MITOCHONDRIAL-DNA AND BINDIN GENE SEQUENCE EVOLUTION AMONG ALLOPATRICSPECIES OF THE SEA-URCHIN GENUS ARBACIA

Sea urchins of the genus Arbacia (order Stirodonta) have discontinuous allopatric distributions ranging over thousands of kilometers. Mitoch ondrial DNA (mtDNA) sequences were used to reconstruct phylogenetic re lationships of four Arbacia species and their geographic populations. There is little evidence of genetic structuring of populations within species, except in two cases at range extremes. The mtDNA sequence dif ferentiation between species suggests that divergence occurred about 4 -9 MYA. Gene sequences encoding the sperm protein bindin and its intro n were obtained and compared with the mtDNA phylogeny. Sea urchins amo ng the well-studied echinoid order Camarodonta, with degrees of mtDNA divergence similar to those of Arbacia species, are known to have rema rkable variation in bindin. However, in Arbacia, little variation in d educed amino acid sequences of bindin was found, indicating that purif ying selection acts on the protein. In contrast, bindin intron sequenc es showed much differentiation, including numerous insertion/deletions . Fertilization experiments performed between a divergent pair of Arba cia species from the Atlantic and Pacific Oceans revealed no evidence of blocks to gamete recognition. In Arbacia, fertilization specificiti es may have evolved relatively slowly as a result of extensive gene fl ow within species, greater functional constraint on the bindin polypep tide, or reduced selective pressure for species recognition in singly occurring species.