Recombination on 'non-recombining' sex chromosomes: empirical insights from the blackspotted stickleback

One hallmark of sex chromosome evolution is recombination suppression driven by inversions. The resulting non-recombining sex-linked regions accumulate deleterious mutations and functionally degenerate. The question therefore arises: why does selection not favor the reestablishment of recombination? Two (non-exclusive) explanations for this have been proposed: 1) selection to maintain linkage between the Primary Sex Determination Locus (PSDL) and sexually antagonistic loci - loci with alleles that are beneficial to one sex but detrimental to the other; and 2) the rarity of reversions - structural mutations that reverse the inversion that initially suppressed recombination. Empirical perspectives are sorely needed to evaluate the relevance of these models. Here, we characterised the intermediate stages of sex chromosome evolution in the blackspotted stickleback (Gasterosteus wheatlandi). We generated high-quality, phased assemblies and annotations for the X and Y chromosomes, as well as population-level sequencing. Using these data, we inferred distinct recombination loss events and their associated inversions. Most significantly, we find support from multiple lines of evidence for a [~]2 Mb recombination event that likely occurred via double crossover within an existing non-recombining inversion. This result highlights that rare recombination can occur within sex-linked inversions. Simulations suggest that such histories of gene flux may allow for a novel empirical framework, adapted from work on autosomal inversions, to infer evolutionary processes at work within sex-linked non-recombining regions and distinguish between models to explain the maintenance of recombination suppression on sex chromosomes. Significance StatementSex chromosomes have evolved countless times across eukaryotes and often lose the ability to recombine, causing them to degenerate over time. Why then, does recombination not restart to purge harmful mutations? By generating a high-quality genome assembly of the blackspotted stickleback, we discovered the strong empirical evidence of a sex chromosome undergoing a double-crossover event within a "non-recombining" region, which reset the degenerative process. This finding challenges the view that sex chromosome decay is a one-way street. Furthermore, using forward evolutionary simulations, we demonstrate that the genetic signatures left by such events could help identify the selective forces acting within non-recombining sex chromosomes, thereby explaining the maintenance of recombination suppression.