Parthenogenesis in squamata
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Parthenogenesis Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and development ...
is a mode 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 fu ...
in which offspring are produced by females without the genetic contribution of a male. Among all the sexual
vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
, the only examples of true parthenogenesis, in which all-female populations reproduce without the involvement of males, are found in squamate reptiles (
snakes Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more joi ...
and
lizards Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia althou ...
). There are about 50 species of lizard and 1 species of snake that reproduce solely through parthenogenesis (obligate parthenogenesis). It is unknown how many sexually reproducing species are also capable of parthenogenesis in the absence of males (facultative parthenogenesis), but recent research has revealed that this ability is widespread among squamates.


Mechanisms

Parthenogenesis can result from either full cloning of the mother's genome, or through the combination of haploid genomes to create a "half-clone". Both mechanisms of parthenogenesis are seen in reptiles.


Full-cloning

Females can produce full clones of themselves through a modification of the normal
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...
process used to produce haploid
egg cells The egg cell, or ovum (plural ova), is the female reproductive cell, or gamete, in most anisogamous organisms (organisms that reproduce sexually with a larger, female gamete and a smaller, male one). The term is used when the female gamete is ...
for sexual reproduction. The female's germ cells undergo a process of premeiotic genome doubling, or endoreduplication, so that two consecutive division cycles in the process of meiosis result in a diploid, rather than haploid, genome. Whereas homologous chromosomes pair and separate during meiosis I in sexual species, identical duplicate sister chromosomes, produced through premeiotic replication, pair and separate during meiosis I in true parthenotes. Pairing of identical sister chromosomes, in comparison to the alternative of pairing homologous chromosomes, maintains
heterozygosity Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...
in obligate parthenotes. Meiosis II involves the separation of sister chromatids in both sexual and parthenogenetic species. This method of parthenogenesis is observed in obligate parthenotes, such as lizards in the genus ''
Aspidoscelis ''Aspidoscelis'' is a genus of whiptail lizards in the family Teiidae. Taxonomy The nomenclature for the genus ''Aspidoscelis'' was published by T.W. Reeder et al. in 2002. Many species that were formerly included in the genus '' Cnemidophorus'' ...
'' and ''
Darevskia ''Darevskia'' is a genus of wall lizards of the family Lacertidae living in the Caucasus, Iran and Turkey in forest and grassy habitats with numerous rock outcrops. Among rock lizards, 7 parthenogenetic species are known. Description These are ...
'', and also in certain facultative parthenotes like the Burmese python.


Half-cloning

Another mechanism typically observed in facultative parthenote reptiles is terminal fusion, in which a haploid polar body produced as a byproduct of normal female meiosis fuses with the egg cell to form a diploid nucleus, much as a haploid sperm cell fuses its nucleus with that of an egg cell to form a diploid genome during sexual reproduction. This method of parthenogenesis produces offspring that are
homozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...
at nearly all genetic loci, and inherit approximately half of their mother's genetic diversity. This form of parthenogenesis can produce male as well as WW-genotype females. Because the meiosis process proceeds normally in species employing this mechanism, they are capable of both sexual and asexual reproduction, as in the Komodo dragon and several species of snakes.


Types of parthenogenesis


True parthenogenesis

"True" parthenogenesis is a form of asexual reproduction in all-female species that produce offspring without any male involvement.


