, D. D., and V. N. Beliaeva. “Povyshenie vykhoda diploidnykh ginogennykh lichinok u v’iuna (Misgurnis fossilia L.) primeneniem temperaturnykh shokov.” Biul. Moskovskogo ob-va ispytatelei prirody: Old. biol, 1965, vol. 70, no. 5.
MacGregor, H. C., and T. M. Uzzell. “Gynogenesis in Salamanders Related to Ambystoma jeffersonianum.” Science, 1964, vol. 143, no. 3,610.

B. L. ASTAUROV

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