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日长(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信号，包括果蝇在内的许多节肢动物物种会探测到秋季白天的缩短，以启动它们冬眠(滞育)的冬季反应。北半球几种昆虫物种的种群研究表明，对日长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，因为尽管白天仍然很长，但北方种群变得更早变冷了。昼长在调节昼夜24小时的行为和生理节律方面也起着重要作用。北方人在夏天暴露在极长的日光下，实验室的研究表明，这种明暗制度可以扰乱昼夜行为。几项研究表明，北方人口通过降低生物钟的光敏感度来应对这一挑战。我们最近分析了一种名为永恒(TIM)的基因的自然多态，该基因控制着果蝇24小时的行为和生理昼夜节律。TIM蛋白对光敏感，并调节生物钟对光的反应。我们发现，7-10,000年前在南欧出现的一种新的TIM等位基因正在通过定向选择在整个欧洲传播。这种新的TIM变异体具有减弱的昼夜和光周期光反应，结果是在欧洲等季节性环境中，它比携带TIM基因的苍蝇更早预测即将到来的冬天，因此更有可能在这些恶劣的条件下生存下来。。其原因是，在光的刺激下，新的TIM蛋白在物理上不容易与一种名为隐花色素(CREY)的光感受器相互作用。因此，新突变的Tim基因已经在种群中传播，因为它改变了光周期行为，这在欧洲赋予了它的竞争优势，即自然选择的达尔文主义。我们现在已经观察到，哭泣和一种新发现的蛋白质日，它与哭泣相互作用，并抑制其光敏性，显示出异常高水平的序列变异。哭泣蛋白中的一些变化会使感光器对光的反应变得不那么灵敏。我们将研究这些哭声和白天的变种在欧洲是如何分布的，以及它们是否显示出被自然选择维持的特征。我们还将把这种方法扩展到调节典型视觉通路的视紫红质(Rh)基因，在对六个基因中的哪一个可能与滞育更相关的功能进行评估后。我们将研究不同的哭声、白昼和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合性的影响。通过将遗传变异(和空间分布)与表型变异联系起来，我们将确定种群如何适应不同的热和光周期环境。我们的工作将对开发评估气候变化的生物标记物产生明显的影响。

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.