Growth, stress and acclimation responses to fluctuating temperatures in field and domesticated populations of Manduca sexta

Diurnal fluctuations in temperature are ubiquitous in terrestrial environments, and insects and other ectotherms have evolved to tolerate or acclimate to such fluctuations. Few studies have examined whether ectotherms acclimate to diurnal temperature fluctuations, or how natural and domesticated populations differ in their responses to diurnal fluctuations. We examine how diurnally fluctuating temperatures during development affect growth, acclimation and stress responses for two populations of Manduca sexta: a field population that typically experiences wide variation in mean and fluctuations in temperature, and a laboratory population that has been domesticated in nearly constant temperatures for more than 300 generations. Laboratory experiments showed that diurnal fluctuations throughout larval development reduced pupal mass for the lab but not the field population. The differing effects of diurnal fluctuations were greatest at higher mean temperature (30 °C): here diurnal fluctuations reduced pupal mass and increased pupal development time for the lab population, but had little effect for the field population. We also evaluated how mean and fluctuations in temperature during early larval development affected growth rate during the final larval instar as a function of test temperature. At an intermediate (25 °C) mean temperature, both the lab and field population showed a positive acclimation response to diurnal fluctuations, in which subsequent growth rate was significantly higher at most test temperatures. In contrast at higher mean temperature (30 °C), diurnal fluctuations significantly reduced subsequent growth rate at most test temperatures for the lab population, but not for the field population. These results suggest that during domestication in constant temperatures, the lab population has lost the capacity to tolerate or acclimate to high and fluctuating temperatures. Population differences in acclimation capacity in response to temperature fluctuations has not been previously demonstrated, but they may be important for understanding the evolution of reaction norms and performance curves.

温度日波动在陆地环境中无处不在，昆虫及其他变温动物（ectotherms）已演化出耐受或适应此类波动的能力。目前鲜有研究探讨变温动物是否会适应温度日波动，以及自然种群与驯化种群对温度日波动的响应存在何种差异。本研究以两个种群的烟草天蛾（Manduca sexta）为实验材料，探究发育阶段的温度日波动对其生长、适应能力与应激响应的影响，实验对象分为两组：一组为野外种群，其生存环境通常伴随温度均值与波动幅度均较大的变动；另一组为实验室驯化种群，已在近乎恒定的温度条件下驯化超过300代。实验室实验结果显示，幼虫发育全程经历温度日波动会降低实验室种群的蛹重，但对野外种群无显著影响。温度日波动的差异化效应在较高平均温度（30℃）下最为显著：此时温度日波动会降低实验室种群的蛹重并延长其蛹发育时长，但对野外种群几乎无影响。本研究还探究了幼虫早期发育阶段的温度均值与波动，如何随测试温度变化，对末龄幼虫的生长速率产生影响。在中等平均温度（25℃）条件下，实验室种群与野外种群均对温度日波动表现出正向适应响应：此时多数测试温度下的后续生长速率均显著提升。而在较高平均温度（30℃）条件下，温度日波动会在多数测试温度下显著降低实验室种群的后续生长速率，但对野外种群无此效应。上述结果表明，在恒定温度条件下的驯化过程中，实验室种群已丧失耐受或适应高温与温度波动的能力。此前尚未有研究证实种群对温度波动的适应能力存在差异，但此类差异或许对理解反应规范（reaction norms）与性能曲线（performance curves）的演化具有重要意义。