Genetic variability and gene flow in populations of Crassostrea virginica (Gmelin) from the northern Gulf of Mexico

Allelic variation at 23 presumptive gene loci was assessed among 10 populations of Crassostrea virginica from the northern Gulf of Mexico, extending from Biloxi Bay, Mississippi, westward to Galveston Bay, Texas. Ten of the 23 loci surveyed proved to be monomorphic across all population samples. The percentage of polymorphic loci among populations ranged from 30.4-39.1 with a mean of 34.8. Allelic variation at 6 gene loci, Aat-1, Icdh-1, Icdh-2, Mdh-1, Mdh-2, and Iddh, was limited to the occurrence of 1 or a few rare alleles usually in the heterozygous condition. Allelic diversity was consistently highest among all populations at 5 loci, Gpi, Lap-1, Mpi, Pgdh, and Pgm-1. Estimates of average heterozygosity among populations appeared to be slightly lower than reported for eastern Gulf coastal and Atlantic populations of this species. Gene flow among populations, as measured by the average number of migrant individuals/generation (Nm), was quite high (Nm = 7.25). The allelic frequency differences observed did not represent a discernible geographic pattern; differentiation due to local selective pressures is a more likely explanation