Lizards

There are at least eight parthenogenetic species of Caucasian rock lizard in the genus ''
Darevskia ''Darevskia'' is a genus of wall lizards of the family Lacertidae living in the Caucasus, Iran and Turkey in forest and grassy habitats with numerous rock outcrops. Among rock lizards, 7 parthenogenetic species are known. Description These are ...
''.Darevskii IS. 1967. Rock lizards of the Caucasus: systematics, ecology and phylogenesis of the polymorphic groups of Caucasian rock lizards of the subgenus ''Archaeolacerta''. Nauka: Leningrad n Russian: English translation published by the Indian National Scientific Documentation Centre, New Delhi, 1978Tarkhnishvili DN (2012) Evolutionary History, Habitats, Diversification, and Speciation in Caucasian Rock Lizards. In: Advances in Zoology Research, Volume 2 (ed. Jenkins OP), Nova Science Publishers, Hauppauge (NY), p.79-120 This genus is unique in containing the only known monoclonal parthenogenetic species, ''Darevskia rostombekovi,'' where the entire species originates from a single
hybridization Hybridization (or hybridisation) may refer to: *Hybridization (biology), the process of combining different varieties of organisms to create a hybrid *Orbital hybridization, in chemistry, the mixing of atomic orbitals into new hybrid orbitals *Nu ...
event. In all other cases of unisexual reptilian species that have been examined, multiple separate asexual lineages are present. As true parthenotes, ''
Darevskia ''Darevskia'' is a genus of wall lizards of the family Lacertidae living in the Caucasus, Iran and Turkey in forest and grassy habitats with numerous rock outcrops. Among rock lizards, 7 parthenogenetic species are known. Description These are ...
'' do not require stimulation from sperm to reproduce. The best-known and perhaps most evolutionarily derived example of parthenogenesis in reptiles occurs within the Teiid genus of whiptail lizards known as ''
Aspidoscelis ''Aspidoscelis'' is a genus of whiptail lizards in the family Teiidae. Taxonomy The nomenclature for the genus ''Aspidoscelis'' was published by T.W. Reeder et al. in 2002. Many species that were formerly included in the genus '' Cnemidophorus'' ...
''. This genus contains at least 13 truly parthenogenetic species, which originate from hybridization events between sexual ''Aspidoscelis'' species.Moore, Michael C., Joan M. Whittier, and David Crews. "Sex steroid hormones during the ovarian cycle of an all-female, parthenogenetic lizard and their correlation with pseudosexual behavior." General and comparative endocrinology 60.2 (1985): 144-153. Parthenogenetic whiptails are unusual in that they engage in female-female courtship to induce ovulation, with one non-ovulating female engaging in courting behavior normally seen in males while the ovulating female assumes the typical female role. While
sex hormone Sex hormones, also known as sex steroids, gonadocorticoids and gonadal steroids, are steroid hormones that interact with vertebrate steroid hormone receptors. The sex hormones include the androgens, estrogens, and progestogens. Their effects ar ...
levels in parthenogenetic '' Aspidoscelis uniparens'' mimic the cycles seen in their sexual relatives, their nervous systems appear to have evolved unique responses to female sex hormones. Male-like behavior in ''A. uniparens'' is correlated with high
progesterone Progesterone (P4) is an endogenous steroid and progestogen sex hormone involved in the menstrual cycle, pregnancy, and embryogenesis of humans and other species. It belongs to a group of steroid hormones called the progestogens and is the m ...
levels. This female-female pseudocopulation has also been found to enhance fecundity. A triploid parthenogenetic species in the genus ''
Aspidoscelis ''Aspidoscelis'' is a genus of whiptail lizards in the family Teiidae. Taxonomy The nomenclature for the genus ''Aspidoscelis'' was published by T.W. Reeder et al. in 2002. Many species that were formerly included in the genus '' Cnemidophorus'' ...
'', formerly part of ''
Cnemidophorus ''Cnemidophorus'' is a genus of lizards in the family Teiidae. Species in the genus ''Cnemidophorus'' are commonly referred to as whiptail lizards or racerunners. The genus is endemic to South America, Central America, and the West Indies. Ta ...
'', has been fertilized with sperm from a sexual species in the same genus to produce a new tetraploid parthenogenetic species in laboratory experiments. Such experiments provide evidence that even truly parthenogenetic species are still capable of incorporating new genetic material. There are six parthenogenetic
gecko Geckos are small, mostly carnivorous lizards that have a wide distribution, found on every continent except Antarctica. Belonging to the infraorder Gekkota, geckos are found in warm climates throughout the world. They range from . Geckos ar ...
species in five
genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...
: ''
Hemidactylus garnotii The Indo-Pacific gecko (''Hemidactylus garnotii'') also known as Garnot's house gecko, the fox gecko, or the Assam greyish brown gecko, is a species of gecko found in India, the Philippines, Southeast Asia, Australia, and throughout Polynesia. A ...
'' (Indo-Pacific house gecko), '' Hemidactylus vietnamensis'' (Vietnamese house gecko), '' Hemiphyllodactylus typus'' (dwarf tree gecko), ''
Heteronotia binoei ''Heteronotia binoei'', also known commonly as the Bynoe's gecko, is a species of lizard in the family Gekkonidae. The species is endemic to Australia. One of Australia's least habitat-specific geckos, it occurs naturally across much of the cou ...
'' (Binoe’s gecko), '' Nactus pelagicus'' (pelagic gecko), and '' Lepidodactylus lugubris'' (mourning gecko). The often quoted parthenogeneetic species ''N. arnouxi'' is ''nomen rejectum'' (ICZN 1991) and therefore a synonym of ''N. pelagicus'', while ''Gehyra ogasawarisimae'' is a misidentified ''L. lugubris''. The gecko ''Lepidodactylus lugubris'' is a parthenogenetic species also known to engage in female-female copulation. The species consists of a number of clonal genetic lineages thought to arise from different hybridization events. Surprisingly, parthenogenetic females of this species occasionally produce male offspring, which are thought to be the result of non-genetic hormonal inversions. While these males are anatomically normal, they produce abnormal
sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
and are sterile. The fecundity of both parthenogenetic and sexual races of the gekkonid lizard ''
Heteronotia binoei ''Heteronotia binoei'', also known commonly as the Bynoe's gecko, is a species of lizard in the family Gekkonidae. The species is endemic to Australia. One of Australia's least habitat-specific geckos, it occurs naturally across much of the cou ...
'' were compared. These races occur together in areas of the Australian arid zone. Under laboratory conditions, the parthenogenetic geckos had about a 30% lower fecundity than their sexual progenitors. Parthenotes are also found in two species of the
night lizard Night lizards (family Xantusiidae) are a group of small scincomorph lizards, averaging from less than to over snout–vent length. Most species are viviparous (live-bearing), with the exception of those in the genus '' Cricosaura''. The family ...
genus ''
Lepidophyma ''Lepidophyma'' is a genus of lizards, commonly called tropical night lizards. The genus ''Lepidophyma'' (Greek for "warty scales") is one of three genera of night lizards (family Xantusiidae), which are a group of viviparous (live-bearing) l ...
''. Unlike most parthenogenetic reptiles, ''Lepidophyma'' lizards show very low genetic
heterozygosity Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...
, suggesting a non-hybrid origin.


