Genetic diversity and effective population size in mass selection lines of Pacific oyster (Crassostrea gigas)

Allozymes and microsatellite loci were used to characterise the levels and patterns of genetic variation in four successive mass selection lines of Tasmanian hatchery produced stocks of Pacific oysters. These were compared with two feral populations from Tasmania and two endemic populations from Japan. Allozyme allele numbers and heterozygosities were relatively uniform across all collections. Microsatellite allele numbers declined in the mass selection lines but heterozygosities were less affected. Observed and Hardy–Weinberg expected heterozygosities were quite similar for the allozyme loci although there were some small but significant deficiencies of heterozygotes in the mass selection lines. Observed heterozygosity was substantially less than expected values for most microsatellite loci in nearly all collections, probably due to the presence of non-amplifying alleles. Estimated effective population sizes from temporal variance in allzele frequencies were about one half that of sex ratio corrected broodstock count estimates, and is likely due to high variance in family size and skewed sex ratios in parents of the final generation. Our mass selection lines have clearly lost some variation. The existing selection program in Australia focuses more on family selection and pair matings than mass selection. If mass selection continues with the current collections, we would suggest augmenting broodstock numbers and undertaking several ‘mini’ spawnings between equal numbers of males and females to increase the effective population sizes of our mass selected oysters.