Can sexual conflict drive transitions to asexuality? Female resistance to fertilization in a facultatively parthenogenetic insect.

Facultatively parthenogenetic animals could help reveal the role of sexual conflict in the evolution of sex. Although each female can reproduce both sexually (producing sons and daughters from fertilized eggs) and asexually (typically producing only daughters from unfertilized eggs), these animals often form distinct sexual and asexual populations. We hypothesized that asexual populations are maintained through female resistance as well as the decay of male traits. We tested this via experimental crosses between individuals descended from multiple natural sexual and asexual populations of the facultatively parthenogenic stick insect Megacrania batesii. We found that male-paired females descended from asexual populations produced strongly female-biased offspring sex ratios resulting from reduced fertilization rates. This effect was not driven by incompatibility between diverged genotypes but, rather, by both genotypic and maternal effects on fertilization rate. Furthermore, when females from asexual populations mated and produced sons, those sons had poor fertilization success when paired with resistant females, consistent with male trait decay. Our results suggest that resistance to fertilization resulting from both maternal and genotypic effects, along with male sexual trait decay, can hinder the invasion of asexual populations by males. Sexual conflict could thus play a role in the establishment and maintenance of asexual populations.