Constraint and divergence in the evolution of male and female recombination rates in fishes.

Recombination is a fundamental feature of sexual reproduction across eukaryotes, yet recombination rates are highly variable both within and between species. In particular, sex differences in recombination rate between males and females (heterochiasmy) are more often the rule than the exception, but despite the prevalence of heterochiasmy the ultimate causes of global patterns of heterochiasmy remain unclear. Here, we assemble a comprehensive dataset of sex-specific recombination rate estimates for 61 fish species, and combine this with information on sex determination, fertilization mode, and sexual dimorphism to test competing theories for the causes and evolution of heterochiasmy. We find that sex differences in recombination rate are evolutionary labile, with frequent shifts in the direction and magnitude of heterochiasmy. This rapid turnover does not appear to be driven by simple neutral processes and is inconsistent with nonadaptive explanations for heterochiasmy, including biological sex differences in meiosis. Although patterns of heterochiasmy across the phylogeny indicate a potential role for adaptive processes, we are unable to directly link variation in heterochiasmy with proxies for sexual selection or sexual conflict across species, indicating that these effects-if present-are either subtle or complex. Finally, we show evidence for correlated rates of recombination rate evolution between males and females, indicating the potential for genetic constraints and sexual conflict over the recombination landscape.