parthenogenesis

parthenogenesis [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. It 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. 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.

parthenogenesis is a type of asexual reproduction in which the offspring develops from unfertilized eggs. It is particularly common amongst arthropods and rotifers, can also be found in some species of fish, amphibians, birds, and reptiles, but not in mammals. Parthenogenetic development also occurs in some plants species, such as roses and orange trees.

Most animal species that reproduce parthenogenetically also display a phase of sexual behaviour and sexual reproduction. In most cases, parthenogenetic reproduction occurs when environmental conditions are favourable and there is plenty of food that can sustain the generation of large numbers of individuals in a short period of time. When external conditions change and food supplies become less abundant, or when the environment becomes unpredictable, these species shift to a sexual mode of reproduction. Although sexual reproduction is considerably slower and generates fewer organisms, it gives rise to individuals containing variations in their genetic material. Some of these individuals might be at an advantage over their predecessors, because they might be more able to adapt to new conditions.

In some species of insects, such as the aphids, parthenogenetic reproduction occurs in the spring and summer, when conditions are favourable for rapid population growth. As time goes by and conditions become less favourable, the parthenogenetically born individuals mate and lay fertilized eggs. These eggs hatch the following spring, when conditions are again favourable for another cycle of parthenogenetic reproduction.

In some species of ants, bees, and wasps, the ability to reproduce both sexually and asexually is part of the mechanism establishing sexual differences. Usually, females develop from unfertilized eggs, containing only half of the genetic material of the mother, whereas males develop from fertilized eggs, containing the genetic contributions of both mother and father.

In other species of insects, such as the rotifers, females produce unfertilized eggs that develop into females during the spring and summer. This process goes on for several generations. During the autumn, smaller eggs are laid, which develop into individuals lacking a digestive system, but capable of secreting sperm. These individuals mate with females, who then produce highly resistant, fertilized eggs that remain viable during long periods of unfavourable conditions. These eggs hatch in the following spring, giving rise only to females, who then engage in a new period of parthenogenetic reproduction.

Silvia Frenk

parthenogenesis Development of a female sex cell or gamete without fertilization. Since there is no involvement of a male gamete, it leads to the production of offspring that are genetically identical to the mother. This process occurs naturally among some plants and invertebrates, such as aphids.