Environmental heterogeneity generates opposite gene-by-environment interactions for two fitness-related traits within a population

Theory predicts that environmental heterogeneity offers a potential solution to the maintenance of genetic variation within populations, but empirical evidence remains sparse. The livebearing fish Xiphophorus variatus exhibits polymorphism at a single locus, with different alleles resulting in up to five distinct melanistic “tailspot” patterns within populations. We investigated the effects of heterogeneity in two ubiquitous environmental variables (temperature and food availability) on two fitness-related traits (upper thermal limits and body condition) in two different tailspot types (wildtype and upper cut crescent). We found gene-by-environment (GxE) interactions between tailspot type and food level affecting upper thermal limits (UTL), as well as between tailspot type and thermal environment affecting body condition. Exploring mechanistic bases underlying these GxE patterns, we found no differences between tailspot types in hsp70 gene expression despite significant overall increase...