The Pacific oyster Crassostrea gigas was repeatedly and massively introduced to North America from Japan starting over 100 years ago and has established large, self-recruiting populations in the Pacific Northwest of the U.S. and Canada. A previous study of mtDNA variation revealed little population genetic structure among populations from British Columbia and Washington State. Samples from that study, more recent samples from two of the same localities, and a sample from Japan were typed at 52 mapped, coding, single-nucleotide polymorphisms (SNPs) assayed by high-resolution melting (HRM) to investigate spatial and temporal genetic variation. Little variation is detected among North American populations, which are, as a group, distinct, perhaps adaptively so, from oysters in Hiroshima, Japan. Significant excesses of heterozygotes with respect to random mating expectations and of pairwise linkage disequilibria reveal, however, that North American populations are not in genetic equilibrium. Moreover, genetic changes over 10 to 21 years in two localities are substantial, suggesting that temporal variance in allelic frequencies per generation is as large as spatial variance. These results illustrate the peril of assuming stability of population structure in connectivity or seascape genetic analyses. Because migration and locus-specific selection can be ruled out as causes of non-equilibrium population structure, random genetic drift is the most parsimonious explanation. This implies effective population sizes (N_e) on the order of hundreds to a few thousand, orders of magnitude smaller than the natural abundance (N) of this oyster. These low N_e/N ratios are compatible with the hypothesis of sweepstakes reproductive success.

Biographical Sketch

Dennis Hedgecock received a B.S. in Biology from St. Mary's College, California, in 1970, and a Ph.D. in Genetics, from the University of California, Davis, in 1974. Hedgecock joined the USC College as the first Paxson H. Offield Professor of Fisheries Ecology in 2003, following a nearly 30-year career at UC Davis. He is a past Head of the Marine Environmental Biology Section of the Department of Biological Sciences. Hedgecock has published over 120 scholarly articles on the population, quantitative, evolutionary and conservation genetics of marine fish and shellfish, primarily Pacific oysters, white seabass and Pacific salmon. He was selected as the inaugural recipient of the Ken Chew Endowed Visiting Professorship in Aquaculture at the University of Washington in 2007. Hedgecock was elected Fellow of the American Association for the Advancement of Science in 1986, and is currently a member of several other scientific societies, including the American Genetics Association, the Genetics Society of America, the National Shellfisheries Association, and the Society for the Study of Evolution. He serves on the editorial boards of Aquaculture, the Journal of Shellfish Research, and the Journal of Experimental Marine Biology and Ecology.