Parthenogenesis

Parthenogenesis (/ˌpɑːrθɪnoʊˈdʒɛnɪsɪs, -θɪnə-/;[1][2] from the Greek παρθένος, parthénos, 'virgin' + γένεσις, génesis, 'creation'[3]) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization.

Parthenogenesis occurs naturally in some plants, algae, invertebrate animal species (including nematodes, some tardigrades, water fleas, some scorpions, aphids, some mites, some bees, some Phasmatodea, and parasitic wasps), and a few vertebrates, such as some fish, amphibians, and reptiles.

This type of reproduction has been induced artificially in animal species that naturally reproduce through sex, including fish, amphibians, and mice.

Depending on the mechanism involved in restoring the diploid number of chromosomes, parthenogenetic offspring may have anywhere between all and half of the mother's alleles.

[4][5] It occurs naturally in some plants, algae, invertebrate animal species (including nematodes, some tardigrades, water fleas, some scorpions, aphids, some mites, some bees, some Phasmatodea, and parasitic wasps), and a few vertebrates, such as some fish, amphibians, reptiles,[6][7][8] and birds.

[9][10][11] This type of reproduction has been induced artificially in a number of animal species that naturally reproduce through sex, including fish, amphibians, and mice.

The switch between sexuality and parthenogenesis in such species may be triggered by the season (aphid, some gall wasps), or by a lack of males or by conditions that favour rapid population growth (rotifers and cladocerans like Daphnia).

This is because in asexual reproduction a successful genotype can spread quickly without being modified by sex or wasting resources on male offspring who will not give birth.

The criterion for sexuality varies from all cases of restitutional meiosis,[27] to those where the nuclei fuse or to only those where gametes are mature at the time of fusion.

[citation needed] Facultative parthenogenesis occurs when a female can produce offspring either sexually or via asexual reproduction.

[35] One of the best-known examples of taxa exhibiting facultative parthenogenesis are mayflies; presumably, this is the default reproductive mode of all species in this insect order.

A female may undergo facultative parthenogenesis if a male is absent from the habitat or if it is unable to produce viable offspring.

Well documented transitions to obligate parthenogenesis have been found in numerous metazoan taxa, albeit through highly diverse mechanisms.

[42] Some documented species, specifically salamanders and geckos, that rely on obligate parthenogenesis as their major method of reproduction.

This form of asexual reproduction is thought in some cases to be a serious threat to biodiversity for the subsequent lack of gene variation and potentially decreased fitness of the offspring.

[41] Some invertebrate species that feature (partial) sexual reproduction in their native range are found to reproduce solely by parthenogenesis in areas to which they have been introduced.

Examples include several aphid species[44] and the willow sawfly, Nematus oligospilus, which is sexual in its native Holarctic habitat but parthenogenetic where it has been introduced into the Southern Hemisphere.

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

The fertilization event causes intracellular calcium oscillations, and targeted degradation of cyclin B, a regulatory subunit of MPF, thus permitting the MII-arrested oocyte to proceed through meiosis.

Treatment with cycloheximide, a non-specific protein synthesis inhibitor, enhances the development of unfertilised eggs in swine presumably by continual inhibition of MPF/cyclin B.

This is because mammals have imprinted genetic regions, where either the maternal or the paternal chromosome is inactivated in the offspring for development to proceed normally.

[53] As a consequence, research on the induced development of unfertilised eggs in humans is focused on the production of embryonic stem cells for use in medical treatment, not as a reproductive strategy.

[13] In 1955, Helen Spurway, a geneticist specializing in the reproductive biology of the guppy (Lebistes reticulatus), claimed that parthenogenesis may occur (though very rarely) in humans, leading to so-called "virgin births".

The same year, Revazova and ISCC published an article describing how to produce human stem cells that are homozygous in the HLA region of DNA.

With selection of oocyte donors according to HLA haplotype, it would be possible to generate a bank of cell lines whose tissue derivatives, collectively, could be MHC-matched with a significant number of individuals within the human population.

[59] After an independent investigation, it was revealed that the discredited South Korean scientist Hwang Woo-Suk unknowingly produced the first human embryos resulting from parthenogenesis.

Initially, Hwang claimed he and his team had extracted stem cells from cloned human embryos, a result later found to be fabricated.

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.

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.

("green frogs" or "waterfrogs"): Other examples where hybridogenesis is at least one of modes of reproduction include i.e. Parthenogenesis, in the form of reproduction from a single individual (typically a god), is common in mythology, religion, and folklore around the world, including in ancient Greek myth; for example, Athena was born from the head of Zeus.