Acclimation capacity and rate change through life in the zooplankton Daphnia

When a change in the environment occurs, organisms can maintain an optimal phenotypic state via plastic, reversible changes to their phenotypes. These adjustments, when occurring within a generation, are described as the process of acclimation. Whilst acclimation has been studied for more than half a century, global environmental change has stimulated renewed interest in quantifying variation in the rate and capacity with which this process occurs, particularly among ectothermic organisms. Yet, despite the likely ecological importance of acclimation capacity and rate, how these traits change throughout life among members of the same species is largely unstudied. Here we investigate these relationships by measuring acute heat tolerance of the clonally reproducing zooplankter Daphnia magna of different size/age and acclimation status. The heat tolerance of individuals completely acclimated to relatively warm (28°C) or cool (17°C) temperatures diverged during development, indicating that older, larger individuals had a greater capacity to increase heat tolerance. However, when cool acclimated individuals were briefly exposed to the warm temperature (i.e. were ‘heat-hardened’), it was younger, smaller animals with less capacity to acclimate that were able to do so more rapidly because they obtained or came closer to obtaining complete acclimation of heat tolerance. Our results illustrate that within a species, individuals can differ substantially in how rapidly and by how much they can respond to environmental change. We urge greater investigation of the intraspecific relationship between acclimation and development along with further consideration of the factors that might contribute to these enigmatic patterns of phenotypic variation.

Methods
Data collected experimentally in laboratory conditions.

当环境发生改变时，生物体可通过对表型进行可塑性、可逆性的调整，维持最优表型状态。这类发生于单个世代内的调整过程，被称为驯化（acclimation）。尽管驯化的相关研究已开展逾半个世纪，但全球环境变化重新激发了学界对量化该过程速率与容量变异的研究兴趣，尤其聚焦于变温生物（ectothermic organisms）类群。尽管驯化容量与速率极可能具备重要的生态学意义，但同一物种种群内，这些性状随个体生命周期的变化规律仍未得到充分研究。
本研究通过测定不同体型/年龄、且具有不同驯化状态的无性繁殖浮游动物大型溞（Daphnia magna）的急性耐热性，探究上述关联。结果显示，完全驯化于相对高温（28℃）或低温（17℃）环境的个体，其耐热性会随发育进程出现分化，表明年龄更大、体型更大的个体，提升耐热性的容量更强。但当经低温驯化的个体短暂暴露于高温环境（即进行“热硬化（heat-hardened）”处理）时，体型更小、年龄更轻且驯化容量更低的个体，反而能更快完成热硬化——这是因为它们能够达到或接近实现耐热性的完全驯化。
本研究结果表明，同一物种种群内，不同个体对环境变化的响应速率与响应幅度均可能存在显著差异。我们呼吁学界进一步探究驯化与个体发育之间的种内关联，并深入剖析可能导致此类难以解释的表型变异模式的潜在成因。

研究方法
本研究通过实验室可控条件下的实验采集数据。