Assessing the effect of experimental evolution under combined thermal-nutritional stress on larval thermotolerance and thermal plasticity in Drosophila melanogaster

Animals commonly face combinations of thermal and nutritional stress in nature, which will intensify under climate change. While genetic adaptation is necessary to buffer long-term stress, its unclear whether adaptation to combined stress can occur without compromising viability and thermal plasticity. We tested larval thermotolerance and thermal plasticity in Drosophila melanogaster selected under different temperatures (18{degrees}C, 25{degrees}C, and 28{degrees}C) and diets (standard, diluted, and low-protein:high-carbohydrate [P:C]). Basal larval cold tolerance was affected by both protein concentration and temperature; larvae evolved higher basal cold tolerance on the diluted and low P:C diets at 18{degrees}C and 28{degrees}C. Hardening increased cold tolerance for most lines, except those selected at 18{degrees}C and 28{degrees}C on low P:C diets and at 25{degrees}C on standard diets. Basal larval heat tolerance was affected by selection temperature; selection at 25{degrees}C increased heat tolerance. An interaction between selection temperature, selection diet, and hardening treatment affected larval heat tolerance; hardening reduced heat tolerance in most lines, except those selected at 25{degrees}C on low P:C diets and at 28{degrees}C on standard diets. Our results suggest that adaptation to combined stress allows basal cold tolerance and its plasticity to co-evolve, but not heat tolerance, highlighting ectotherms vulnerability to long-term climate change.