Parthenogenesis, Parthenogenesis

In plants, parthenogenesis means development of an embryo from an unfertilized egg cell, and is a component process of apomixis. The offspring produced by parthenogenesis are always female in species that use the XY sex-determination system.

A parthenogenetic offspring is sometimes called a parthenogen. Parthenogenesis is seen to occur naturally in aphids, daphnia, rotifers, and some other invertebrates, as well as in many plants. Komodo dragons and the hammerhead- and blacktip sharks have recently been added to the list of vertebratesalong with several genera of fish, amphibians, and reptilesthat exhibit differing forms of asexual reproduction, including true parthenogenesis, gynogenesis, and hybridogenesis (an incomplete form of parthenogenesis). As with all types of asexual reproduction, there are both costs (low genetic diversity and therefore susceptibility to adverse mutations that might occur) and benefits (reproduction without the need for a male) associated with parthenogenesis.

The offspring may be capable of sexual reproduction, if this mode exists in the species. In many cases, parthenogenesis occurs when one gender (typically the male) is unavailable in the general vicinity.

Because there are so many variables in parthenogenesis, there is little that can be said for sure unless the specific methods of the particular parthenogenetic tendencies of an organism are known.

A forms of reproduction related to parthenogenesis, but that only requires the presence of sperm that do not fertilize an egg, is known as gynogenesis. In hybridogenesis the sperm fertilizes the egg, but its chromosomes are not carried to subsequent generations.

In some of the eggs fertilized by males, however, the fertilization can cause the female genetic material to be ablated from the zygote, in a process called ameiotic parthenogenesis. In this way, males pass on only their genes to become fertile male offspring. This type of reproduction results in a complete separation of the gene pools for females and males. This is the first recognized example of an animal species where both females and males can reproduce clonally resulting in a complete separation of male and female gene pools.

The testing showed the female pup's DNA matched only one female who lived in the tank, and that no male DNA was present in the pup. The pup was not a twin or clone of her mother, but rather, contained only half of her mother's DNA ("automictic parthenogenesis"). This type of reproduction had been seen before in bony fish, but never in cartilaginous fish such as sharks, until this documentation.

Although parthenogenesis may help females who cannot find mates, it does reduce genetic diversity.

However, in 1936, Gregory Goodwin Pincus reported successfully inducing parthenogenesis in a rabbit.

Use of an electrical or chemical stimulus can produce the beginning of the process of parthenogenesis in the asexual development of viable offspring.

A mammal created by parthenogenesis would thus have double doses of maternally imprinted genes and lack paternally imprinted genes, leading to developmental abnormalities if any were present in the genes of the mother. As a consequence, research on human parthenogenesis is focused on the production of embryonic stem cells for use in medical treatment, not as a reproductive strategy.

Initially, Hwang claimed he and his team had extracted stem cells from cloned human embryos, a result which was later found to be fabricated. Further examination of the chromosomes of these cells show indicators of parthenogenesis in those extracted stem cells, similar to those found in the mice created by Tokyo scientists in 2004. Although Hwang deceived the world about being the first to create artificially cloned human embryos, he did contribute a major breakthrough to stem cell research by creating human embryos using parthenogenesis.

In 2006, a group of Italian researchers announced they had achieved the same feat, but have yet to publish their results. Therefore, ISC is the first organization to achieve artificial parthenogenesis that intentionally led to the creation of human parthenogenetic stem cell lines from unfertilized eggs.

Here offspring are produced by the same mechanism as in parthenogenesis, but with the requirement that the egg merely 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 are all female, activation of their eggs requires mating with males of a closely related species for the needed stimulus. 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 rare fertilization of eggs by males, introducing new material to the gene pool, which may result from perhaps only one mating out of a million.

In typical parthenogenesis the individual offspring differ from one another and their mother.

Source: Wikipedia > Parthenogenesis