Evidence for lower plasticity in CTMAX at warmer developmental temperatures

Understanding the capacity for different species to reduce their susceptibility to climate change via phenotypic plasticity is essential for accurately predicting species extinction risk. The climatic variability hypothesis suggests that spatial and temporal variation in climatic variables should select for more plastic phenotypes. However, empirical support for this hypothesis is limited. Here, we examine the capacity for ten Drosophila species to increase their critical thermal maxima (CTMAX) through developmental acclimation and/or adult heat hardening. Using four fluctuating developmental temperature regimes, ranging from 13 to 33 degrees C, we find that most species can increase their CTMAX via developmental acclimation and adult hardening, but found no relationship between climatic variables and absolute measures of plasticity. However, when plasticity was dissected across developmental temperatures, a positive association between plasticity and one measure of climatic variability (temperature seasonality) was found when development took place between 26 and 28 degrees C, whereas a negative relationship was found when development took place between 20 and 23 degrees C. In addition, a decline in CTMAX and egg-to-adult viability, a proxy for fitness, was observed in tropical species at the warmer developmental temperatures (26-28 degrees C); this suggests that tropical species may be at even greater risk from climate change than currently predicted. The combined effects of developmental acclimation and adult hardening on CTMAX were small, contributing to a <0.60 degrees C shift in CTMAX. Although small shifts in CTMAX may increase population persistence in the shorter term, the degree to which they can contribute to meaningful responses in the long term is unclear.