Timeless and diapause in Drosophila

冬眠是一种假死的保护状态，生活在世界温带的动植物每个季节都会冬眠。在昆虫中，这种季节性循环被称为滞育，是由于冬天来临时白昼长度的减少而引起的。这个星球上的另一个伟大的节奏是昼夜节律周期，即每天24小时的周期，我们都认为这是我们的“生物钟”。最近，我们发现了一种叫做“永恒”的基因与季节性滞育周期之间的联系，这种基因控制着遗传学家最喜欢的宠物——果蝇的24小时节律。大约8000年前，也就是苍蝇从非洲入侵欧洲的最后一次冰川期几千年后，意大利东南部出现了永恒基因ls-tim的新突变。这种突变使苍蝇产生了两种TIM蛋白，L-TIM和S-TIM，而古老的S-TIM基因只能产生S-TIM。这种新的突变，ls-tim，在自然选择下传播，因为它使苍蝇适应了欧洲的季节性环境。它通过降低短时间苍蝇滞育机制的光敏感性来做到这一点。因此，即使在相对较长的白天，短时蝇“看到”较短的白天，这刺激它们比短时蝇更早地滞育。苍蝇在欧洲越早进入滞育，就越有可能在即将到来的冬天存活下来。此外，24小时的昼夜节律钟对光线也不太敏感，这在欧洲也是适应性的，那里夏天有非常奇特的光周期，这导致苍蝇的生物节律出现异常。因此，L-TIM减弱了果蝇对昼夜节律和季节行为的光敏性，这也许就是这种新突变在欧洲传播的原因。从生物化学角度来看，L-TIM蛋白对光的敏感度较低，因为它与一种名为隐花色素的昼夜节律光感受器的物理相互作用较弱。L-TIM突变在L-TIM蛋白中产生了S-TIM不共享的位点。这个新位点可能被磷酸化，这是一种可能改变L-TIM形状和稳定性的蛋白质修饰。这可能提供了为什么短时间果蝇行为不同的生化线索。我们将研究这个位点，看看它是否确实被磷酸化了。我们还将研究在产生滞育的短日低温条件下，L-TIM和S-TIM是否会改变它们产生的比例。此外，我们将研究在这些条件下，L-TIM和S-TIM“触摸”果蝇大脑和眼睛中的光感受器隐色素的强度，因为这种物理相互作用为昼夜节律机制提供了光输入。关于生物钟是否有助于滞育的争论已经持续了很长时间。利用这个永恒的基因，我们将从基因上解剖果蝇的神经系统，以研究哪一组神经元对滞育很重要。因为24小时时钟神经元是众所周知的，很容易识别，我们应该能够看到这些细胞是否与苍蝇的冬眠有关。然后，我们可以在滞育条件下观察L-TIM和S-TIM蛋白在L-TIM果蝇的相关细胞中，看看这两种蛋白的行为是否不同。昆虫滞育是控制温带地区医疗和农业害虫的潜在目标，因此我们的工作在未来可能具有实际的附带效益。

Hibernation is a protective state of suspended animation that comes around every season in plants and animals that live in the temperate zones of the world. In insects, this seasonal cycle is called diapause, and is stimulated by the reduction in daylength as winter approaches. The other great rhythm on this planet is the circadian cycle which is the daily 24 hour cycle that we all recognise as our 'body clock'. Recently we have found a connection between a gene called timeless that controls the 24 hour rhythm of the geneticists favourite pet, the fruitfly, and the seasonal diapause cycle. A new mutation in the timeless gene, ls-tim, occurred in south-eastern Italy about 8000 years ago, a few thousand years after the last glaciation when flies invaded Europe from Africa This mutation allows the fly to make two types of TIM protein, L-TIM and S-TIM, whereas the ancient gene, s-tim can only make S-TIM. The new mutation, ls-tim, is spreading under natural selection, because it adapts the fly to the seasonal environment of Europe. It does this by reducing the light sensitivity of the ls-tim fly's diapause mechanism. Thus, even in relatively long days, ls-tim flies 'see' shorter days, and this stimulates them diapause earlier than in s-tim flies. The earlier the fly goes into diapause in Europe, the better its chances of surviving the oncoming winter. In addition, the 24 hour circadian clock of ls-tim is also less light-sensitive and this too is adaptive in Europe, which has very exotic photoperiods in summer which causes the fly's biorhythms to creak. Consequently, L-TIM attenuates the fly's photosensitivity for both circadian and seasonal behaviours, and that is perhaps why this new mutation has spread in Europe. Biochemically, the L-TIM protein is less light sensitive because of its weak physical interaction with a circadian photoreceptor called Cryptochrome. The ls-tim mutation creates a site in the L-TIM protein that is not shared in S-TIM. This new site may be phosphorylated, a protein modification that may alter the shape and stability of L-TIM. This may provide the biochemical clue for why ls-tim flies behave differently. We shall study this site to see whether it is indeed phosphorylated. We shall also investigate whether ls-tim flies change the ratios of the L-TIM and S-TIM they produce under the short days and cold temperature that produce diapause. In addition we will study under these conditions how strongly L-TIM and S-TIM 'touch' the photoreceptor Cryptochrome in the fly's brain and eyes, as this physical interaction provides the light input into the circadian mechanism. There has been a long running debate about whether the circadian clock contributes to diapause. Using the timeless gene, we shall genetically dissect the fly's nervous system in order to investigate which groups of neurons are important for diapause. Because the 24 hour clock neurons are well known and easily identified, we should be able to see whether these cells are relevant for the fly's hibernation. We can then visualise L-TIM and S-TIM protein in the relevant cells of ls-tim flies under diapause conditions, to see whether the two proteins behave differently. Insect diapause is a potential target for the control of medical and agricultural pests in temperate regions, and so our work may have practical spin-offs in future.