POSITIVE SELECTION AND SEQUENCE REARRANGEMENTS GENERATE EXTENSIVE POLYMORPHISM IN THE GAMETE RECOGNITION PROTEIN BINDIN

Bindin is a gamete recognition protein of sea urchins that mediates sp ecies-specific attachment of sperm to an egg-surface receptor during f ertilization. Sequences of bindin from closely related urchins show fi xed species-specific differences. Within species, highly polymorphic b indin alleles result from point substitution, insertion/deletion, and recombination. Since speciation, positive selection favoring allelic v ariants has generated diversity in bindin polypeptides. Intraspecific bindin variation can be tolerated by the egg receptor, which suggests functional parallels between this system and other flexible recognitio n systems, including immune recognition. These results show that polym orphism in mate recognition loci required for rapid evolution of sexua l isolation can arise within natural populations.