Collaborative Research: Experimental Tests of the Adaptive Significance of Ectotherm Thermoregulation

动物的体温对其生物学的各个方面都有深远的影响。极端温度可能会对生理造成损害，甚至是致命的，但即使是中等体温也决定了生理活动和生殖产出的速度。吸热型动物(主要是鸟类和哺乳动物)通过调节生理(例如新陈代谢产热、排汗)来控制体温；而外热型动物(例如线虫、昆虫、鱼类、爬行动物)通常无法做到这一点。然而，体温外的动物可以调整它们的行为(例如，在阳光下呆的时间与在阴凉处的时间)，以获得对体温的显著控制：许多体温动物仔细地在狭窄的水平上调节它们的体温，这随物种的不同而不同。物种的这种温度偏好往往与使生理表现最大化(冲刺速度、消化效率、感官敏锐度)的温度相关，因此，一个经典的假说提出，外胚层动物的温度偏好将进化成与使种群增长率最大化的温度(“适合度”)相匹配。这一适应性假说被广泛接受，是生理生态学的基础，但还从未被直接检验过。这一建议对外温体的热偏好的生物学后果进行了严格的探索。具体地说，它开发了一套完整的实验、理论和比较研究，重点关注两个物种[果蝇(黑腹果蝇)和土壤线虫(秀丽线虫)]，作为经济和生态上重要的动物群体(昆虫和线虫)的一般模型。该项目利用了强大的新方法(由神经生物学家开发)来实验性地操纵热偏好。这两种动物的体温调节精确度将通过手术和突变来操纵，而体温偏好本身将通过人工选择(选择性育种)来改变。然后，将测量由此产生的对健康的影响。如果经典假设成立，那么(例如)温度调节设定值发生变化的个体在温度梯度中的适合度将低于他们在固定温度下的适合度，而对照组个体在两种环境中的适合度都将表现出相似的适合度。该项目还完成了一个新的热偏好理论模型。初步模型表明，波动环境下的最优设定值实际上低于恒定环境下的温度最大化适应值。关于昆虫和蜥蜴的比较数据将被用来挑战该模型的预测。这些研究具有相当大的应用相关性。确定热偏好实际上是否以及如何使种群增长率最大化，这不仅对那些研究热环境对节肢动物和线虫害虫或病媒种群增长的影响的人具有重要意义，而且对需要确定大规模饲养生防剂的最佳温度的成本效益估计的应用昆虫学家也具有重要意义。

The body temperature of an animal has profound influence on all aspects of its biology. Extreme temperatures can be damaging physiologically or even lethal, but even intermediate body temperatures determine rates of physiological activities and of reproductive output. Endothermal animals (mainly birds and mammals) control their body temperatures by adjusting physiology (for example, metabolic heat production, perspiration); but ectothermal animals (e.g., nematodes, insects, fishes, reptiles) are generally unable to do this. Nevertheless, ectotherms can instead adjust their behavior (for example, amount of time spent in sun versus shade) to gain remarkable control over body temperature: many carefully regulate their body temperature at narrow levels ("thermal preferences") that vary from species to species. Such thermal preferences of species often correlate with temperatures that maximize physiological performance (sprint speed, digestive efficiency, sensory acuity), and accordingly a classical hypothesis proposes that thermal preferences of ectotherms will have evolved to match temperatures that maximize rates of population growth ("fitness"). This adaptive hypothesis is widely accepted and is fundamental to physiological ecology, but yet has never been tested directly.This proposal provides a rigorous exploration of the biological consequences of thermal preferences of ectotherms. Specifically, it develops an integrated set of experimental, theoretical, and comparative studies focusing on two species [fruitfly (Drosophila melanogaster), soil nematode (Caenorhabditis elegans)] that serve as general models for economically and ecologically important animal groups (insects and nematodes). The project exploits powerful new methods (developed by neurobiologists) of experimentally manipulating thermal preferences. Thermoregulatory precision of both animals will be manipulated both surgically and via mutation, and thermal preferences themselves will be shifted via artificial selection (selective breeding). Then the resultant impacts on fitness will be measured. If the classical hypothesis holds, then (for example) individuals with shifted thermoregulatory set-points will have reduced fitness in a thermal gradient relative to their fitness at a fixed temperature, whereas control individuals will show similar fitness in both environments. The project also completes a novel theoretical model of thermal preferences. The preliminary model shows that optimal set-points in fluctuating environments are actually lower than the temperature maximizing fitness in a constant environment. Comparative data on insects and lizards will be used to challenge the model's predictions.These studies have considerable applied relevance. Establishing whether and how thermal preferences actually maximize rates of population growth will have crucial implications not only for those studying the effects of the thermal environment on population growth of arthropod and nematode pests or disease vectors, but also for applied entomologists needing to determine cost-effective estimates of optimal temperature for mass rearing of bio-control agents.