Seasonal Adaptation of a Circadian Clock

项目摘要 在这项提案中，我们将使用果蝇作为一个模型系统，以了解昼夜节律(E24小时)系统如何适应环境条件的季节性变化，重点是温度相对未被描述的角色。这一建议的一个主要基础是我们的长期工作，该工作表明，在黑腹盘藻中，关键时钟成分周期(PER)中3‘端内含子(称为dmpi8)的温度依赖剪接是一个显著的“温度传感器”，它调节每日觉醒-睡眠周期的分布，引起季节性适当的反应。像许多白天活动的动物一样，果蝇表现为早晚活动，中间有午睡，大部分在晚上睡觉。在温暖的日子里，dmpi8内含子的剪接效率低下，导致每个RNA水平的缓慢积累，这会增加中午午睡的时间，并可能将炎热的中午时间与干燥相关的风险降至最低。最近的研究表明，多个次优剪接信号[即5‘和3’剪接位点(Ss)]是dmpi8剪接效率的温度敏感性的基础，并赋予了D.Blackogaster将中午午睡延长到下午中晚的能力，这可能有助于其适应温和气候，在那里温暖的白天通常伴随着较长的热期。事实上，在几种自然局限于非洲赤道地区的果蝇物种中，每3‘末端剪接效率的温度依赖变化和每日觉醒活动周期的调整都不存在，这些物种的温度和日长全年几乎没有波动。有趣的是，这些非热反应物种在其每3‘末端的内含子上有很强的5’和3‘s，这可能限制了它们在温带气候中定居的能力。 推测，低温有利于剪接因子与次优剪接信号的相互作用。我们将采取多方面的实验策略，包括生化、分子、细胞培养和全动物方法，以了解调控每个3‘端内含子剪接效率的顺式和反式作用因子，以及它们如何调节果蝇的觉醒-睡眠周期。这将包括新发现的在PER中不同地调节dmpi8剪接的自然多态的特征，并且可能因地理而异。我们还将确定dmpi8剪接的影响是否优先从早晨或晚上的节律性脑中心和/或最近描述的觉醒/睡眠起搏器介导。 通过使用各种自然种群和果蝇物种进行比较研究，这项拟议的工作提供了一个独特的机会，将控制复杂行为的基因表达和神经回路的研究与生态和进化意义结合起来。从更广泛的角度来看，我们的工作表明，在剪接信号水平上进行的自然选择在生命形式的热适应中发挥着重要作用。

Project Summary In this proposal we will use Drosophila as a model system to understand how circadian (E24 hr) systems adapt to seasonal changes in environmental conditions, with an emphasis on the relatively uncharacterized role of temperature. A major foundation of this proposal is based on our long-standing work showing that in D. melanogaster the temperature-dependent splicing of the 3'-terminal intron (called dmpi8) from the critical clock component period (per) is a prominent 'thermosensor' that adjusts the distribution of daily wake-sleep cycles, eliciting seasonably appropriate responses. As in many diurnal animals, Drosophila manifests morning and evening activity bouts separated by a midday siesta and largely sleep during the night. On warm days splicing of the dmpi8 intron is inefficient leading to slow accumulation of per RNA levels, events that enhance midday siesta and likely minimize the risks associated with desiccation during the hot midday hours. Recent progress indicates that multiple suboptimal splicing signals [i.e., 5' and 3' splice sites (ss)] are the basis for the thermosensitivity in the splicing efficiency of dmpi8, and endow D. melanogaster with the ability to prolong its midday siesta into the mid-to-late afternoon, presumably facilitating its adaptation to temperate climates where warm days are typically associated with extended periods of heat. Indeed, temperature-dependent changes in per 3'-terminal splicing efficiency and adjustments in daily wake-activity cycles are absent in several species of Drosophila that are naturally restricted to Afro-equatorial localities, wherein temperature and daylength undergo little fluctuation throughout the year. Intriguingly, these non-thermal responsive species have strong 5' and 3'ss on their per 3'-terminal introns, perhaps limiting their ability to colonize temperate climates. Presumably, low temperatures favor the interaction of splicing factors with suboptimal splicing signals. We will undertake a multi-faceted experimental strategy that includes biochemical, molecular, cell-culture and whole animal approaches to understand the cis- and trans-acting factors regulating the splicing efficiency of per 3'-terminal introns and how they modulate wake-sleep cycles in Drosophila. This will include characterization of newly identified natural polymorphisms in per that differentially regulate dmpi8 splicing and might vary geographically. We will also determine whether the effects of dmpi8 splicing are preferentially mediated from the morning or evening rhythmic brain centers and/or the more recently described arousal/sleep pacemakers. By undertaking comparative studies using a wide variety of natural populations and Drosophila species, this proposed work offers a unique opportunity to integrate studies on gene expression and neural circuits controlling complex behaviors with ecological and evolutionary implications. On a broader perspective, our work suggests that natural selection operating at the level of splicing signals plays an important role in the thermal adaptation of life forms.