Woods H A, Harrison J F
Department of Biology, Arizona State University, Tempe, AZ 85287-1501, USA.
Physiol Biochem Zool. 2001 Jan-Feb;74(1):32-44. doi: 10.1086/319302.
Do organisms make beneficial physiological adjustments in response to environmental change? We examined this question by measuring the effects of short-term (12-36 h) and long-term (larval lifetime) hydric stress on the tobacco hornworm, Manduca sexta. Larvae were reared from the first instar on low-water (69%) or high-water (80%) artificial diets and then transferred early in the fifth instar to the same or opposite diet (2x2 design). Within the subsequent 36 h, we measured 24-h growth rates and three primary determinants of the water budget: water gain via consumption and water loss via evaporation and defecation. Larvae preexposed to low-water diet grew less rapidly on low-water diet than those switched acutely to low-water diet from high-water diet, showing that larvae preexposed to a particular environment do not necessarily acclimate beneficially to that environment. Our data on water fluxes to and from larvae, however, strongly suggest that water-stressed larvae did make beneficial physiological adjustments. Larvae responded to short-term hydric stress by minimizing rates of water excretion, primarily by increasing rates of rectal water absorption. Larvae responded to chronic water stress by significantly reducing rates of evaporative water loss; they also showed additional reductions in fecal water excretion, but these decreases were due to lowered consumption and not to further increases in rate of rectal water absorption. This mismatch between maladaptive acclimation of organismal performance and beneficial adjustment of suborganismal traits can be reconciled by recognizing that organismal physiology is hierarchical: fitness-related performance traits represent the aggregate outcome of numerous, more mechanistic physiological traits. Although chronic exposure to an environment may depress the aggregate effect of these mechanistic traits on performance, organisms are not precluded from making beneficial adjustments to individual traits contributing to performance.
生物体是否会针对环境变化做出有益的生理调节？我们通过测量短期（12 - 36小时）和长期（幼虫整个生命周期）水分胁迫对烟草天蛾（Manduca sexta）的影响来研究这个问题。幼虫从一龄开始在低水分（69%）或高水分（80%）的人工饲料上饲养，然后在五龄初期转移到相同或相反的饲料上（2×2设计）。在随后的36小时内，我们测量了24小时的生长速率以及水分平衡的三个主要决定因素：通过摄食获得的水分以及通过蒸发和排便损失的水分。预先暴露在低水分饲料上的幼虫在低水分饲料上的生长速度比从高水分饲料急性转换到低水分饲料的幼虫慢，这表明预先暴露在特定环境中的幼虫不一定能有益地适应该环境。然而，我们关于幼虫水分通量的数据强烈表明，水分胁迫的幼虫确实做出了有益的生理调节。幼虫通过主要提高直肠水分吸收速率来最小化水分排泄速率，从而对短期水分胁迫做出反应。幼虫通过显著降低蒸发水分损失速率来应对慢性水分胁迫；它们的粪便水分排泄也有额外减少，但这些减少是由于摄食量降低，而不是直肠水分吸收速率的进一步提高。通过认识到生物体生理学是分层的，可以调和生物体性能的适应不良调节与亚生物体特征的有益调节之间的这种不匹配：与适应性相关的性能特征代表了众多更具机械性的生理特征的综合结果。虽然长期暴露在一种环境中可能会抑制这些机械性特征对性能的综合影响，但生物体并不排除对有助于性能的个体特征做出有益调节。
