Parthenogenesis (Partheno-genesis from the Greek παρθενος, "virgin", + γενεσις, "birth") means the growth and development of an embryo or seed without fertilization by a male. Parthenogenesis occurs naturally in some lower plants (called agamospermy), invertebrates (e.g. water fleas, aphids) and some vertebrates (e.g. lizards, salamanders, some fish, and even turkeys). Parthenogenetic populations are typically all-female. As with all types of asexual reproduction, there are both costs and benefits associated with parthenogenesis.
Parthenogenesis is seen as a possible way to clone primates, with the emphasis on human cloning. In April 2004, scientists at Tokyo University of Agriculture used parthenogenesis to successfully create fatherless mice.
The alteration between parthenogenesis and sexual reproduction is called heterogamy. Forms of reproduction related to parthenogenesis but that require the presence of sperm are known as gynogenesis and hybridogenesis.
Asexuality is relatively rare among multicellular organisms, for reasons that are not completely understood. Current hypotheses suggest that, while asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, over the long term sexual reproduction offers a net advantage by allowing more rapid adaptation to changing environments. Asexual lineages can increase their numbers rapidly because (since individuals are always female) everyone can produce viable eggs. In sexual populations half the individuals are male and cannot themselves produce offspring. This means that an asexual lineage will have roughly double the rate of population growth under ideal conditions. Organisms that can reproduce through parthenogenesis are also more able to settle isolated habitats like oceanic islands, as only a single (female) member of the species has to reach the habitat to start the population. Another consequence of asexual reproduction, which may have both benefits and costs, is that offspring are typically genetically identical or nearly identical to their parent. This genetic similarity can be beneficial if the genotype is well suited to a stable environment, but disadvantageous if the environment is changing. For example, if a new predator or pathogen appears and a genotype is particularly defenseless against it, an asexual lineage is more likely to be completely wiped out by it. In contrast, a lineage that reproduces sexually has a higher probability of having at least some members surivive due to the genetic recombination that produces a novel genotype in each individual. Similar arguments apply to changes in the physical environment.
Some species alternate between the sexual and assexual strategies, an ability known as heterogamy, depending on conditions. For example, the freshwater crustacean Daphnia reproduces by parthenogenesis in the spring to rapidly populate ponds, then switches to sexual reproduction as the intensity of competition and predation increases.
Parthenogenesis is a particular form of asexual reproduction in which females produce eggs that develop without fertilization. Parthenogenesis is seen in aphids, rotifers, and some other invertebrates, as well as in some plants. Among vertebrates, there are several genera of fish, amphibians, and reptiles that exhibit differing form of asexual reproduction, including true parthenogenesis, gynogenesis, and hybridogenesis, an incomplete form of parthenogenesis.
Among the reptiles, about fifteen species of whiptail lizard (genus Cnemidophorus) reproduce exclusively by parthenogenesis. These lizards live in the dry and sometimes harsh climate of the southwestern United States and northern Mexico. All these asexual species appear to have arisen through the hybridization of two or three of the sexual species in the genus. The mechanism by which the mixing of chromosomes from two or three species can lead to parthenogenetic reproduction is unknown. Because multiple hybridization events can occur, individual parthenogenetic whiptail species can consist of multiple, independent asexual lineages. Within lineages, there is very little genetic diversity, but different lineages may have quite different genotypes. An interesting aspect to reproduction in these asexual whiptail lizards is that mating behaviors are still seen even though the populations are entirely female. One female plays the role of a male lizard and mounts the female that is about to produce eggs. The reason the animals act this way is due to their hormonal cycles, which cause some to act as males when levels of estrogen are low, and others to take the role of female when estrogen levels are high. Lizards that act out the courtship ritual have greater fecundity than those kept in isolation due to the increase in hormones that accompanies the fake sex. So, even though asexual whiptail lizards populations lack males, they still require sexual stimuli for maximum reproductive success.
A form of asexual reproduction related to parthenogensis is gynogenesis. In gynogenesis, offspring are produced by the same mechanism as in parthenogenesis, but with the requirement that the egg be stimulated by the presence of sperm in order to develop. However, the sperm cell does not contribute any genetic material to the offspring. Since gynogenetic species lack males, activation of the egg requires mating with males of a cloesly related species. Some salamanders of the genus Ambystoma are gynogenetic and appear to have been so for over a million years. It is believed that the success of those salamanders may be due to the rare (perhaps only one mating out of a million) actual fertilization of eggs by a male, introducing new material to the gene pool.
In hybridogenesis reproduction is not completely asexual but instead hemiclonal, with half the genome passing intact to the next generation while the other half is replaced. In hybridogenetic species, females mate with males and both individuals contribute genetic material to the offspring. But when the female offspring produce their own eggs, the eggs contain no genetic material from their father; instead the eggs contains an exact copy of the chromosomes those offspring got from their own mother. This process continues, so that each generation is half (or hemi-) clonal on the mother's side and half new genetic material from the father's side. This form of reproduction is seen in some of the guppy species in the genus Poeciliopsis .