Coggins Bret L, Collins John W, Holbrook Kailea J, Yampolsky Lev Y
Department of Biological Sciences, East Tennessee State University, Johnson, TN, 37601, USA.
J Comp Physiol B. 2017 Dec;187(8):1091-1106. doi: 10.1007/s00360-017-1090-9. Epub 2017 Apr 7.
Examples of phenotypic plasticity-the ability of organisms of identical genotypes to produce different phenotypes in response to the environment-are abundant, but often lack data on the causative physiology and biochemistry. Phenotypes associated with increased protection against or reduced damage from harmful environments may, in fact, be downstream effects of hidden adaptive responses that remain elusive to experimental measurement or be obscured by homeostatic or over-compensatory effects. The freshwater zooplankton crustacean Daphnia drastically increases its heat tolerance as the result of acclimation to high temperatures, an effect often assumed to be based on plastic responses allowing better protection against oxidative stress. Using several geographically distant Daphnia magna genotypes, we demonstrate that the more heat tolerant individuals have a higher total antioxidant capacity (TAC) both in the comparison of heat-acclimated vs. non heat-acclimated females and in the comparison of females to age- and body size-matched males, which show lower heat tolerance than females. However, experimental manipulations of hypothesized antioxidant pathways by either glutathione addition or glutathione synthesis inhibition had no effect on heat tolerance. Lipid peroxidation (LPO), contrary to expectations, did not appear to be a predictive measure of susceptibility to thermal damage: LPO was higher, not lower, in more heat tolerant heat-acclimated individuals after exposure to a lethally high temperature. We hypothesize that LPO may be maintained in Daphnia at a constant level in the absence of acute exposure to elevated temperature and increase as a by-product of a possible protective antioxidant mechanism during such exposure. This conclusion is corroborated by the observed short-term and long-term changes in phospholipid composition that included an increase in fatty acid saturation at 28 °C and up-regulation of certain long-chain polyunsaturated fatty acids. Phospholipid composition was more strongly affected by recently experienced temperature (4-day transfer) than by long-term (2 generations) temperature acclimation. This is consistent with partial loss of thermal tolerance after a short-term switch to a reciprocal temperature. As predicted under the homeoviscous adaptation hypothesis, the more heat tolerant Daphnia showed lower membrane fluidity than their less heat tolerant counterparts, in comparison both between acclimation temperatures and among different genotypes. We conclude that thermal tolerance in Daphnia is influenced by total antioxidant capacity and membrane fluidity at high temperatures, with both effects possibly reflecting changes in phospholipid composition.
表型可塑性（即基因型相同的生物体在响应环境时产生不同表型的能力）的例子大量存在，但往往缺乏关于因果生理和生物化学的数据。事实上，与增强对有害环境的保护或减少有害环境造成的损害相关的表型，可能是隐藏的适应性反应的下游效应，这些效应仍然难以通过实验测量，或者被稳态或过度补偿效应所掩盖。淡水浮游动物甲壳类水蚤由于适应高温，其耐热性会大幅提高，这种效应通常被认为是基于可塑性反应，从而能更好地抵御氧化应激。我们使用了几种地理上相隔较远的大型溞基因型，发现在比较热适应雌体与非热适应雌体时，以及在比较雌体与年龄和体型匹配但耐热性低于雌体的雄体时，耐热性更强的个体具有更高的总抗氧化能力（TAC）。然而，通过添加谷胱甘肽或抑制谷胱甘肽合成对假定的抗氧化途径进行实验操作，对耐热性没有影响。与预期相反，脂质过氧化（LPO）似乎并不是热损伤易感性的预测指标：在暴露于致死高温后，耐热性更强的热适应个体中的LPO更高，而非更低。我们推测，在没有急性暴露于高温的情况下，水蚤体内的LPO可能维持在恒定水平，并在这种暴露期间作为一种可能的保护性抗氧化机制的副产物而增加。观察到的磷脂组成的短期和长期变化证实了这一结论，这些变化包括在28°C时脂肪酸饱和度增加以及某些长链多不饱和脂肪酸的上调。磷脂组成受近期经历的温度（4天转移）的影响比受长期（2代）温度适应的影响更大。这与短期切换到相反温度后耐热性部分丧失是一致的。正如在适应性膜粘度假说下所预测的那样，在适应温度之间以及不同基因型之间进行比较时，耐热性更强的水蚤比耐热性较弱的水蚤表现出更低的膜流动性。我们得出结论，水蚤的耐热性受高温下的总抗氧化能力和膜流动性的影响，这两种效应可能都反映了磷脂组成的变化。
