Circadian clocks schedule biological functions at a specific time of the day. Full comprehension of the clock function requires precise understanding of their entrainment to the environment. The phase of entrained clock is plastic, which depends on different factors such as the period of endogenous oscillator, the period of the zeitgeber cycle (T), and the proportion of light and darkness (day length). The circadian clock of fruit fly Drosophila melanogaster is able to entrain to a wide range of T-cycles and day lengths. Here, we investigated the importance of the neuropeptide Pigment-Dispersing Factor (PDF) for entrainment by systematically studying locomotor activity rhythms of Pdf ⁰ mutants and wild-type flies under different T-cycles (T22 to T32) and different day lengths (8, 12, and 16 hour [h]). Furthermore, we analysed PERIOD protein oscillations in selected groups of clock neurons in both genotypes under T24 and T32 at a day length of 16 h. As expected, we found that the phase of Drosophila’s evening activity and evening neurons advanced with increasing T in all the day lengths. This advance was much larger in Pdf ⁰ mutants (~7 h) than in wild-type flies causing (1) pronounced desynchrony between morning and evening neurons and (2) evening activity to move in the morning instead of the evening. Most interestingly, we found that the lights-off transition determines the phase of evening neurons in both genotypes and that PDF appears necessary to delay the evening neurons by ~3 h to their wild-type phase. Thus, in T32, PDF first delays the molecular cycling in the evening neurons, and then, as shown in previous studies, delays their neuronal firing rhythms to produce a total delay of ~7 h necessary for a wild-type evening activity phase. We conclude that PDF is crucial for appropriate phasing of Drosophila activity rhythm.

生物钟在一天中的特定时间安排生物功能。对时钟功能的全面理解需要精确理解它们对环境的影响。夹带时钟的相位是可塑性的，这取决于不同的因素，例如内生振荡器的周期、时代周期（T）的周期以及明暗比例（日长）。果蝇 Drosophila melanogaster的生物钟能够调节范围广泛的 T 周期和昼长。在这里，我们通过系统地研究Pdf ^{0 的}运动活动节律，研究了神经肽色素分散因子 (PDF) 对夹带的重要性突变体和野生型果蝇在不同的 T 周期（T22 到 T32）和不同的日长（8、12 和 16 小时 [h]）下。此外，我们在 16 小时的日长下分析了 T24 和 T32 两种基因型中选定的时钟神经元组中的 PERIOD 蛋白振荡。正如预期的那样，我们发现果蝇夜间活动的阶段和夜间神经元随着 T 在所有白天长度的增加而推进。这种进步在Pdf ^{0 中}要大得多突变体（~7 小时）比在野生型果蝇中导致 (1) 早晚神经元之间明显的不同步和 (2) 晚间活动在早晨而不是晚上移动。最有趣的是，我们发现熄灯转变决定了两种基因型中晚间神经元的相位，并且 PDF 似乎需要将晚间神经元延迟约 3 小时到其野生型阶段。因此，在 T32 中，PDF 首先延迟夜间神经元中的分子循环，然后，如之前的研究所示，延迟它们的神经元放电节律，以产生野生型夜间活动阶段所需的约 7 小时的总延迟。我们得出结论，PDF 对于果蝇活动节律的适当定相至关重要。