Insect species with a wide distribution offer a great opportunity to trace latitudinal variation in the photoperiodic regulation of traits important in reproduction and stress tolerances. We measured this variation in the photoperiodic time-measuring system underlying reproductive diapause in Drosophila montana, using a Nanda-Hamner (NH) protocol. None of the study strains showed diel rhythmicity in female diapause proportions under a constant day length (12 h) and varying night lengths in photoperiods ranging from 16 to 84 h at 16°C. In the northernmost strains (above 55°N), nearly all females entered diapause under all photoperiods and about half of them even in continuous darkness, while the females of the southern strains showed high diapause proportions only in the circadian 24 h photoperiod. Significant correlation between the strains’ mean diapause proportions in ≥ 24 h photoperiods and critical day length (CDL; half of the females enter diapause) suggests at least partial causal connection between the traits. Interestingly, females of the northern strains entered diapause even in ≤ 24 h photoperiods, where the night length was shorter than their critical night length (24 h - CDL), but where the females experienced a higher number of Light:Dark cycles than in 24 h photoperiods. NH experiments, performed on the control and selection lines in our previous selection experiment, and completed here, gave similar results and confirmed that selection for shorter, southern-type CDL decreases female diapausing rate in non-circadian photoperiods. Overall, our study shows that D. montana females measure night length quantitatively, that the photoperiodic counter may play a prominent but slightly different role in extra short and extra long photoperiods and that northern strains show high stability against perturbations in the photoperiod length and in the presence of LD cycles. These features are best explained by the quantitative versions of the damped external coincidence model.

分布广泛的昆虫物种为追踪繁殖和胁迫耐受性重要性状的光周期调节的纬度变化提供了绝佳的机会。我们使用 Nanda-Hamner (NH) 方案测量了蒙大拿州果蝇生殖滞育背后的光周期时间测量系统的这种变化。在 16°C 下，在恒定的日长（12 小时）和不同的夜间长度（16 至 84 小时的光周期）下，所有研究菌株均未表现出雌性滞育比例的昼夜节律性。在最北的品系（高于55°N）中，几乎所有雌性在所有光周期下都进入滞育，其中约一半甚至在持续黑暗的情况下也进入滞育，而南部品系的雌性仅在昼夜24小时光周期中表现出较高的滞育比例。品系在≥24小时光周期内的平均滞育比例与临界日长（CDL；一半雌性进入滞育）之间的显着相关性表明这些性状之间至少存在部分因果关系。有趣的是，北方品系的雌性即使在≤ 24小时的光周期下也会进入滞育，其中夜间长度短于其临界夜间长度（24小时-CDL），但雌性经历的光：暗循环次数高于24小时h 光周期。 NH 实验在我们之前的选择实验中在对照和选择系上进行，并在这里完成，得到了类似的结果，并证实了对较短的南方型 CDL 的选择降低了非昼夜节律光周期中雌性滞育率。总的来说，我们的研究表明D. 蒙大拿州雌性定量测量夜间长度，光周期计数器可能在超短和超长光周期中发挥突出但略有不同的作用，并且北方品系对光周期长度和 LD 周期存在的扰动表现出高度稳定性。这些特征可以通过阻尼外部符合模型的定量版本得到最好的解释。