By Isabelle Hall
The phenomenon of parthenogenesis, also known as ‘virgin birth’ (from the Greek – parthenos, meaning “virgin”, and genesis, meaning “birth” or “origin”), refers to the development of an embryo without fertilisation. This is considered to be another form of asexual reproduction, which includes processes such as binary fission and budding (Dudgeon et al., 2017). However, it has also been described as an incomplete form of sexual reproduction, distinct from asexual methods. Parthenogenesis occurs widely among numerous invertebrate species, including nematodes, aphids, and parasitic wasps. Cases have also been observed in vertebrates, such as sharks and lizards (Ramachandran & McDaniel, 2018).
Species may exhibit obligate or facultative parthenogenesis. The former refers to those incapable of completing sexual reproduction. Facultative parthenogenesis occurs more frequently among vertebrates: in such species, an embryo may also develop through fertilisation, completing sexual reproduction. Some argue that many of the cases in vertebrates which have been described as facultative parthenogenesis are in fact accidental, generally occurring as a result of reproductive errors (van der Kooi & Schwander, 2015).
For cases of parthenogenesis which have commonly been interpreted as facultative, the main trigger for this adaptive response appears to have been lack of access to a potential mate. This has been observed in captive environments, in which females have had no contact with males over the course of their reproductive lifetime. In another case, a female switched from sexual reproduction to parthenogenesis following a period of separation from a mate (Dudgeon et al., 2017).
Transitions from sexual reproduction to parthenogenesis also appear to be influenced by specific environmental factors. In birds, such conditions may include food availability, hormones, temperature, and infections caused by viruses or bacteria (Ramachandran & McDaniel, 2018).
Parthenogenesis can proceed through a number of different pathways. Terminal fusion automixis involves the fusion of an ovum with a polar body, a haploid cell produced during oogenesis. This restores the diploid number of chromosomes. Another type of automictic parthenogenesis may occur, in which two gametes fuse. Additionally, post-meiotic doubling can restore diploidy through duplication of the chromosome set (Mirzaghaderi & Hörandl, 2016). While these automictic pathways involve meiosis, parthenogenesis may also proceed through apomixis, in which diploid germ line cells undergo mitosis. This form is more common in plants (Holtcamp, 2009).
In other cases, restoration of the diploid number of chromosomes is not necessary, and a haploid individual can develop. This is observed among members of the order Hymenoptera, such as the honeybee, Apis mellifera. Haploid males of this species arise from unfertilised eggs released by the queen, demonstrating a process known as arrhenotokous parthenogenesis (Oldroyd et al., 2008).
No cases of parthenogenesis have been reported in mammals. It is thought that such a method of reproduction would not be possible in members of this group due to genetic and developmental factors, including genomic imprinting. This is a process which occurs during gametogenesis, in which DNA is marked by specific epigenetic modifications. Such alterations include methylation of DNA and various histone modifications. These epigenetic imprints are maintained throughout fertilisation and development, and are only erased in primordial germ cells in preparation for reprogramming.
The genes impacted by this process are termed ‘imprinted genes’, and exhibit monoallelic expression in a parent-of-origin-specific manner. Consequently, among species in which genomic imprinting occurs, presence of a maternal genome and a paternal genome are required in the embryo. Previous experiments with mice have shown that gynogenetic embryos (formed from a zygote containing two maternal pronuclei) and androgenetic embryos (containing two paternal pronuclei) result in abnormal development (Li & Sasaki, 2011). Genomic imprinting is considered a significant factor in the apparent inability of mammals to reproduce via parthenogenesis, as it is widely observed among eutherian mammals, as well as in marsupials (Renfree, Suzuki & Kaneko-Ishino, 2013).
Within the field of conservation biology, the discovery of parthenogenesis in sharks raised questions about their declining numbers, and whether this method of reproduction may assist survival. However, it does not present an ideal solution: it results in far fewer pups being born within each litter compared to sexual reproduction (up to 15 for sexual reproduction, vs. 1 pup from each observed case of parthenogenesis). Additionally, there is concern regarding genetic diversity, which would decrease within shark populations utilising parthenogenesis. This can lead to a reduced ability to adapt to changing environmental conditions (Holtcamp, 2009).
Further research into parthenogenesis may reveal more about the significance of its role in nature, and the conditions which can initiate it within certain species.
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