The transmission of microsatellite alleles in Australian and North American stocks of the Pacific oyster (Crassostrea gigas): Selection and null alleles

Variation, transmission, and selection at 24 microsatellite loci are studie d in five experimental families of the Pacific oyster (Crassostrea gigas). Two families are from naturalized North American stocks, and three come fro m Australian stock. As expected, there are multiple alleles at these loci a nd their segregating variation is reduced to four alleles or less in full s ib progeny groups. Two to 21 loci were tested per family. Eight of the 24 l oci have only codominant alleles, but 16 loci also have non-amplifying or n ull alleles. Of the 172 (43 x 3) parental sequences that were progeny teste d, 30 (17%) were null alleles. Null alleles segregate in both Australian an d North American stocks and their presence is heterogeneous among crosses. Overall null allele frequency in North American crosses was estimated to be 11% (eight of the 72 alleles progeny tested), just significantly less than the 22% (22 of the 100 alleles progeny tested) in the Australian stocks (P = 0.04). After accounting for nulls in genetic hypotheses, selection in th e form of significant deviations from Mendelian expectations is observed in 16 of 43 progeny tests (37%). There is no systematic association between n ull alleles and selection, but analysis of dominance by sequential C-rests reveals non-additive kinds of zygotic selection. This has also been recorde d in two other oyster species and the blue mussel. It appears that null all eles at microsatellites and selection near generic markers are expected phe nomena when studying transmission of genetic markers in bivalve molluscs. T he implications of these results fur breeding, aquaculture and population g enetics are discussed.