Do sex differences in autosomal recombination rates facilitate divergence?

Abstract

Recombination rate varies within and between individuals. One form of such variations is seen between sexes in dioecious populations, with males typically exhibiting a smaller recombination rate than females. This is true both for sex chromosomes and autosomes (so-called heterochiasmy). Although a large body of theory exists on the role of sex chromosomes in adaptation and population divergence, much less is known about the role of heterochiasmy. Recently, it has been suggested that heterochiasmy can facilitate local adaptation and divergence, but if, and when this is true has not been systematically studied theoretically to date. Here we use Individual-based simulations to assess the effect of sex differences in autosomal recombination rates on the process of divergence and adaptation in populations subject to divergent selection and migration. We found evidence supporting that sex differences in autosomal recombination rate between adaptive loci can facilitate, and especially maintain, divergence, but this is true only under very limited conditions, involving strong selection, high sex-averaged effective recombination rates and relatively high rates of migration compared to the strength of selection. We further found that this effect, when present, is typically weak but is amplified in cases of highly polygenic adaptation in comparison to cases with a few adaptive loci of strong effect. We conclude that, in most cases, sex differences in autosomal recombination rate alone are unlikely to noticeably contribute to the process of adaptation and divergence. Further studies are needed to evaluate their effect in combination with other processes not considered in the present study, such as assortative mating between the alike mates, or recombination suppression in heterozygotes. TeaserIn dioecious populations, recombination rate typically differs between males and females. This is true both for sex chromosomes and autosomes. While much theoretical research has focused on understanding how recombination rate differences in sex chromosomes shape local adaptation and divergence, we lack theoretical knowledge of the potential role of sex differences in autosomal recombination rates. Recombination has a dual role in local adaptation. Strong recombination can effectively purge deleterious alleles, but it can also break apart beneficial allele complexes (and vice versa for weak recombination). Thus, one may expect that in the presence of both strong and weak recombination exhibited by females, and males, respectively, population divergence can be efficiently facilitated. But is this true? Here, we study this question theoretically using computer simulations. Our main finding is that sex differences in autosomal recombination can facilitate divergence, but this effect is typically weak and present only under very stringent conditions.