Origin, Phylogeography and Evolutionary Ecology of a Gynogenetic Species (Poecilia formosa) and its Sexual Hosts (P. latipinna and P. mexicana)
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Unisexual (all female) vertebrates are extremely rare hybrids that comprise about 1% of all extant vertebrate species. Many selective forces act against unisexual individuals, and the persistence of these species, which reproduce via complete or modified parthenogenesis, has been of interest to evolutionary biologists for many decades. Theoretically, given their mode of reproduction, and, therefore, the lack of genetic recombination, unisexual vertebrate lineages should be unable to respond quickly to selective pressures, as well as, accumulate deleterious mutations over time. Additionally, when the unisexual species is sperm-dependent (relies on males of a closely related species to reproduce but does not recombine the genomes), additional pressures are added. For example, these females need to compete with both conspecific and heterospecific females to access mates, and, at the same time, selection should act on heterospecific males to avoid interspecific matings. Additionally, in the case of sperm-dependent unisexual females, the usually considered benefit of having a two-fold reproductive advantage of asexuals over sexually reproducing individuals becomes a cost. If resource differentiation has not occurred between the asexual hybrid and the sexual species (usually a parent species), then the fast population growth of the asexual population could cause asexual individuals to competitively exclude sexual individuals from resources. If this occurs, the asexual species will soon go extinct because males will be no longer available.
Unisexual-bisexual mating complexes, which are composed by a sperm-dependent unisexual species and its sexual host, are present in several taxa and have persisted longer than it would be predicted by theory. Several non-mutually exclusive hypotheses have been proposed to provide mechanisms that aid the maintenance of these rare mating complexes. The work herein relates the results of experiments that were performed to test these hypotheses using the unisexual-bisexual mating system of Poecilia formosa –P. mexicana and P. latipinna as a study system. No empirical support was found for the hypotheses, however, interesting and surprising aspects of the genomic architecture of P. formosa were discovered. The persistence of P. formosa might be due to its high genetic variation and might not be as paradoxical as theory would predict.