Daylength is a major environmental condition that varies seasonally and predictably along a latitudinal cline, where higher latitudes exhibit greater ranges in total daylengths. Generally, the circadian clock acts as a network of genes whose expression dynamics are known to control daily rhythms in response to daylength, and it enables the control of many physiological processes such as growth and development. While well studied in many model animals, the influence of daylength variation on phenotypic evolution is poorly examined in marine species. In this study we demonstrate that two allopatric populations of the intertidal crustacean Tigriopus californicus exhibit plastic and divergent phenotypic responses to changes in daylength. Using common-garden experiments, we discovered that shorter daylengths promoted decreased adult body size and faster growth rates in the two divergent populations, suggesting a plastic response to shortened days. In addition, the higher-latitude population exhibited a faster growth rate at any daylength condition, indicating a fixed response, possibly as a result of adaptation to respective natural light regimes. Gene expression profiles of several circadian clock genes, monitored throughout the day by quantitative polymerase chain reaction, revealed that the key core clock genes reach higher daily transcription maxima in the southern population compared to the northern population, pointing to divergent strategies used to respond to changes in daylength. Many modifier genes to the circadian clock showed similar plastic responses to the different daylengths, supporting the existence of at least some conserved gene expression across both populations. Ultimately, our results suggest that photoperiod and daylength exert a potent selective pressure underexplored in marine systems and warranting further future research.

中文翻译：

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桡足类Tigriopus californicus异域种群不同光周期下生长和基因表达的表型变异

昼长是一种主要的环境条件，沿纬度线随季节变化且可预测，其中较高的纬度在总日长中表现出更大的范围。通常，生物钟充当一个基因网络，其表达动态已知可根据日长控制日常节律，并且它能够控制许多生理过程，例如生长和发育。虽然在许多模型动物中得到了很好的研究，但在海洋物种中很少研究日长变化对表型进化的影响。在这项研究中，我们证明潮间带甲壳动物 Tigriopus californicus 的两个异域种群对日长变化表现出可塑性和不同的表型反应。使用普通花园实验，我们发现，在两个不同的人群中，较短的日照促进了成年体型的减小和更快的生长速度，这表明对缩短的日照产生了可塑性的反应。此外，高纬度人群在任何日长条件下都表现出更快的增长速度，这表明响应是固定的，这可能是适应各自自然光照条件的结果。通过定量聚合酶链反应全天监测的几个生物钟基因的基因表达谱显示，与北方人群相比，南方人群的关键核心时钟基因达到更高的每日转录最大值，表明用于应对变化的不同策略在白天。生物钟的许多修饰基因对不同的日长表现出相似的可塑性反应，支持在两个种群中至少存在一些保守的基因表达。最终，我们的研究结果表明，光周期和日长会施加一种在海洋系统中未充分探索的强大选择压力，值得进一步研究。