Nutritional optima for life-history traits vary with temperature and across locally-adapted populations

As the climate changes, populations must overcome more frequent and more extreme exposure to a wide range of stressors. However, our knowledge of how locally-adapted populations respond to combinations of stressors remains incomplete. Recent studies show that elevated temperatures can interact with nutrition to accentuate the negative effects of a poor diet, suggesting higher costs of nutritional stress when individuals experience temperatures outside of their locally-adapted conditions. This can translate into reduced nutrient optima under thermal stress in life-history trait landscapes, a hypothesis that remains to be tested. Here, we used the Geometric Framework for Nutrition to test this hypothesis using two locally-adapted populations of Drosophila melanogaster from opposing ends of a well-characterised adaptive gradient along the east coast of Australia (tropical vs. temperate). We found that the negative effects of nutritional stress were significantly greater in the tropical population under warmer temperatures. In contrast, the temperate population was able to utilise a broader nutritional space to maintain high viability and a large wing size across the range of fluctuating temperatures. Our findings reveal the ways in which local adaptation impacts how populations navigate and explore the nutritional space in response to increasingly stressful thermal conditions. These data suggest that certain populations may be better able to cope with increasingly stressful and variable environments, while others may be more vulnerable to local extinctions.