Breaking the parthenogenesis fertilization barrier: direct and indirect selection pressures promote male fertilization of parthenogenetic females

Abstract

All else being equal, females would double their fecundity through parthenogenetic reproduction. On the other hand, males should be subject to positive selection pressures to coerce parthenogenetic females into sexual fertilization, because the twofold advantage of parthenogenesis is achieved at the expense of the male genetic contribution. Interestingly, although male coercion superficially imposes the cost of sex on parthenogenetic females, it would confer a reproductive benefit even on the parthenogens. This is because females fertilized by coercive males gain indirect reproductive success via sons inheriting coercion, who would succeed in mating with other parthenogens in the next generation (sons’ effect). In this study, using two mathematical models, I show for the first time that the indirect sons’ effect of male coercion plays an important role in the maintenance of sex in potentially parthenogenetic species. The first model, which compares the fitness of a female reproducing parthenogenetically with that of a female mating with a coercive male, demonstrates that the sons’ effect can outweigh the cost of sex and resolve sexual conflict over reproductive modes. My second model of population genetics, which analyses the dynamics of coercion and parthenogenesis, shows that the occurrence of parthenogenetic reproduction is suppressed in the presence of the sons’ effect of male coercion. These results indicate that the sons’ effect of male coercion helps to maintain sexual reproduction at an evolutionary time scale, as well as offset the twofold cost of males in the invasion phase of the coercion.