Unisexual Vertebrate Species Can Be Ancient

Snake species in many families have been shown to be capable of facultative parthenogenesis, as have some lizards (Varanus). Only one species of snake has been shown to be an all female species, the Flower Pot Snake or Brahminy blindsnake, Ramphotyphlops braminus. But there is another suspected of being an obligate parthenospecies, Dussumier’s Mud Snake, Enhydris dussumierii, a homalopsid from southern India (Murphy, 2007).

Relatively few organisms have abandoned the ability to reproduce sexually. It has long been thought that genetic recombination during meiosis was important in preventing species from accumulating harmful mutations and increasing the probability of survival. However, about 90 species of vertebrates are known to be unisexual, that is reproduction occurs only asexually. A few fish, some amphibians and some squamates are known to be unisexual. Conventional wisdom suggested these were dead end lineages, genetic lines that would be short-lived. Unisexual species were thought to be quite young, formed in an instant through hybridization of closely related species. And, they were considered to be accidental - not every hybridization event could produce a new species. Many of the unisexual species, particularly the whiptail lizards, Aspidoscelis (formerly Cnemidophorus), are obligate pathenospecies, so those harmful mutations will accumulate over time. They were also thought to be maladapted for competing with their parent species and congeners and therefore would be short-lived species in evolutionary time.

Now, Ke Bi and James Bogart (2010) from the University of Guelph, in Ontario have examined the complete mitochondrial genomes as well as markers of a unisexual forms related to Barbour’s Mole Salamander, Ambystoma barbouri. The results supported a monophyletic lineage for the unisexual form that was derived from A. barbouri in western Kentucky. The molecular clock suggests the unisexual lineage originated in the Pliocene, about 5 MYA. Long-lived unisexual species create some confusion.

Lampert and Scharti (2010) asked the question, are the hypotheses about the consequences of absence of genetic recombination wrong, or are the age estimates in error? They suggest the ideas about gene recombination and the dates are both most likely correct and that the solution lies in the specialized ways unisexual vertebrates have circumvented the loss of meiotic recombination in their nuclear genomes. They describe female parthenogenesis as “leaky” and note three ways in which new genetic material can enter a parthenogenetic lineage. In gynogenesis meiotic reduction of chromosomes does not occur, but sperm are needed to trigger embryonic growth, and while the paternal DNA is usually excluded, it is not 100% effective. In a related process, kleptogenesis the maternal genome is on occasion exchanged for the paternal genome, these produce triploid individuals with one set of chromosomes from one parent and two sets from the other. In hybridogenesis haploid oocytes are produced without meiosis and the egg is fertilized by sperm from a closely related species but the male genome is only present in a single generation, and then is excluded from the next generation.

References

Bi, K. and J. P. Bogart. 2010. Time and time again: unisexual salamanders (genus Ambystoma) are the oldest unisexual vertebrates. Biology BMC 2010 10:238.

Lampert, K. P. and M. Scharti. 2010. A little bit is better than nothing: the incomplete parthesogensis of salamanders, frogs and fish. BMC Biology 2010, 8:78.

Murphy, J.C. 2007. Homalopsid Snakes, Evolution in the Mud. Malabar: Krieger Publishing.