Widespread effects of early embryonic thermal stress on morphology, physiology and performance across the lifespan in D. melanogaster
Authors: Sara Helms Cahan; Andrew Stoloff; Katie Bora; Collin Brown; Caela Flanagan
Affiliation: University of Vermont
Keywords: p. physiology; m. adaptation
Environmental temperature has profound effects on every aspect of the development and performance of poikilothermic organisms, and thermal limits play an important role in setting species geographic distributions. The most obvious effect of temperature extremes is mortality, which is used to determine the extent of the thermal safety margin, the difference between environmental temperatures and the thermal limits of the organism. However, significant negative impacts on performance may occur well below the LT50 value, particularly for thermally sensitive life stages. To investigate the importance of sublethal effects, we compared multiple aspects of survival, development and thermal performance of developing D. melanogaster Canton-S eggs or larvae exposed to unstressed or moderately stressful low and high developmental temperatures (18°C, 25°C and 30°C) after one hour, 24 hours, or 60 hours post-laying. One-hour embryos displayed widespread deficits that were largely absent in flies transferred later in development, with somewhat different elements of performance impaired under cold and heat stress. Adult-to-pupal survival of cold-stressed early embryos was 70% of survival in unstressed conditions but was not reduced at 30°C; however, heat-stressed embryos suffered a 50% mortality rate during the pupal stage, and of those that emerged successfully, 35% displayed a crumpled-wing phenotype that rendered them unable to fly, and 30% showed significant locomotion deficits in an adult climbing assay. Adult thermal performance was also impacted, with both cold-stressed and heat-stressed early embryos displaying a ~0.3°C reduction in critical thermal maximum, while cold-stressed embryos showed substantially reduced capacity to improve their critical thermal minimum through developmental acclimation. Altogether, these delayed effects of early thermal stress summed across the lifespan have the potential to reduce mean fitness below levels needed for population replacement, which may have important implications for prediction of population persistence under projected climate change.