The molecular evolution of sperm bindin in six species of sea urchins (Echinoida: Strongylocentrotidae)

The acrosomal protein bindin attaches sperm to eggs during sea urchinfertilization. Complementary to ongoing functional biochemical studies, Itake a comparative approach to explore the molecular evolution of bindin ina group of closely related free-spawning echinoid species. Two alleles ofthe mature bindin gene were sequenced for each of six species in the seaurchin family Strongylocentrotidae. The nucleotide sequences diverged by atleast 1% per Myr at both silent and replacement sites. Two short sectionsflanking the conserved block show an excess of nonsynonymous substitutions.Each is homologous to a region that had been identified as a target ofselection in other sea urchin comparisons. A large proportion of thebindin-coding sequence consists of a highly variable repeat region. Bindinsequences, even including the large intron, could not resolve the branchingorder among five of the species.