Lack of Character Displacement in the Male Recognition Molecule, Bindin, in Altantic Sea Urchins of the Genus Echinometra

Bindin, a protein involved in sea urchin sperm-egg recognition and adhesion, is under positive selection in genera with sympatric species but evolves neutrally in genera in which all species are allopatric. This pattern has led to suggestions that reinforcement may be the source of the observed selection. Reproductive character displacement, or increased divergence of reproductive characters in areas where closely related species overlap, is often a consequence of reinforcement and has been shown to be present in one Indo-Pacific species of the genus Echinometra. In the Atlantic species of the same genus, positive selection has been shown to act on bindin of Echinometra lucunter. To examine whether the source of this selection is reinforcement, we determined variation on the first exon of bindin in E. lucunter in the Caribbean, where it is sympatric with Echinometra viridis, and in the rest of the Atlantic, where E. viridis is absent. There was no differentiation between bindin sequences from the two geographic regions; similar levels of positive selection were found to be acting in both areas. The similarities were not due to gene flow; mitochondrial DNA from the two regions indicates that E. lucunter populations most likely originated in the Atlantic and have not exchanged genes with Caribbean populations for approximately 200,000 years. The lack of evidence of stronger selection on bindin of E. lucunter in areas of sympatry with its sister species suggests that the source of selection is not reinforcement. Processes acting within species, such as sexual selection, sperm competition, or sexual conflict, are more likely to be involved in the evolution of this molecule.