Linear reaction norms of thermal limits in Drosophila: predictable plasticity in cold but not in heat tolerance

1. Thermal limits of ectotherms have been studied extensively and are believed to be evolutionarily constrained, leaving ectotherms at risk under future climate change. Phenotypic plasticity may extend the thermal limits, but we lack detailed characterizations of thermal limit reaction norms as well as an understanding of the interspecific variation in these reaction norms. 2. Here, we investigated the interspecific variation in phenotypic plasticity of thermal limits in 13 Drosophila species. We obtained high-resolution reaction norms for upper and lower thermal limits across the permissive developmental thermal range (125-30 degrees C). The estimated phenotypes were then associated (while accounting for phylogeny) with climatic parameters from the species' distributional range. 3. All species showed linear reaction norms for cold tolerance (CTmin) and heat tolerance (CTmax) across developmental acclimation temperatures. We observed strong beneficial cold acclimation to lower temperatures in all species. Conversely, the heat acclimation response was non-existent in some species, and decreasing or increasing with increasing developmental acclimation temperatures in other species. The degree of phenotypic plasticity of CTmin and CTmax was related neither to the basal thermal limits (trade-off hypothesis) nor to climatic parameters connected to latitudinal distributions (latitudinal hypothesis). 4. A substantial and linear developmental plasticity of lower thermal limits is a general characteristic of Drosophila species, which allows for straightforward application in species distribution models. In general, upper thermal limits also show linear norms of reaction, but their adaptive significance is limited and highly variable among species, making general predictions across species rather impossible. 5. High-resolution estimates of norms of reaction of thermal limits can considerably increase our understanding of the capacity of ectotherms to acclimate to different thermal environments. However, our understanding of the environmental drivers of the evolution of phenotypic plasticity and thus of the interspecific differences remains ambiguous, potentially constrained by limited microclimate information.