Seasonal timing and molecular evolution of circadian photoresponsive genes in Drosophila

在温带地区，许多高等生物利用温度和昼长（光周期）的变化来预测即将到来的季节。昼长被认为是比温度更可靠的线索，许多节肢动物物种，包括果蝇，会察觉到秋天白天的缩短，从而开始冬眠（滞育）冬季反应。对北半球几种昆虫物种的种群研究表明，对白昼长度变化的反应与纬度有关，因此，北方种群在一年中比南方种群更早开始滞育，因为即使白天仍然很长，气温也会更早变冷。白天长度在调节行为和生理的24小时昼夜节律中也起着重要作用。北方人口在夏季暴露在极长的白天，实验室的研究表明，这种明暗制度会破坏昼夜节律行为。几项研究表明，北方人对这种挑战的反应是降低生物钟对光线的敏感度。我们最近分析了一种名为“永恒”（tim）的基因的自然多态性，这种基因控制着果蝇24小时的行为和生理昼夜节律。TIM蛋白对光很敏感，并调节生物钟对光的反应。我们发现，7-1万年前在南欧出现的一种新的时间等位基因，通过定向选择在整个欧洲传播。这种新的tim变体具有减弱的昼夜节律和光周期光响应，因此在欧洲这样的季节性环境中，它比携带tim基因的苍蝇更早地预测到即将到来的冬天，因此在这些恶劣的条件下有更好的生存机会。这是因为新的TIM蛋白在受到光的刺激时，不容易与一种叫做隐色素（CRY）的光感受器发生物理反应。因此，新的突变基因通过其改变的光周期行为在种群中传播，这在欧洲使其具有竞争优势，即自然选择的达尔文主义。我们现在已经观察到，CRY和一个新发现的蛋白质DAY，与CRY相互作用，并抑制其光敏性，显示出异常高水平的序列变异。CRY蛋白中的一些变化使光感受器的光反应减弱。我们将研究这些CRY和DAY变异是如何在欧洲分布的，以及它们是否显示出由自然选择维持的特征。在对六个基因中哪一个可能与滞育更相关进行功能评估后，我们还将把这种方法扩展到介导典型视觉通路的视紫红质（Rh）基因。我们将研究不同的cry， day和Rh变异对滞育和昼夜节律光敏感性的功能后果，并研究它们对达尔文适应度的影响。通过将遗传变异（和空间分布）与表型变异相关联，我们将确定种群如何适应不同的热环境和光周期环境。我们的工作将对开发评估气候变化的生物标志物具有明确的意义。

In temperate regions, many higher organisms use changes in temperature and daylength (photoperiod) to anticipate the coming season. Daylength is considered a more reliable cue than temperature, and numerous arthropod species, including fruitflies, detect the autumnal shortening of the day to initiate their hibernating (diapause) winter response. Population studies from the northern hemisphere in several insect species have demonstrated that the response to the change in daylength is correlated with latitude, so that northern populations initiate diapause earlier in the year than southern populations, because it gets colder earlier, even though days are still long. Daylength also plays an important role in regulating the daily circadian 24 h rhythms of behaviour and physiology. Northern populations are exposed to extremely long daylengths in the summer, and studies in the laboratory reveal that such light-dark regimes can disrupt circadian behaviour. Several studies have shown that northern populations respond to this challenge by reducing the light-sensitivity of their clock. We have recently analysed natural polymorphism in a gene called timeless (tim) that controls the 24-hour circadian rhythm of behaviour and physiology in the fruit fly. TIM protein is sensitive to light and mediate how the clock responds to light. We have found that a new allele of tim that arose in southern Europe 7-10,000 years ago, is spreading throughout Europe by directional selection. This new tim variant has an attenuated circadian and photoperiodic light response, with the result that in a seasonal environment such as Europe, it anticipates the oncoming winter earlier than flies carrying the tim gene and therefore has a better chance of surviving these harsh conditions. . The reason for this is that the new TIM protein, on stimulation by light, does not physically interact as readily, with a photoreceptor called cryptochrome (CRY). Thus the new mutant tim gene has spread through the population because of its altered photoperiodic behaviour, which in Europe, gives it a competitive advantage, ie Darwinism by natural selection. We have now observed that CRY, and a newly identified protein DAY, which interacts with CRY, and acts to repress its photoceptiveness, show unusually high levels of sequence variation. Some of these changes in the CRY protein make the photoreceptor less photoresponsive. We will examine how these CRY and DAY variants are distributed in Europe, and whether they show the signatures of being maintained by natural selection. We will also extend this approach to the rhodopsin (Rh) genes, which mediate the canonical visual pathway, after a functional assessment of which of the six genes may be the more relevant to diapause. We shall examine the functional consequences of the different cry, day and Rh variants on diapause and circadian light sensitivity, and also study their implications for Darwinian fitness. By correlating genetic variation (and spatial distribution) with phenotypic variation we will identify how populations adapt to different thermal and photoperiodic environments. Our work will have clear implications for developing biological markers for assessing climate change.