Data from: Cold adaptation increases rates of nutrient flow and metabolic plasticity during cold exposure in Drosophila melanogaster

Metabolic flexibility is an important component of adaptation to stressful environments, including thermal stress and latitudinal adaptation. A long history of population genetic studies suggest that selection on core metabolic enzymes may shape life histories by altering metabolic flux. However, the direct relationship between selection on thermal stress hardiness and metabolic flux has not previously been tested. We investigated flexibility of nutrient catabolism during cold stress in Drosophila melanogaster artificially selected for fast or slow recovery from chill coma (i.e. cold-hardy or -susceptible), specifically testing the hypothesis that stress adaptation increases metabolic turnover. Using 13C-labelled glucose, we first showed that cold-hardy flies more rapidly incorporate ingested carbon into amino acids and newly synthesized glucose, permitting rapid synthesis of proline, a compound shown elsewhere to improve survival of cold stress. Second, using glucose and leucine tracers we showed that cold-hardy flies had higher oxidation rates than cold-susceptible flies before cold exposure, similar oxidation rates during cold exposure, and returned to higher oxidation rates during recovery. Additionally, cold-hardy flies transferred compounds among body pools more rapidly during cold exposure and recovery. Increased metabolic turnover may allow cold-adapted flies to better prepare for, resist and repair/tolerate cold damage. This work illustrates for the first time differences in nutrient fluxes associated with cold adaptation, suggesting that metabolic costs associated with cold hardiness could invoke resource-based trade-offs that shape life histories.

代谢灵活性（Metabolic flexibility）是生物体适应包括热胁迫、纬度适应在内的胁迫环境的重要组成部分。长期群体遗传学研究提示，对核心代谢酶的选择可通过调控代谢流塑造物种的生活史。然而，此前尚未有研究验证冷胁迫耐受性选择与代谢流之间的直接关联。本研究以经人工选育、可快速或缓慢从冷昏迷（chill coma）中恢复的黑腹果蝇（Drosophila melanogaster）——即耐寒或冷敏感品系——为对象，探究其在冷胁迫期间的营养分解代谢灵活性，专门验证"胁迫适应可提升代谢周转"这一假说。研究首先利用13C标记葡萄糖（13C-labelled glucose）发现，耐寒品系果蝇可更快地将摄入的碳整合进入氨基酸及新合成的葡萄糖中，从而快速合成脯氨酸（proline）——此前已有研究表明该化合物可提升果蝇的冷胁迫存活率。其次，通过葡萄糖示踪剂与亮氨酸示踪剂实验，研究发现耐寒品系果蝇在冷暴露前的氧化速率高于冷敏感品系，冷暴露期间二者氧化速率相近，而在恢复阶段耐寒品系的氧化速率再次回升至更高水平。此外，耐寒品系果蝇在冷暴露与恢复阶段，体内各代谢池间的物质转运速率更快。提升的代谢周转或许可帮助冷适应果蝇更好地应对、抵御冷胁迫损伤，并完成损伤修复或耐受。本研究首次揭示了与冷适应相关的营养流差异，表明与冷耐受性相关的代谢成本可能引发基于资源的权衡效应，进而塑造物种的生活史。