Selection against early maturity in large rainbow trout Oncorhynchus mykiss: the quantitative genetics of sexual dimorphism and genotype-by-environment interactions

In rainbow trout Oncorhynchus mykiss, females typically mature at a later age than males, and males mature either early or late. When farmed rainbow trout is produced to obtain large fillets of high quality, especially early maturation of males reduces economical benefits. To examine whether it is possible to change the degree of this sexual dimorphism using selective breeding, we estimated sex-specific heritabilities, genetic correlations between sexes, and cross-environment correlations between two production environments for age at maturity and body weight. Extensive data set including individuals in five generations measured in brackish and fresh water in a split-family design was utilised to estimate the genetic parameters. The timing of maturity ( h 2=0.12–0.56) and body weight ( h 2=0.20–0.27) exhibited significant heritabilities, allowing genetic changes in trait means in response to selection. Genetic change in sexual dimorphism of body weight is, however, strongly constrained, as shown by the high genetic correlations between sexes ( r A≥0.95). For timing of maturity, the heritabilities of males and females differed, but the genetic correlation between sexes was high and positive ( r A=0.70). Although sexual dimorphism of age at maturity is less strongly contained than that of body weights, we conclude that no rapid genetic changes can be achieved in the sexual dimorphism of age at maturity through selective breeding. However, breeders may delay the timing of maturity in both sexes to avoid the drawbacks associated with the early maturation. Cross-environment correlations for the timing of maturity and body weight were highly positive ( r A≥0.61), indicating weak genotype-by-environment interactions and allowing parallel genetic changes in two environments where rainbow trout is farmed.