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）是生物应对各类胁迫环境（包括温度胁迫与纬度适应性演化）的核心组成部分。长期以来的群体遗传学研究表明，对核心代谢酶的选择可通过改变代谢通量（metabolic flux）重塑生物的生活史策略。然而，此前尚未有研究验证温度胁迫抗性的选择与代谢通量之间的直接关联。本研究以经人工选育、可快速或缓慢从冷昏迷中恢复的黑腹果蝇（Drosophila melanogaster）为实验对象，探究其在冷胁迫过程中的营养分解代谢灵活性，专门验证“胁迫适应性可提升代谢周转率”这一科学假说。首先，利用13C标记葡萄糖（13C-labelled glucose）开展示踪实验，结果显示抗寒果蝇可更快速地将摄入的碳源整合进入氨基酸与新合成的葡萄糖中，从而快速合成脯氨酸——已有研究表明该化合物可显著提升生物的冷胁迫存活率。其次，通过葡萄糖与亮氨酸（leucine）示踪剂实验，本研究发现：冷暴露前，抗寒果蝇的氧化速率显著高于冷敏感果蝇；冷暴露期间二者氧化速率无显著差异；而在恢复阶段，抗寒果蝇的氧化速率再次回升至更高水平。此外，在冷暴露与恢复阶段，抗寒果蝇体内各代谢库之间的化合物转运速率也更为迅速。更高的代谢周转率可帮助冷适应果蝇更好地完成胁迫前准备、抵御冷胁迫并修复/耐受冷损伤。本研究首次揭示了与冷适应相关的营养通量差异，表明与冷胁迫抗性相关的代谢成本可能引发基于资源分配的生活史权衡效应，进而塑造生物的生活史策略。