Parthenogenesis is a form of asexual reproduction found in various plant and animal species, allowing females to produce viable eggs without fertilization. It is rare in complex animals and can result in low genetic diversity and harmful mutations. Some species can use both sexual and asexual means to reproduce, while others, such as the New Mexico whiptail lizard, rely solely on parthenogenesis. Mammalian parthenogenesis has been artificially induced but often leads to abnormal development. Human births from parthenogenesis are unlikely due to ethical concerns, but the process can be used for experimentation.
Parthenogenesis is a form of asexual reproduction in which females can produce viable eggs without fertilization by males. It is an ability possessed by a wide variety of plant and animal species, including most non-vascular plants (liverworts, hornworts and mosses), a genus of succulent plants in South Africa, assorted invertebrates, particularly some arthropods ( water fleas, daphnia, rotifers, aphids, some bees, some scorpions and parasitic wasps), and some vertebrates (some reptiles, fish and, in some cases, birds and sharks). In general, parthenogenesis is rare – most animals capable of asexual reproduction are very simple – sponges or cnidarians (jellyfish, corals, etc.), too simple to have genders.
The process of parthenogenesis should not be confused with the quality of hermaphroditism, a species with both male and female reproductive parts. In most hermaphroditic species, reproduction is still sexual and requires two participants. Parthenogenesis is the only way complex animals can clone themselves. In some cases, it can lead to offspring that are genetically distinct from the parents, depending on whether the egg is haploid or diploid. Like any other form of reproduction, parthenogenesis has its advantages (reproduction without the need for a male) and weaknesses (low genetic diversity, susceptibility to harmful mutations that persist across generations).
Many species that can undergo parthenogenesis are also capable of using sexual means to reproduce. This shuffles genes through recombination, increasing genetic diversity beyond a necessary threshold. In some reptiles, notably the New Mexico whiptail, reproduction occurs solely through parthenogenesis. These species tend to be polyploid, as they have more than two sets of chromosomes, meaning their genomes are combinations of two or more lizard species in the same genus. The mechanism by which mixing of chromosomes between species leads to parthenogenesis is unknown, but is what is observed. Polyploidy occurs in some hybrids between two or more species.
Although no natural cases of mammalian parthenogenesis are known, it has been artificially induced in rabbits (Gregory Pincus, 1936), mice (Tokyo University of Agriculture, 2004), and monkeys. Induced parthenogenesis in mice and monkeys often leads to abnormal development, because the mother’s chromosomes are imprinted twice in the offspring’s genome, and many mammalian genomes are completely dependent on a mix of genes from both sexes for smooth development. Consequently, for ethical reasons, human births from parthenogenesis are unlikely to be prosecuted, although the process can be used to create embryos for experimentation. South Korean scientist Hwang Woo-Suk achieved this in 2007 while attempting to be the first to clone a human. He was unsuccessful at cloning, but did produce viable human embryos made via artificial parthenogenesis.
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