No evidence for overdominance at the phosphoglucomutase-2 locus in Pacific oysters (Crassostrea gigas) from New Zealand

Overdominance for enzyme specific activity at the phosphoglucomutase-2 (PGM-2) locus has been demonstrated for wild, adult Pacific oysters (Crassostrea gigas) that are formed from heterozygous combinations of the allele at highest frequency and any other allele at this locus. Overdominant genotypes have been shown to have higher catalytic capacities than other genotypes, and consequently it was proposed that this effect might account for genotype-dependent weight differences among Pacific oysters. From an aquaculture perspective, it would be relatively easy for a selective breeding programme to exploit a genotype-dependent effect on production traits (growth rate, wet body weight) if this phenomenon could be demonstrated among hatchery-reared oysters. However, a previous test of this failed to detect either an overdominant effect or a genotype-dependent effect among wild juvenile oysters (9 months of age), among adults of this same batch 12 months later, or among juveniles (5 months of age) from 8 full-sib hatchery-reared families. In the present paper, we extend this work by examining the relationship between PGM-2 genotype and production traits in adult market-size Pacific oysters from five full-sib families specifically selected because they contain putatively overdominant and other genotypic combinations. Within-families, there were significant weight differences among genotypes, but there was no evidence of a consistent PGM-2 genotype-dependent effect, overdominant or otherwise, on wet tissue weight, dry shell weight or total weight (=wet tissue weight plus dry shell weight). Among families, tests of pooled data showed no significant difference between putative overdominant and other genotypes for wet tissue weight, dry shell weight or total weight. We conclude that there is no evidence on which to base a Pacific oyster-breeding programme using PGM-2 as a marker for increased production.