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parthenogenesis (pärˌthənōjĕnˈəsĭs) [Gr.,=virgin birth], in biology, a form of reproduction in which the ovum develops into a new individual without fertilization. Natural parthenogenesis has been observed in many lower animals (it is characteristic of the rotifers), especially insects, e.g., the aphid. In many social insects, such as the honeybee and the ant, the unfertilized eggs give rise to the male drones and the fertilized eggs to the female workers and queens. Parthenogenesis has also been observed in some snakes, fish, and monitor lizards. The phenomenon is rarer among plants (where it is called parthenocarpy) than among animals. Unusual patterns of heredity can occur in parthenogenetic organisms. For example, offspring produced by some types are identical in all inherited respects to the mother.
The phenomenon of parthenogenesis was discovered in the 18th cent. by Charles Bonnet. In 1900, Jacques Loeb accomplished the first clear case of artificial parthenogenesis when he pricked unfertilized frog eggs with a needle and found that in some cases normal embryonic development ensued. Artificial parthenogenesis has since been achieved in almost all major groups of animals, although it usually results in incomplete and abnormal development. The parthenogenetic marble crayfish, believed to have been bred from the American slough crayfish by German pet traders in the 1990s, has become a pest in Europe and Africa. Numerous mechanical and chemical agents have been used to stimulate unfertilized eggs. In 1936, Gregory Pincus induced parthenogenesis in mammalian (rabbit) eggs by temperature change and chemical agents. No successful experiments with human parthenogenesis have been reported.
reproduction in which the female sex cell, or ovum, develops without fertilization.
Parthenogenesis is a type of sexual reproduction—more accurately, of unisexual reproduction—that arose in the process of evolution among dioecious forms. In those species where parthenogenesis either always or occasionally gives rise exclusively to females, one of the most important survival values of parthenogenesis is an accelerated reproductive rate of the species, which occurs because all new individuals are capable of having offspring. In those cases where fertilized ova develop into females and unfertilized ones develop into males, for example, in bees, parthenogenesis regulates the numerical ratio of the sexes. Parthenogenetic species and strains are often polyploid and arise as a result of hybridization between taxonomically distant species; such hybridization gives rise to heterosis and great viability (seeHETEROSIS).
Parthenogenesis should not be confused with asexual types of reproduction, for instance, cleavage and budding, which always involve somatic organs and cells. It is the normal method of reproduction for certain organisms; it can also be artificially induced for experimental purposes by the action of various irritants on an unfertilized ovum that normally requires fertilization in order to become a zygote.
In animals. The original form of parthenogenesis—rudimentary parthenogenesis—arises in many species of animals when their ova are not fertilized. As a rule, a zygote that arose through rudimentary parthenogenesis completes only the initial stages of embryonic development; however, development can occasionally attain the concluding stages, in which case the phenomenon is called accidental parthenogenesis. Complete natural parthenogenesis, by which a fully developed organism arises from an unfertilized ovum, is found among all types of invertebrates. It is the usual reproductive mechanism in arthropods, especially insects. Among vertebrates it has been discovered in fishes and certain amphibians. Complete parthenogenesis is especially frequent in reptiles: no fewer than 20 strains and species of lizards and geckos, for example, reproduce by this method. Among birds, turkeys exhibit a strong tendency toward parthenogenesis; through artificial selection it is thus possible to raise turkeys so that only sexually mature males will result. In mammals only isolated cases of natural rudimentary parthenogenesis are known, but fully developed rabbits have been experimentally reproduced through artificial parthenogenesis.
Parthenogenesis may be obligate, in which case the ova are exclusively capable of parthenogenetic development, or facultative, in which case the ova may develop either by parthenogenesis or as a result of fertilization. Many Hymenoptera exhibit facultative parthenogenesis; for example, male bees, or drones, develop from unfertilized ova, while queens and worker bees develop from fertilized ova.
