Differences in Competitive Efficiency between a Sexual Parasite and Its Host Provide Insight into the Maintenance of a Sperm-Dependent Vertebrate Species

Abstract

ABSTRACT Theory predicts that unisexual females have a two-fold reproductive advantage over sexual species because of the benefit of not producing males. Consequently, unisexuals species should competitively exclude sexual species whenever their niches overlap substantially. When the unisexual species is sperm-dependent (gynogenetic), competitive exclusion of the host sperm donor can lead to extinction of both species. Thus, persistent gynogenetic-sexual species complexes are paradoxical. Models examining competition predict that gynogentic species will persist if the sexually reproducing species are more efficient at competing for resources than the unisexual species. We tested these models using the persistent complex of the gynogenetic Poecilia formosa and its parental, sexual host, Poecilia latipinna. We examined the effect of conspecific and heterospecific females on foraging abilities of P. latipinna and P. formosa. When foraging with a conspecific, the foraging ability was similar between the two species, whereas the presence of P. latipinna negatively affected the foraging ability of the gynogenetic P. formosa. Conversely, the presence of P. formosa did not have any effect on the foraging ability of its sexual host. These results are consistent with previously proposed models on the maintenance of unisexual-bisexual mating systems and provide further explanation for the maintenance of unisexual, sperm-dependent species.