Polymorphism in abalone fertilization proteins is consistent with the neutral evolution of the egg's receptor for lysin (VERL) and positive Darwinianselection of sperm lysin

The evolution of species-specific fertilization in free-spawning marine inv ertebrates is important for reproductive isolation and may contribute to sp eciation. The biochemistry and evolution of proteins mediating species-spec ific fertilization have been extensively studied in the abalone (genus Hali otis). The nonenzymatic sperm protein lysin creates a hole in the egg vitel line envelope by species-specifically binding to its egg receptor VERL. The divergence of lysin is promoted by positive Darwinian selection. In contra st, the evolution of VERL does not depart from neutrality. Here, we cloned a novel nonrepetitive region of VERL and performed an intraspecific polymor phism survey for red (Haliotis rufescens) and pink (Haliotis corrugata) aba lones to explore the evolutionary farces affecting VERL. Six statistical te sts showed that the evolution of VERL did not depart from neutrality. Inter estingly, there was a subdivision in the VERL sequences in the pink abalone and a lack of heterozygous individuals between groups, suggesting that the evolution of assortative mating may be in progress. These results are cons istent with a model which posits that egg VERL is neutrally evolving, perha ps due to its repetitive structure, while sperm lysin is subjected to posit ive Darwinian selection to maintain efficient interaction of the two protei ns during sperm competition.