Sexual conflict as a constraint on asexual reproduction: an empirical review

Theory predicts that facultatively asexual animals, which can leverage the advantages of both sexual and asexual reproduction, should outcompete obligately sexual and obligately asexual animals. Yet, paradoxically, obligate sexual reproduction predominates in many animal lineages, while the most flexible form of facultative asexuality (i.e. facultative parthenogenesis) appears to be rare. Recent theoretical work suggests that sexual conflict could help to resolve this paradox. Males that coercively fertilise females' eggs may, in the process, prevent alleles for parthenogenesis from spreading by limiting opportunities for asexual reproduction. Coercive males may also inhibit asexual reproduction by making resistance to sex disproportionately costly for females. In this review, we outline evidence of interactions with males that could impose costs on parthenogenetic females or hinder their ability to reproduce parthenogenetically in diverse animal taxa. The evidence suggests that such interactions between the sexes have the potential to mediate sexual conflict over mating and reproductive mode, both within facultative species and between closely related sexual and asexual taxa. However, the relative costs of sex and parthenogenesis are clearly context dependent, and much remains unknown. The most direct evidence for male inhibition of parthenogenesis comes from stick insects, but several other systems offer promising avenues for further investigation. Further research on the costs of mating and resistance in such systems could shed light on the reasons for the puzzling rarity of facultative parthenogenesis in nature.