GENOTYPE-ENVIRONMENT INTERACTION FOR JUVENILE GROWTH IN THE HARD CLAM MERCENARIA MERCENARIA (L.)

Offspring from half-sib and full-sib families of the hard clam, Mercenaria mercenaria were reared in five locations along the Atlantic Coast to test for the presence of genotype-environment interaction for juvenile growth rate. Location effects upon growth rate variation were prevalent; of the genetic effects, the additive genetic by location variance was predominant with the nonadditive genetic by location component contributing to a lesser degree to the interaction variance. The additive and nonadditive variation over all environments was negligible. Genotype-environment interaction was found to be at least partially due to a change in the amount of genetic variation expressed at each location; with significant additive variation detected at Charleston and Georgetown, SC sites and significant nonadditive variation at Millsboro, DE. Genetic covariance/correlation analysis indicated that reversals in relative family performance across locations were prevalent, implying the possibility of habitat specialization among genotypes. In addition, graphical analysis produced no evidence of a ubiquitously superior genotype. These analyses suggest that genotype-environment interaction should act to constrain the evolution of juvenile growth rate in Mercenaria, preserve any heritable variation associated with this trait and may lead to the development of phenotypic plasticity for growth.