Abstract
Sex chromosomes carry the sex-determining locus, causing them to be differently transmitted to and from females and males. These differences lead them to be selected upon in different ways, and hence they are predicted to become enriched for sexually- and parentally-antagonistic genes. Sexually-antagonistic genes have opposing fitness effects in females versus in males; parentally-antagonistic genes have opposing fitness effects when inherited maternally versus paternally. Sexually-antagonistic selection can drive sex determination transitions, whereby an autosome pair becomes a sex chromosome pair in lieu of the ancestral sex chromosomes. Whether parentally-antagonistic selection can similarly drive sex determination transitions remains unknown. I present a model to investigate the potential for transitions in sex determination through parentally-antagonistic selection as compared to sexually-antagonistic selection. This model assumes an ancestral sex-chromosomal sex-determining locus linked to a parentally- or sexually-antagonistic gene, and an autosomal parentally- or sexually-antagonistic gene in whose vicinity a novel sex-determining gene arises. I find that parentally-antagonistic selection can promote the spread of novel sex-determining genes as well as maintain ancestral sex-determining genes when the invasion of the novel sex-determining gene would involve transitions from male to female heterogamety (or vice versa), similar to sexually-antagonistic selection. Transitions between male and female heterogamety are, however, more likely when the ancestral sex-determining locus is linked to a parentally-antagonistic locus. Consequently, parentally-antagonistic selection can enable some highly unusual evolutionary patterns not encountered in other evolutionary models of sex determination. These results provide novel insights into why some sex-determining mechanisms may be so evolutionary labile.