Allometric plasticity and the evolution of environment-by-environment (ExE) interactions during a rapid range expansion of a dung beetle

Plastic responses to environmental conditions may themselves depend on other environmental conditions, but how such environment-by-environment (ExE) interactions may impact evolution remains unclear. We investigate how temperature shapes the nutritional polyphenism in horn length in a beetle and test whether "allometric plasticity" (a form of ExE) predicts latitudinal differentiation during a rapid range expansion. Rearing populations under common garden conditions demonstrates that increased temperatures reduce the body size threshold separating two male morphs in all populations but also that the magnitude of temperature-dependent changes in allometry diverged across recently established populations. Furthermore, we found a latitudinal increase in the threshold in the species' exotic range at one of the temperatures, suggesting that allometric plasticity in response to temperature may predict evolved clinal differences. Our findings demonstrate that ExE interactions can be similar in magnitude to GxE interactions and that allometric plasticity and its evolution may impact population's responses to environmental changes.