In a single species, reproduction by parthenogenesis often alternates with bisexual reproduction; in such cases the external appearance of parthenogenetic generations differs from that of the sexual generation. For instance, one generation in aphids of the genus Chermes is winged, while the succeeding generation is wingless; the two generations also feed on different plants. In some gall wasps, parthenogenetic and bisexual generations are so different that they have been assigned to separate species and even genera. In the usual arrangement, which is observed in many aphids, daphnids, and rotifers, summer parthenogenetic generations consist only of females, and fall generations consist both of males and females; the fall generations leave fertilized ova for the winter.
Many species of animals exhibit constant parthenogenesis; they are capable of prolonged reproduction even though males are absent. Some species simultaneously have both generations of parthenogenetic females and bisexual generations; the latter represent the original form of the species. When these two types of generation inhabit different geographical areas, they are said to exhibit geographical parthenogenesis. Examples of geographically parthenogenetic organisms are casebearer moths and many beetles, myriapods, mollusks, rotifers, daphnids, and lizards.
Several types of parthenogenesis are distinguished according to the sex of the resultant offspring. Arrhenotoky is parthenogenesis in which only males develop from unfertilized ova; this is observed in bees and other hymenopterous insects, in ticks and mites, and in parthenogenetic turkey strains. Thelytoky, in which only males develop, is the most common type of parthenogenesis. Deuterotoky, in which both males and females develop, is observed in the form of accidental parthenogenesis in butterflies and in the alternating generations of daphnids, rotifers, and aphids.
The cytogenetic mechanism by which the unfertilized ovum matures is of great significance, for ultimately it is precisely on this mechanism that the genetic makeup, or genotype, of the parthenogenetic embryo depends. Furthermore, the most important hereditary characters of the embryo, including sex, heterosis, and homozygosis, also depend on this mechanism (seeHETEROSIS; HOMOZYGOSIS). Specifically, four basic mechanisms are recognized: in meiotic parthenogenesis, the ovum undergoes meiosis, and its number of chromosomes is halved; meiosis does not occur in ameiotic parthenogenesis; zygotic parthenogenesis is a type of ameiotic parthenogenesis in which the number of chromosomes that is characteristic of the species is preserved; and automixic parthenogenesis is a type of meiotic parthenogenesis in which the chromosome number is somehow reconstituted after reduction, usually by merging of the ovum’s nucleus with the nucleus of the polar body.
Parthenogenesis is also classified as generative, or haploid, and somatic, which may be diploid or polyploid. In generative parthenogenesis a haploid number (n) of chromosomes is observed in the dividing cells of the body. This case is relatively rare and occurs in combination with arrhenotoky; for instance, drone bees are haploid males. With somatic parthenogenesis an initial diploid number (2n) or polyploid number (3n, 4n, 5n, and rarely 6n or 8n) of chromosomes is observed in the dividing cells of the body. Often several strains within a species exist that are characterized by multiple numbers of chromosomes; such strains constitute a polyploid series. Polyploidy, which is very rare among bisexual animals, occurs very frequently in parthenogenetic animals. Polyploid dioecious species apparently originated through a combination of parthenogenesis and hybridization between taxonomically distant species. Paedogenesis is parthenogenetic reproduction in the larval state.
Artificial parthenogenesis in animals was first accomplished by the Russian zoologist A. A. Tikhomirov. In 1886 he showed that unfertilized eggs of the Asiatic silkworm can be induced to develop by chemical and physical irritants, such as solutions of strong acids and friction. Subsequently, artificial parthenogenesis was produced by J. Loeb and other scientists, mostly in marine invertebrates (sea urchins, starfishes, worms, and mollusks), in some amphibians (frogs), and even in a mammal (rabbit). In the late 19th and early 20th centuries, experiments in artificial parthenogenesis received special attention from biologists, who hoped to uncover the basis of fertilization by studying this physicochemical model of ovum activation.
Artificial parthenogenesis is induced by subjecting the egg to hypertonic or hypotonic solutions (osmotic parthenogenesis), by injecting the egg with a needle moistened with hemolymph (traumatic parthenogenesis of amphibians), by introducing severe temperature changes, especially heating (thermal parthenogenesis), and by subjecting the ovum to acids or alkalies. Artificial parthenogenesis usually does not succeed in producing an organism that can grow past the initial stages of development; however, although complete development is rare, it has been achieved, even with vertebrates (frog, rabbit).
