PROTEIN POLYMORPHISMS IN THE HARD CLAMS MERCENARIA MERCENARIA AND MERCENARIA CAMPECHIENSIS

¹ Environmental Protection Agency, National Marine Water Quality Laboratory, West Kingston, Rhode Island 02892

Four gene loci were characterized by polyacrylamide disc-gel electrophoresis of four enzymes in the hard clams, M. mercenaria and M. campechiensis. Four populations of M. mercenaria and two populations of M. campechiensis were selected from a transect between Prince Edward Island, Canada and Tampa Bay, Florida. Three of the four enzymes were polymorphic in all six populations. M. mercenaria averaged 2.6 alleles per locus for four populations while M. campechiensis averaged 3.5 alleles per locus for two populations. Malate dehydrogenase—NAD form (MDH) had a single allele predominating in all populations. A second rare allele appeared in the Maine and South Carolina populations of M. mercenaria. Polymorphism was pronounced at an esterase (EST) locus but the data showed no patterned shift along the transect. North to south clines were observed at both the tetrazolium oxidase (TO) and the lactate dehydrogenase (LDH) loci. The different patterns observed for each locus suggest a complex of selective processes probably of differing adaptive significance. In the portion of the gene pools studied these species were found to have many alleles in common (12 of 16). Commonality of alleles, homology of chromosomes, ease of hybridization, and intergrades in the field suggest that these species have not yet reached reproductive isolation.