Snakes

The brahminy blindsnake is a triploid obligate parthenote and the only snake species known to be obligately parthenogenetic.


Facultative parthenogenesis

Facultative parthenogenesis is the type of parthenogenesis when a female individual can reproduce via both sexual and asexual reproduction.Bell, G. (1982). The Masterpiece of Nature: The Evolution and Genetics of Sexuality, University of California Press, Berkeley, pp. 1- 635 (see page 295). Females can produce viable offspring with or without genetic contribution from a male, and such an ability may, just like true parthenogens, enable colonization of new habitats by single female animals. Facultative parthenogenesis is extremely rare in nature, with only a few examples of animal taxa capable of facultative parthenogenesis, of which none are vertebrate taxa. Facultative parthenogenesis is often incorrectly used to describe cases of accidental or spontaneous parthenogenesis in normally sexual animals, including many examples in squamata. For example, many cases of accidental parthenogenesis in sharks, some
snakes Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more joi ...
,
Komodo dragon The Komodo dragon (''Varanus komodoensis''), also known as the Komodo monitor, is a member of the monitor lizard family Varanidae that is endemic to the Indonesian islands of Komodo, Rinca, Flores, and Gili Motang. It is the largest extant ...
s and a variety of domesticated birds were widely perpetuated as facultative parthenogenesis. These cases should, however, be considered accidental parthenogenesis, given the frequency of asexually produced eggs and their hatching rates are extremely low, in contrast to true facultative parthenogenesis where the majority of asexually produced eggs hatch. In addition, asexually produced offspring in vertebrates exhibit extremely high levels of sterility, highlighting that this mode of reproduction is not adaptive. The occurrence of such asexually produced eggs in sexual animals can be explained by a meiotic error, leading to automictically produced eggs.


