Gynogenesis, a form of

parthenogenesis Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...

, is a system of

asexual reproduction Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the f ...

that requires the presence of sperm without the actual contribution of its

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

for completion. The paternal DNA dissolves or is destroyed before it can fuse with the egg. The egg cell of the organism is able to develop, unfertilized, into an adult using only maternal genetic material. Gynogenesis is often termed "sperm parasitism" in reference to the somewhat pointless role of male gametes. Gynogenetic species, "gynogens" for short, are

unisexual Dioecy ( ; ; adj. dioecious, ) is a characteristic of certain species that have distinct unisexual individuals, each producing either male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproduction is ...

, meaning they must mate with males from a closely related bisexual species that normally reproduces sexually. Gynogenesis is a disadvantageous mating system for males, as they are unable to pass on their DNA. The question as to why this reproductive mode exists, given that it appears to combine the disadvantages of both asexual and sexual reproduction, remains unsolved in the field of evolutionary biology. The male equivalent to this process is

androgenesis Androgenesis is a system of asexual reproduction that requires the presence of eggs and occurs when a zygote is produced with only paternal nuclear genes. During standard sexual reproduction, one female parent and one male parent each produce h ...

where the father is the sole contributor of DNA.Androgenesis: where males hijack eggs to clone themselves
/ref>

Taxonomic range

Most gynogenetic species are fishes or amphibians. Among the fishes, Amazon mollies ('' Poecilia formosa'') require the sperm of closely related male '' Poecilia latipinna'' to engage in gynogenesis. ''P. latipinna'' males prefer to mate with females of their own species. This presents a problem for ''P. formosa'', as they must compete for males who do not favour them. However, those ''P. formosa'' successful in finding a mate make up the deficit by producing twice as many female offspring as their competitors. Among salamanders, the '' Ambystoma platineum'', a unisexual

mole salamander The mole salamanders (genus ''Ambystoma'') are a group of Salamandroidea, advanced salamanders endemic to North America. The group has become famous due to the study of the axolotl (''A. mexicanum'') in research on neoteny, paedomorphosis, and t ...

, is hybrid of sexually reproducing '' A. jeffersonianum'' and '' A. laterale''. ''A. platineum'' individuals normally live in proximity to either of these parent species, so as to access their sperm.

Gynogenesis with haplodiploidy

The ant '' Myrmecia impaternata'' is a hybrid of '' M. banksi'' and '' M. pilosula''. In ants, sex is determined by the

haplodiploidy Haplodiploidy is a sex-determination system in which males develop from unfertilized eggs and are haploid, and females develop from fertilized eggs and are diploid. Haplodiploidy is sometimes called arrhenotoky. Haplodiploidy determines the s ...

system: unfertilized eggs result in haploid males, while fertilized eggs result in diploid females. In this species – its specific epithet ''impaternata'' meaning 'fatherless' – the queen reproduces through sexual interaction, yet not fertilization, with gynogenetically produced females, and males reared from fatherless eggs. Since these males are haploid, they are genetically identical to one of the two parent species, but are produced by a queen of ''M. impaternata''. The queens therefore have no need to mate parasitically with males of either parent species. This situation is unique.

Evolutionary origin

Two evolutionary pathways may be considered to explain how and why gynogenesis evolved. The single-step pathway involves multiple changes taking place simultaneously:

meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...

must be interrupted, one gender's gametes eradicated, and a

gender formation must arise. The second option involves multiple steps: a sexual generation is formed with a strongly biased

sex ratio A sex ratio is the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually. However, many species deviate from an even sex ratio, ei ...

, and because of Haldane's rule the species evolves towards loss of sexuality, with selection preferential towards the gynogen. Experimenters who attempted unsuccessfully to induce ''P. formosa'' by hybridizing its genetic ancestors concluded that the evolutionary origin of ''P. formosa'' was not from the simple hybridization of two specific genomes, but the movement of certain alleles.

References

{{reflist, 30em Asexual reproduction in animals Sexual reproduction Zoology Parasitism

Taxonomic range

Gynogenesis with haplodiploidy

Evolutionary origin

See also

References