The evolutionary persistence of the gynogenetic Amazon molly, Poecilia formosa.

摘要

One of the central questions in evolutionary biology over the last two decades has been the persistence of sexual reproduction. Theoretical studies have identified a range of genetic and ecological mechanisms that favor sexual reproduction and predict that asexually reproducing lineages should not persist over evolutionary time. Gynogenetic lineages are unisexual (all-female), reproduce asexually, and face an additional obstacle to evolutionary persistence; reproduction is dependent on sperm from a related sexual species. Therefore a gynogenetic lineage must coexist with a species it can parasitize sexually. Simple population models incorporating reproductive success and mate discrimination predict gynogenetic lineages should rapidly become extinct. More complex population models predict evolutionary persistence in the presence of ecological niche-partitioning or density-dependent mating. Poecilia formosa, the Amazon molly, is a gynogenetic lineage of livebearing fish that coexists with the sexual species, P. mexicana or P. latipinna. This thesis describes three studies that empirically investigate factors that could affect evolutionary persistence of gynogenetic Poecilia formosa : mate discrimination, interspecific competition for mates, and frequency-dependent reproductive success. The first study asks if the ability of P. formosa to attract heterospecific P. latipinna and P. mexicana males is related to its hybrid origin or is a result of coevolution as posited by previous researchers. The results from laboratory behavioral tests indicate mate attraction ability is present at the moment of hybridization and it is not necessary to assume a coevolutionary process. The second study measures antagonistic behavior of P. formosa and P. mexicana females and shows they do not differ. The final study uses semi-natural experimental ponds to ask whether reproductive success of P. formosa is negatively frequency-dependent and if it is sufficient to explain coexistence of P. formosa with its P. latipinna host. The results indicate when frequency of P. formosa is high, its reproductive success is low and vice versa. This causes their frequency in a population to oscillate, which prevents extinction of themselves and their hosts. This is the first empirical study to show the importance of frequency-dependent reproductive success in the evolutionary persistence of gynogenetic lineages.