Maternal effects of egg size in brown trout (Salmo trutta): norms of reaction to environmental quality

The magnitude of fitness variation caused by maternal effects and, thus, th e adaptive significance of maternal traits may depend on environmental qual ity, generating crossing reaction norms among offspring phenotypes that sha pe life-history evolution. By manipulating intraclutch variation in egg siz e and comparing siblings we examined the maternal effects of egg size on of fspring performance and tested for the existence of reaction norms to envir onmental quality using the brown trout Salmo trutta. When sibling groups of small and large eggs were reared separately in a hatchery environment init ial size differences disappeared rapidly. However, in semi-natural environm ents and under direct competition, juveniles from large eggs experienced gr owth and survival advantages over siblings from small eggs. Moreover, disti nct reaction norms existed, with the differences in performance of juvenile s from small and large eggs being most pronounced in the poorer growth envi ronments. Our results provide the first direct evidence, to our knowledge, for a causal relationship between egg size and fitness-related traits in fi shes, independent of potentially confounding genetic effects. Moreover, the y indicate that previous studies have been biased by experimental condition s that excluded competitive asymmetries and environmental variability. The existence of reaction norms indicates a shift in optimal egg size across gr adients of environmental quality that probably shapes the evolution of this trait.