Parthenogenesis is a type of asexual reproduction in which offspring (progeny) arise from unfertilized eggs. The phenomenon is known among many invertebrate and vertebrate groups, including some species of nematodes, gastropods, crustaceans, insects (especially honey bees and wasps), fishes, amphibians and reptiles. Few species reproduce solely by parthenogenesis and it is more common that episodes of parthenogenesis alternate with periods of sexual reproduction. Parthenogenesis may occur for several generations and then be followed at some point by sexual reproduction. During the sexual period males develop, produce sperm, and mate with females to fertilize their eggs. In some species, parthenogenesis actually appears to be an adaptation for survival in times of stress or serious population decline.
In situations where unfertilized eggs usually perish and only occasionally develop by parthenogenesis, the condition is known as exceptional parthenogenesis, while in those species where unfertilized eggs regularly develop to produce progeny, the condition is known as normal or physiological parthenogenesis. In faculatative parthenogenesis, the eggs develop whether they are fertilized or unfertilized. Several insect species demonstrate facultative parthenogenesis and their unfertilized eggs develop into only one sex, the other sex arising from fertilized eggs. Thus, for example, in the honey bee (Apis mellifica), the males (drones) always develop from unfertilized eggs (arrhenotoky), whereas fertilized eggs develop either into sexually mature females (queens) or into female workers, depending on the nutrition of their respective larvae. A population of honey bees can become a degenerate "drone brood" if the available stock of sperm becomes exhausted, or if a queen is not fertilized. The population suffers a similar fate if a worker lays eggs in the absence of a queen. An alternative situation exists in the shield bug where only females hatch from unfertilized eggs (thelytoky), while in aphids, for example, unfertilized eggs develop into either males or females (amphitoky or deuterotoky).
Cyclic parthenogenesis or heterogony is a form of life cycle in which parthenogenic and sexual reproduction alternate. In rotifer worms and water fleas for example, certain eggs are capable of fertilization and if fertilized lie dormant through the winter. Such eggs are large, richly endowed with yolk and develop more slowly. In contrast, eggs that are laid parthenogenically in large numbers during the summer are small, have little yolk and develop rapidly. Especially complicated examples of heterogony have been discovered in aphids and gall wasps in which different generations of parthenogenically produced females exhibit a pronounced polymorphism and also have different modes of existence. If sexual reproduction is omitted from the cycle, for example because the climate is favorable as may happen in the case of the vineyard pest, Pylloxera vitifoliae, then there is a continual succession of generations of parthenogenic females.
Parthenogenesis may be subdivided with regard to the chromosome complement of cells. In haploid parthenogenesis, the progeny arise from haploid eggs. Thus honey bee drones are haploid, while the workers and queens are diploid. Haploid parthenogenesis has also been observed in some flowering plants e.g. thorn apple, tobacco, rice, maize and wheat. Diploid parthenogenic progeny can arise by various mechanisms, for example the haploid egg nucleus may fuse with the second polar body or two haploid cleavage nuclei may fuse. Alternatively, the egg may be diploid because meiosis does not occur completely during egg formation. Diploid parthenogenesis is much more common than haploid parthenogenesis and is known in insects such as the gall wasp and also in the roundworms and flukes as well as in various flowering plants such as the dandelion.
In addition to natural parthenogenesis, it is possible to induce artificial parthenogenesis. This has been achieved with representatives of practically all animal phyla, including amphibians and mammals. Nonfertilized eggs can be induced to develop by a variety of means, for example by exposure to hyper-or hypotonic solutions of salts, diluted acids, alkalis, toxins, narcotics, and also by physical methods such as temperature changes, ultraviolet or radium irradiation, brief electrical stimulation and puncture with a glass or platinum needle. In gynogenesis the sperm enters and activates the egg, but degenerates without its nucleus fusing with that of the egg. This can be performed experimentally in sea urchin or amphibian eggs, which are allowed to be penetrated by sperm from the same species, followed by destruction of the sperm nucleus by irradiation. Alternatively, egg development is induced by penetration (without fertilization) by sperm of different species. The counterpart of gynogenesis is androgenesis (male parthenogenesis), in which progeny develop from a male gamete. Artificial androgenesis has been performed with sea urchin and amphibian sperm, by allowing them to penetrate denucleated egg fragments of the same species, and also by damaging the egg nucleus of a normal sperm-penetrated and activated egg by radium irradiation. Artificial androgenesis has also been performed in mouse sperm by allowing egg penetration by the sperm nucleus, then removing the female nucleus by suction.
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