Developmental Plasticity in Butterfly Eyespot Mutants: Variation in Thermal Reaction Norms across Genotypes and Pigmentation Traits

SIMPLE SUMMARY: The temperature experienced during organismal development can affect adult phenotypes. This phenomenon is called thermal developmental plasticity and is common for insect pigmentation. Plasticity can help organisms cope with environmental heterogeneity, such as that across yearly seasons, and might also impact how populations deal with environmental perturbation, such as climate change. Because plasticity can be adaptive and heritable, it is liable to evolve. The evolution of plasticity will depend on the availability of genetic variation contributing to variation in plasticity, and on the integration across plastic traits, which might limit opportunities for their independent evolutionary change. Here we address these two topics by focusing on thermal plasticity in butterfly wing pattern elements called eyespots, which are composed of concentric rings of different colors. We found differences in plasticity between genotypes and concluded that mutants of strong effect on pigmentation can also affect thermal plasticity therein. We also found differences in plasticity between distinct pigmentation features, suggesting plasticity can evolve somewhat independently for those features. ABSTRACT: Developmental plasticity refers to the property by which a genotype corresponds to distinct phenotypes depending on the environmental conditions experienced during development. This dependence of phenotype expression on environment is graphically represented by reaction norms, which can differ between traits and between genotypes. Even though genetic variation for reaction norms provides the basis for the evolution of plasticity, we know little about the genes that contribute to that variation. This includes understanding to what extent those are the same genes that contribute to inter-individual variation in a fixed environment. Here, we quantified thermal plasticity in butterfly lines that differ in pigmentation phenotype to test the hypothesis that alleles affecting pigmentation also affect plasticity therein. We characterized thermal reaction norms for eyespot color rings of distinct Bicyclus anynana genetic backgrounds, corresponding to allelic variants affecting eyespot size and color composition. Our results reveal genetic variation for the slope and curvature of reaction norms, with differences between eyespots and between eyespot color rings, as well as between sexes. Our report of prevalent temperature-dependent and compartment-specific allelic effects underscores the complexity of genotype-by-environment interactions and their consequence for the evolution of developmental plasticity.