 Kaguya is one success from 460 attempts at growing embryos. (c) T. Kono 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), where it can be used for reproduction in the absence of males.
Parthenogenesis is also 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.
Parthenogenesis
Parthenogenesis (from Greek meaning "virgin birth") is a peculiar form of asexual reproduction in which the egg from the female remains unfertilized but still produces viable offspring. A high stress environment will often cause sexual reproduction to be a more successful process due to selecting factors having different effects upon genetically different offspring, so some organisms (for example, the freshwater crustacean Daphnia) are heterogamic, able to reproduce both by parthenogenesis and sexual reproduction. Often heterogamic organisms are able to switch between reproductive strategies based upon the level of predation or competition that they experience in their environment. Parthenogenesis is seen in aphids, rotifers, some plants, and some other lower order animals. Among vertebrates, there are several genera of fish, amphibians, and reptiles that utilize a more complex form of parthenogenesis involving a coupling of the females egg with the usually discarded polar body, resulting in diploid zygotes after the occurrence of meiosis.
About fifteen species of whiptail lizard (Cnemidophorus uniparens) procreate exclusively by parthenogenesis. These lizards live in the very dry and sometimes harsh climate of the southwestern United States and northern Mexico. At some point along their evolutionary history, natural selection excluded males from the population of whiptail lizards, apparently because there was reproductive success with an entirely female population. What is unique about the whiptail lizards is the mating ritual they partake in before carrying out their reproduction. Although they are entirely female, they still imitate the behavior of species that reproduce sexually, by one female playing the role of a male lizard and mounting the female that is about to produce eggs. The reason these lizards 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 on account of the increase in hormones that accompanies the fake sex. So, even though whiptail lizards have erased the male population entirely, they still require sexual stimuli for maximum reproductive success.
As with all types of asexual reproduction, there are advantages and disadvantages associated with parthenogenesis. Advantages include the capacity of all members or all females to be able to produce eggs; this increases reproductive efficiency, as they can reproduce without wasting time or energy searching for mates. Organisms that can reproduce through parthenogenesis are 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. Offspring of parthenogenesis often have a very similar genetic makeup to their parent, resulting in both beneficial and harmful mutations being passed on with greater regularity. The genetic similarity can also cause problems with predation. If a predator or pathogen invades their population and their genetic makeup is particularly defenseless against it, they are more likely to be completely exterminated by it, since they will all exhibit the same vulnerability. To contrast, a population that reproduces sexually has a higher probability for different members to survive a certain predator due to the genetic recombination that results in differences from one generation to the next.
Gynogenesis
A special form of parthenogenesis is gynogenesis. Species that reproduce by gynogenesis (e.g., some salamanders of the genus Ambystoma) can perform only parthenogenesis, and consist entirely of females. They do, however, have sexual contact with the males of a closely related species. The sperm from these males is not used to fertilize the egg; it just stimulates egg development without any exchange of genetic material. It is believed that the success of those salamanders is 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.
See also
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