A large-scale method of producing fully developed, parthenogenetic Asiatic silkworms was worked out in 1936 by B. L. Astaurov. He heated unfertilized ova, which were extracted from the female, to 46°C for 18 minutes. This method makes it possible to exclusively obtain female silkworms that are genetically identical to the parent female and to each other. The diploid, triploid, and tetraploid clones that are thus obtained can be parthenogenetically reproduced for an unlimited period after the initial parthenogenesis. They also retain their original heterozygosity and hybrid vigor. Through selection, clones have been obtained that reproduce by parthenogenesis just as easily as bisexual breeds reproduce by fertilization, that is, over 90 percent of activated ova hatch, and up to 98 percent of these survive. Clearly, parthenogenesis is of great interest to sericulturists.
In plants. Constant parthenogenesis is the usual type among seed and spore plants, although single instances of facultative parthenogenesis have been discovered in some species of the genus Hieracium and in the meadow rue Thalictrum purpurascens. As a rule, the sex of parthenogenetically reproducing plants is female. In dioecious plants parthenogenesis is associated with the absence or extreme rarity of male plants; in monoecious plants it is associated with degeneration of male flowers and absent or sterile pollen. As in animal parthenogenesis, generative, or haploid, parthenogenesis is distinguished from somatic parthenogenesis, which may be diploid or polyploid. Generative parthenogenesis is found in the genera Cutleria, Spirogyra, and Ectocarpus among algae and in Saprolegnia, Mucor, and Endomyces among fungi. In flowering plants it is observed only under experimental conditions and can be induced in many plants, including tobacco, Carpis, the cotton plant, and cereals. Somatic parthenogenesis is found in algae of the genus Chara and Cocconeis; in Marselia Drummond among ferns; and in higher flowering plants, for example, Alchemilla, Hieracium, Chondrilla, Antennaria, and Taraxacum. As with animals, polyploid parthenogenesis is found in plants; however, polyploidy in plants is not a characteristic of parthenogenetic species, since it is also common in bisexual plants.
Apogamy, in which the embryo develops not from the ovum but from other cells of the gametophyte, and apomixis are methods of reproduction that are closely related to parthenogenesis in plants (seeAPOMIXIS). Artificial parthenogenesis has been produced in some algae and fungi by hypertonic solutions and also by short-term heating of female generative cells. In research conducted from 1935 to 1948, the Austrian scientist E. Tschermak artificially produced parthenogenesis in many flowering plants, including cereals and legumes, by irritating the stigma with killed or foreign pollen, or with some other powdery substances, for example, talc, flour, and chalk. In 1972, E. M. Vermel’ obtained diploid currants, tomatoes, and cucumbers by inducing parthenogenesis with dimethyl sulfoxide. Certain specific methods of development in both animals and plants—gynogenesis and androgenesis—are also considered examples of parthenogenesis. Here, the ovum is activated to develop by a penetrating sperm cell of its own species or of a closely related species, but the nuclei of the ovum and of the sperm do not merge, true fertilization does not take place, and the embryo develops with only a female (gynogenesis) or male (androgenesis) nucleus (seeGYNOGENESIS; ANDROGENESIS).
REFERENCESAstaurov, B. L. Iskusstvennyi partenogenez u tutovogo shelkopriada (eksperimental’noe issledovanie). Moscow-Leningrad, 1940.
Astaurov, B. L. Tsitogenetika razvitiia tutovogo shelkopriada i ee eksperimental’nyi kontrol’. Moscow, 1968.
Tyler, A. ”Iskusstvennyi partenogenez.” In Nekotorye problemy sovremennoi embriofiziologii. Moscow, 1951. (Translated from English.)
Astaurov, B. L., and Iu. S. Demin. “Partenogenez u ptits.” Ontogenez, 1972, vol. 3, no. 2.
Rostand, J. La Parthénogenèse animale. Paris, 1950.
B. L. AUSTAUROV