Bradshaw W E, Holzapfel C M, Davison T E
Department of Biology, University of Oregon, Eugene, OR 97403-1210, USA e-mail:
Oecologia. 1998 Dec;117(4):486-495. doi: 10.1007/s004420050684.
The mosquito, Wyeomyia smithii, enters a larval dormancy or diapause that is initiated, maintained, and terminated by photoperiod. The median or critical photoperiod regulating diapause increases from 12 h of light per day along the Gulf of Mexico, USA (30° N), to over 15 h in southern Canada (49° N). Photoperiodic time measurement in W. smithii comprises both rhythmic and hourglass (interval timer) components. Using interrupted-night and resonance experiments, we show that both the rhythmic and hourglass components are prominent in the southern (ancestral) populations and that the influence of the rhythmic component declines with increasing latitude, while the hourglass component remains strong in northern (derived) populations. Previously, it has been shown that the genetic differences in critical photoperiod between northern populations and their southern ancestors involve not only the additive (independent) effects of genes, but also gene-gene interaction (epistasis). We therefore conclude that adaptive evolution of W. smithii has probably involved the progressive epistatic masking of the ancestral rhythmic component resulting in photoperiodic time measurement in northern populations accomplished principally through a day-interval timer. A comparison of W. smithii with previous studies indicates that the decline in critical photoperiod with increasing latitude represents an overall decrease in response to light rather than a shift in the timing of photosensitivity among arthropods in general. We propose that the underlying functional components of photoperiodic time measurement, as well as the overt photoperiodic response, are either homologous or are themselves responding directly to selection over latitudinal gradients in seasonality.
斯氏伊蚊会进入一种幼虫休眠或滞育状态，这种状态由光周期启动、维持并终止。调节滞育的中位或临界光周期从美国墨西哥湾沿岸（北纬30°）每天12小时光照增加到加拿大南部（北纬49°）超过15小时。斯氏伊蚊的光周期时间测量包括节律性和沙漏型（间隔定时器）成分。通过中断夜间和共振实验，我们发现节律性和沙漏型成分在南部（祖先）种群中都很突出，并且节律性成分的影响随着纬度增加而下降，而沙漏型成分在北部（衍生）种群中仍然很强。此前已经表明，北部种群与其南部祖先在临界光周期上的遗传差异不仅涉及基因的加性（独立）效应，还涉及基因-基因相互作用（上位性）。因此，我们得出结论，斯氏伊蚊的适应性进化可能涉及对祖先节律性成分的渐进上位性掩盖，导致北部种群的光周期时间测量主要通过日间隔定时器完成。将斯氏伊蚊与先前的研究进行比较表明，临界光周期随纬度增加而下降代表了对光反应的总体降低，而不是一般节肢动物光敏性时间的转变。我们提出，光周期时间测量的潜在功能成分以及明显的光周期反应要么是同源的，要么它们本身直接响应季节性纬度梯度上的选择。
