Selection and Demographic History Shape the Molecular Evolution of the Gamete Compatibility Protein Bindin in Pisaster Sea Stars

Reproductive compatibility proteins have been shown to evolve rapidly under positive selection leading to reproductive isolation, despite the potential homogenizing effects of gene flow. I characterize the gene that encodes the gamete compatibility protein, bindin, for three broadcast spawning sea star species in the genus Pisaster, in a species-level comparison of bindin gene structure and molecular evolution. I then use phylogeographic patterns and variation in life history characteristics between P. ochraceus and P. brevispinus to test predictions about selection acting on bindin divergence among conspecific populations. I discover that divergence in the repetitive bindin domain structure may be partly influenced by concerted evolution within species. I find modest evidence of positive selection acting on P. ochraceus bindin alleles that is consistent with sexual conflict favoring selection for intraspecific bindin polymorphism, and can be explained in a demographic context of recent population expansions, coupled with the homogenizing effects of concerted evolution.