REGIONAL SURVEY OF GENE FREQUENCIES IN THE MUD SNAIL NASSARIUS OBSOLETUS

¹ Department of Biology, Juniata College, Huntingdon, Pennsylvania 16652, and Department of Zoology, University of Reading, Reading, England

Six gene loci were characterized by polyacrylamide gel electrophoresis of 5 protein systems in the mud snail, Nassarius obsoletus. Snail populations collected at 11 sites along an Atlantic coast transect from Cape Cod, Massachusetts to Beaufort, North Carolina were surveyed for genetic variability at these loci. Four loci from malate dehydrogenase, tetrazolium oxidase, and leucine aminopeptidase systems are monomorphic for the same alleles througout the transect. Two loci from lactate dehydrogenase (Lc-1) and general protein ((Gp-1) systems are polymorphic in all populations.

The Lc-1 locus has 2 common and 1 rare codominant alleles and the Gp-1 locus segregates for 2 codominant alleles. Alleles of each locus segregate independently of those of the other. Chi-square homogeneity tests indicate homogeneity of allele and genotype frequency throughout the transect at both loci. All populations conform to Hardy-Weinberg equilibrium values, as do pooled populations without indication of Wahlund's effect. Populations partitioned into size (and hence age) classes are also homogeneous in allele frequency.

The remarkable geographic homogeneity of allele frequency may be due to (1) mechanisms of balanced polymorphism that are insensitive to local variations in environment, or (2) extensive gene flow. The potentialities of widespread pelagic dispersal in N. obsoletus argue strongly for gene flow; however the 2 agents of gene stablization are mutually complementary and may have acted in concert throughout populations.