Mitochondrial DNA and bindin gene sequence evolution among allopatric species of the sea urchin genus Arbacia

Sea urchins of the genus Arbacia (order Stirodonta) have discontinuousallopatric distributions ranging over thousands of kilometers.Mitochondrial DNA (mtDNA) sequences were used to reconstruct phylogeneticrelationships of four Arbacia species and their geographic populations.There is little evidence of genetic structuring of populations withinspecies, except in two cases at range extremes. The mtDNA sequencedifferentiation between species suggests that divergence occurred about 4-9MYA. Gene sequences encoding the sperm protein bindin and its intron wereobtained and compared with the mtDNA phylogeny. Sea urchins among thewell-studied echinoid order Camarodonta, with degrees of mtDNA divergencesimilar to those of Arbacia species, are known to have remarkable variationin bindin. However, in Arbacia, little variation in deduced amino acidsequences of bindin was found, indicating that purifying selection acts onthe protein. In contrast, bindin intron sequences showed muchdifferentiation, including numerous insertion/deletions. Fertilizationexperiments performed between a divergent pair of Arbacia species from theAtlantic and Pacific Oceans revealed no evidence of blocks to gameterecognition. In Arbacia, fertilization specificities may have evolvedrelatively slowly as a result of extensive gene flow within species,greater functional constraint on the bindin polypeptide, or reducedselective pressure for species recognition in singly occurring species.