Correlated evolution between heat tolerance and thermal performance curves in Drosophila subobscura

Global warming imposes important challenges for ectotherm organisms, which can avoid the negative effects of thermal stress via evolutionary adaptation of their upper thermal limits (CTmax). In this sense, the estimation of CTmax and its evolutionary capacity is crucial to determine the vulnerability of natural populations to climate change. However, these estimates depend on the thermal stress intensity and it is not completely clear whether this thermal stress intensity can impact the evolutionary response of CTmax and thermal reaction norms (i.e. thermal performance curve, TPC). Here we performed an evolutionary experiment by selecting high heat tolerance using acute and chronic thermal stress in Drosophila subobscura. After artificial selection, we found that knockdown temperatures (a CTmax proxy) evolved in selected lines compared to control lines, whereas the realized heritability and evolutionary rate change of heat tolerance did not differ between acute-selected and chronic-selected lines. From TPC analysis, we found acute-selected lines evolved a higher optimal performance temperature (Topt) compared to acute-control lines, whereas this TPC parameter was not different between chronic-selected and chronic-control lines. The evolutionary response of Topt caused a displacement of entire TPC to high temperatures suggesting a shared genetic architecture between heat tolerance and high-temperature performance, which only arose in the acute-selected lines. In conclusion, thermal stress intensity has important effects on the evolution of thermal physiology in ectotherms, indicating that different thermal scenarios conduce to similar evolutionary responses of heat tolerance but do not for thermal performance. Therefore, thermal stress intensity could have important consequences on the estimations of the vulnerability of ectotherms to global warming.