An Ecological Model for the Maintenance of Sex and Geographic Parthenogenesis

This paper discusses the maintenance of sexual reproduction, using a model that considers a heterogeneous environment subdivided into patches, with the fitness of sexual and asexual species varying among them. Each patch produces a proportion of the total offspring, which are then randomly dispersed to all patches, where selection occurs. The sufficient conditions for coexistence is described, in terms of rates of increase and intra-specific and inter-specific competition coefficients. The paper also considers the circumstances under which sex can spread when it is rare or resist invasion when it is common. Finally, the phenomenon of geographic parthenogenesis is addressed and different patterns of coexistence that can account for this empirically observed phenomenon is discussed. The reproductive advantage conferred by asexual reproduction will be overcome if the greater genetic diversity of the sexual species is translated into a decrease in the strength of intra-specific interactions and/or a reduced competitive effect of asexuals on sexuals. The competitive edge conferred by sexual reproduction does not have to hold for all patches; what is required is a competitive edge at least in those patches that contribute the most to population growth. Coexistence between closely related sexual and asexual species is possible even under strong inter-specific competition. This result contrasts with those obtained from previous ecological models on the maintenance of sex, all of which require reduced competitive interactions for coexistence. Geographic parthenogenesis will occur if one of the species is favored in one of the geographic locations while the other is able to maintain itself at high density in the other geographic location, either through competitive abilities (sexuals) or reproductive abilities (asexuals). The pattern of geographic parthenogenesis most likely to occur in nature is that of partial asexual refugium, where asexuals are excluded from some of the geographic locations but remain at high density in locations where the strength of inter-specific interaction is not too high.