Environmental dependence of mutational (co)variances of adaptive traits

Standing genetic variation, and capacity to adapt to environment change, will ultimately depend on the fitness effects of mutations across the range of environments experienced by contemporary, panmictic, populations. We investigated how mild perturbations in diet and temperature affect mutational (co)variances of traits that evolve under climatic adaptation, and contribute to individual fitness in Drosophila serrata. We assessed egg-to-adult viability, development time and wing size of 64 lines that had diverged from one another via spontaneous mutation over 30 generations of brother-sister mating. Our results suggested most mutations have directionally concordant (i.e., synergistic) effects in all environments and both sexes. However, elevated mutational variance under reduced macronutrient conditions suggested environment-dependent variation in mutational effect sizes for development time. We also observed evidence for antagonistic effects under standard versus reduced macronutrient conditions, where these effects were further contingent on temperature (for development time) or sex (for size). Diet also influenced the magnitude and sign of mutational correlations between traits, although this result was largely due to a single genotype (line), which may reflect a rare, large effect mutation. Overall, our results suggest environmental heterogeneity and environment-dependency of mutational effects could contribute to the maintenance of genetic variance.