The coevolution of sex-specific dominance and allelic diversity at sexually antagonistic loci

Sexually antagonistic (SA) selection can promote genetic diversity. Current theory predicts biallelic polymorphism in SA loci primarily under strong selection or dominance reversal between the sexes. Yet, selection is often weak, several candidate SA loci harbour more than two alleles, and the prevalence of dominance reversal remains unclear. We present a model in which allelic effects at a quantitative-trait locus coevolve with sex-specific promoter affinities that determine allelic dominance, under distinct female and male phenotypic optima. We show that gradual coevolution between allelic values and promoter affinities can generate a positive feedback between beneficial sex-specific dominance and allelic divergence. Importantly, this feedback does not require an established balanced polymorphism, but can be initiated during selective sweeps, during evolutionarily transient protected polymorphism, or from variation maintained by mutation-selection-drift balance. Under weak or asymmetric selection, repeated diversification can generate polyallelic polymorphism, with several alleles forming dominance hierarchies that reduce segregation load. These results hold for both autosomal and X-linked loci and do not require complete dominance reversal. To assess these findings, we analyse segregating genetic variation in three insect populations and find stronger signals of polyallelic polymorphism in candidate SA loci and those showing sex-specific dominance, while genes with the strongest signals are enriched for functions associated with known SA phenotypes.