The effects of sexual selection on functional and molecular reproductive divergence during experimental evolution in seed beetles

Sexual selection can be an engine of divergent evolution between closely related lineages, as a result of idiosyncratic coevolution of male and female reproductive traits. The possibility that this can contribute to speciation has ample support from comparative studies but very few experimental evolution studies have addressed the role of sexual selection in very early stages of divergent evolution. Here, we use experimental evolution to study divergent evolution between replicate lines of the seed beetle <i>Acanthoscelides obtectus</i> evolving under strong or weak sexual selection for >190 generations. We first confirm that the experimental regimes employed resulted in marked differences in the strength of sexual selection. We then indirectly assess the degree of divergent evolution of those male and female traits that affect postmating sexual selection, by crossing replicate lines. We find that lines evolving under strong sexual selection are more divergent in reproductive traits, as evidenced by a stronger male × female interaction for male sperm competition success. Finally, we assess the degree of divergent evolution in the expression of candidate genes for male seminal fluid proteins and female reproductive proteins. We find that lines evolving under strong sexual selection are more divergent in the expression of reproductive proteins, providing a possible causal mechanism contributing to the results seen in the reproductive phenotype. Our findings provide evidence for more divergent evolution of reproductive traits under stronger sexual selection, in line with the tenet that sexual selection may promote divergence even in the absence of environmental differences between populations.