A major life-history locus underlies genotype-by-environment variation in growth across water temperatures

Rapid environmental change is exposing organisms to conditions that do not match those under which they evolved, making it increasingly important to understand how genetic variation shapes phenotypic responses to environmental stress. Since most phenotypic traits arise from interactions between genetic variation and the environments experienced throughout an organisms lifetime, understanding the genetic architecture of these interactions is central to predicting how populations will respond to novel environments. While genotype-by-environment interactions (GxE) are well studied in quantitative genetics, identifying specific loci that contribute to environmentally dependent trait expression remains rare. Salmonids already exhibit a wide portfolio of plastic life-history strategies, reflecting adaptation to highly heterogeneous environments, yet it remains unclear whether known major-effect loci involved in life-history regulation also contribute to variation in plastic responses to environmental change. One such major-effect locus is the transcription factor six6, which has been repeatedly associated with variation in age at maturity across multiple populations of rainbow trout (Oncorhynchus mykiss). Since maturation timing is closely linked to growth trajectories and patterns of energetic allocation during early development, allelic variation at this locus may also influence growth responses to warming conditions. Here, we test this hypothesis using a common-garden experiment in which 6 months old juvenile rainbow trout were reared under current and warming (+2{degrees}C) temperature regimes. By quantifying genotype-specific reaction norms across environments, we show that six6 genotype contributes to environmentally dependent variation in growth and body composition, with individuals heterozygous for the six6 locus showing a distinct and steeper response to warming relative to homozygotes. These findings provide evidence that a major life-history gene shapes plastic responses to thermal stress in juvenile rainbow trout, with novel implications for how standing genetic variation at in large-effect loci may influence population-level responses to climate warming.