Sexual Conflict over the Maintenance of Sex: Effects of Sexually Antagonistic Coevolution for Reproductive Isolation of Parthenogenesis

Kazutaka Kawatsu,Wayne M Getz

doi:10.1371/journal.pone.0058141

Abstract

Sexual reproduction involves many costs. Therefore, females acquiring a capacity for parthenogenetic (or asexual) reproduction will gain a reproductive advantage over obligately sexual females. In contrast, for males, any trait coercing parthenogens into sexual reproduction (male coercion) increases their fitness and should be under positive selection because parthenogenesis deprives them of their genetic contribution to future generations. Surprisingly, although such sexual conflict is a possible outcome whenever reproductive isolation is incomplete between parthenogens and the sexual ancestors, it has not been given much attention in the studies of the maintenance of sex. Using two mathematical models, I show here that the evolution of male coercion substantially favours the maintenance of sex even though a female barrier against the coercion can evolve. First, the model based on adaptive-dynamics theory demonstrates that the resultant antagonistic coevolution between male coercion and a female barrier fundamentally ends in either the prevalence of sex or the co-occurrence of two reproductive modes. This is because the coevolution between the two traits additionally involves sex-ratio selection, that is, an increase in parthenogenetic reproduction leads to a female-biased population sex ratio, which will enhance reproductive success of more coercive males and directly promotes the evolution of the coercion among males. Therefore, as shown by the individual-based model, the establishment of obligate parthenogenesis in the population requires the simultaneous evolution of strong reproductive isolation between males and parthenogens. These findings should shed light on the interspecific diversity of reproductive modes as well as help to explain the prevalence of sexual reproduction.

Highlights

Sex is an inefficient way to reproduce as it involves many costs [1,2,3] and should be vulnerable to invasions by parthenogenetic reproduction
Because frequency of parthenogenetic reproduction in the population depends on the level of reproductive isolation, the values of male coercion and female barrier are associated with the densities of males and female or population sex ratio, and will affect selection pressure for the two traits in the generation
The analysis of adaptive-dynamics model demonstrates that the resultant antagonistic coevolution for reproductive isolation fundamentally results in one of the two outcomes depending on the value of male potential reproductive rate (PRR), and males persist under both outcomes

Summary

Introduction

Sex is an inefficient way to reproduce as it involves many costs [1,2,3] and should be vulnerable to invasions by parthenogenetic reproduction. The fact, is paradoxical: obligate sex prevails among anisogamous animals [5,6], in which males produce smaller gametes by definition. Most studies have focused on genetic mixing or recombination that is specific to sexual reproduction, and such theories showed that recombination does confer some benefits on sexual species [7,8,9]. Recent advances revealed that the benefits are insufficient to prevent the invasions of asexuality [11,12], and the maintenance of sexual reproduction under the cost-of-males condition remains an enigma in evolutionary biology [1,2,13]

Methods

Results

Conclusion