ABSTRACT
Recently, the wide spread Indo-west Pacific sea urchin, Echinometra mathaei, has been shown to consist of at least 4 separate cryptic species, distributed patchily throughout the tropical Pacific. On most islands, two or more closely related Echinometra species can be found in the same habitat. Between these sympatric Echinometra, reproductive isolation is due to failure of eggs and sperm from different species to cross fertilize. We have isolated and sequenced the gene for binding, a protein involved in sperm egg attachment in sea urchins, and have compared the sequences among newly formed Echinometra species. Between species, binding sequences are very different at both nucleotide and amino acid levels. There are a large number of insertion and deletion mutations, and a greater number of replacement substitutions than silent substitutions. Each species shows several unique binding mutations. However, the binding sequences are also highly polymorphic within species. Even at the amino acid level, different binding alleles within a population can be very different from one another.
