Seasonal timing and molecular evolution of circadian photoresponsive genes in Drosophila

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.