How Are Marine Invertebrate Sperm Surface Proteins Different from Immune System Genes?
In sea urchins, the sperm surface protein bindin mediates binding to the egg. The protein is not only highly divergent between species but also highly polymorphic within species.36-38 The high polymorphism within species could suggest a situation of sexual conflict with male bindin proteins being in a state of male-to-male competition for the eggs. In a sexual conflict scenario where males control the results of fertilization, the bindin proteins for which females have not yet coevolved proper defense would be more successful. However, Palumbi39 showed that sea urchins could be grouped into different clades according to the sequence of their bindin alleles, and that males are more efficient at fertilizing eggs of the same genotype, suggesting female cryptic choice scenario as opposed to sexual conflict. Male-to-female coadaptation within the population, where different bindin alleles work better with akin partners, apparently have maintained high polymorphism and the system seems to be