Gynogenesis

Gynogenesis Gynogenesis, a form of parthenogenesis, is a system of asexual reproduction that requires the presence of sperm without the actual contribution of its DNA for completion. The paternal DNA dissolves or is destroyed before it can fuse with the egg. Th ...
is a form of asexual reproduction in which female eggs are activated by male sperm, but no male genetic material is contributed to offspring. While this mode of reproduction has not been observed in reptiles, it occurs in several salamander species of the genus '' Ambystoma''.


Hybridogenesis

Hybridogenesis is a variation of parthenogenesis in which males mate with females, but only the mother's genetic material is propagated by these offspring to their own young. While this form of reproduction has not been observed in reptiles, it does occur in frogs of the genus ''
Pelophylax ''Pelophylax'' is a genus of true frogs widespread in Eurasia, with a few species ranging into northern Africa. This genus was erected by Leopold Fitzinger in 1843 to accommodate the green frogs of the Old World, which he considered distinct fro ...
''.


Evolution


Origin

In all parthenogenetic reptile species studied to date, chromosomal evidence supports the theory that parthenogenesis arose through a
hybridization Hybridization (or hybridisation) may refer to: *Hybridization (biology), the process of combining different varieties of organisms to create a hybrid *Orbital hybridization, in chemistry, the mixing of atomic orbitals into new hybrid orbitals *Nu ...
event, although members of the genus ''
Lepidophyma ''Lepidophyma'' is a genus of lizards, commonly called tropical night lizards. The genus ''Lepidophyma'' (Greek for "warty scales") is one of three genera of night lizards (family Xantusiidae), which are a group of viviparous (live-bearing) l ...
'' may be exceptions to this rule. The original hybridization event is believed to occur between two related species and is often followed by backcrossing to either parent species to create triploid parthenogenetic offspring. As no crosses of two sexual species in captivity have ever produced parthenogenetic offspring, it is unclear how a hybridization event would actually lead to asexual reproduction. It is possible that parthenogenesis evolved as a way of overcoming sterility due to improper chromosomal pairing and segregation during meiosis in hybrids, and that rare hybrid individuals that could premeiotically duplicate their chromosomes could escape hybrid sterility by reproducing through parthenogenesis. The ability to premeiotically duplicate chromosomes would be selected for in this scenario as it would be the only option for successful reproduction.


Selective advantage

While it is often assumed that parthenogenesis is an inferior evolutionary strategy to sexual reproduction because parthenogenetic species lack the ability to complement genetic mutations through outcrossing or are unable to incorporate new genetic material, research on parthenogenetic species has gradually revealed a number of advantages to this mode of reproduction. Triploid unisexual geckos of the species ''
Heteronotia binoei ''Heteronotia binoei'', also known commonly as the Bynoe's gecko, is a species of lizard in the family Gekkonidae. The species is endemic to Australia. One of Australia's least habitat-specific geckos, it occurs naturally across much of the cou ...
'' have greater endurance and aerobic capacity than their diploid ancestors, and this advantage may be the result of polyploidy and a form of
hybrid vigor Heterosis, hybrid vigor, or outbreeding enhancement is the improved or increased function of any biological quality in a hybrid offspring. An offspring is heterotic if its traits are enhanced as a result of mixing the genetic contributions of ...
. It has also been observed that obligate parthenotes are often found at high altitudes and in sparse or marginal habitats, a pattern known as "geographical parthenogenesis," and their distribution in suboptimal territories may be a result of their increased colonization ability. A single parthenogenetic individual can colonize a new territory and produce offspring, while for a sexual species multiple individuals would need to occupy a new habitat and come into contact with each other for mating in order for successful colonization to occur. A parthenogenetic species can undergo a more rapid population increase than a sexual species because all parthenotes are female and produce offspring, while in sexual species half of all individuals are males and do not give birth to offspring. Additionally, laboratory experiments have revealed that even obligate parthenotes retain the capability of incorporating new genetic material through sexual reproduction to form new parthenogenetic lineages, and the ability to outcross on occasion may explain the lengthy evolutionary persistence of some parthenogenetic species.


References

{{reflist Vertebrate parthenogenesis