Positive Darwinian selection in gamete recognition proteins of Strongylocentrotus sea urchins

Gamete recognition proteins commonly experience positive Darwinian selection and evolve more rapidly than nonreproductive proteins, but the selective forces responsible for their adaptive diversification remain unclear. We examined the patterns of positive selection in the cognate interacting pair of proteins formed by sperm bindin and its egg receptor (EBR1) and in two regions of the sea urchin sperm receptor for egg jelly suREJ3 gene (exons 22 and 26) among four species of Strongylocentrotus sea urchins (S.similar to purpuratus, S.similar to droebachiensis, S.similar to pallidus and S.similar to franciscanus). The signatures of selection differed at each reproductive protein. A strong signal of positive selection was detected at bindin in all lineages even though the species compared had highly variable gamete traits and experience different intensities and forms of sexual selection and sexual conflict in nature. Weaker selection was observed at EBR1 but the small region studied precluded a clear understanding of the extent of sexual conflict between bindin and the EBR1 protein. At the suREJ3 locus, diversifying selection was observed in exon 22 but not exon 26, suggesting that these regions experience different selective pressures and evolutionary constraints. Positive selection was also detected within S.similar to pallidus at suREJ-22 because of the presence of 12 amino acid replacement mutations segregating at frequencies >0.10. Our results suggest that sexual conflict may be the predominant evolutionary mechanism driving the rapid diversification of reproductive proteins between, and polymorphism within, strongylocentrotid sea urchins.