BACKGROUND In many birds, day length (= photoperiod) regulates reproductive cycle. The photoperiodic environment varies between different seasons and latitudes. As a consequence, species at different latitudes may have evolved separate photoperiodic strategies or modified them as per their adaptive need. We studied this using house sparrow as a model since it is found worldwide and is widely investigated. In particular, we examined whether photoperiodism in house sparrows (Passer domesticus) at 27 degrees N, 81 degrees E shared features with those exhibited by its conspecifics at high latitudes. RESULTS Initial experiment described in the wild and captive conditions the gonad development and molt (only in captives) cycles over a 12-month period. Both male and female sparrows had similar seasonal cycles, linked with annual variations in day length; this suggested that seasonal reproduction in house sparrows was under the photoperiodic control. However, a slower testis and attenuated follicular growth among captives indicated that other (supplementary) factors are also involved in controlling the reproductive cycle. Next experiment examined if sparrows underwent seasonal variations in their response to stimulatory effects of long day lengths. When birds were transferred every month over a period of 1 year to 16 hours light:8 hours darkness (16L:8D) for 17-26 weeks, there was indeed a time-of-year effect on the growth-regression cycle of gonads. The final experiment investigated response of house sparrows to a variety of light-dark (LD) cycles. In the first set, sparrows were exposed for 31 weeks to photoperiods that were close to what they receive in between the period from sunrise to sunset at this latitude: 9L:15D (close to shortest day length in December), 12L:12D (equinox, in March and September) 15L:9D (close to longest day length in June). They underwent testicular growth and regression and molt in 12L and 15L photoperiods, but not in 9L photoperiod. In the second set, sparrows were exposed for 17 weeks to photoperiods with light periods extending to different duration of the daily photosensitivity rhythm (e.g. 2L:22D, 6L:18D, 10L:14D, 14L:10D, 18L:6D and 22L:2D). Interestingly, a slow and small testicular response occurred under 2L and 10L photoperiods; 6L:18D was non-inductive. On the other hand, 14L, 18L and 22L photoperiods produced testicular growth and subsequent regression response as is typical of a long day photostimulation. CONCLUSION Subtropical house sparrows exhibit photoperiodic responses similar to that is reported for its population living at high latitudes. This may suggest the conservation of the photoperiodic control mechanisms in birds evolved over a long period of time, as a physiological strategy in a temporally changing environment ensuring reproduction at the best suited time of the year.

中文翻译：

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控制亚热带麻雀（Passer domesticus）的年度生殖周期：为鸟类的光周期控制机制提供了证据。

背景技术在许多鸟类中，日长（=光周期）调节生殖周期。光周期环境在不同季节和纬度之间变化。结果，不同纬度的物种可能已经进化出单独的光周期策略或根据其适应性需求对其进行了修改。我们以麻雀为模型研究了它，因为它已在世界范围内发现并受到广泛研究。特别是，我们研究了北纬27度，东经81度的麻雀（Passer domesticus）的光周期性是否与高纬度的物种具有相同的特征。结果最初的实验描述了在野生和圈养条件下12个月内性腺的发育和蜕皮（仅在圈养下）周期。雄麻雀和雌麻雀都有相似的季节性周期，并与日长的年度变化有关。这表明麻雀的季节性繁殖处于光周期控制之下。然而，圈养者的睾丸速度减慢和卵泡生长减弱表明，其他（补充）因素也参与了生殖周期的控制。下一个实验检查了麻雀对长日照刺激的响应是否经历了季节性变化。当在每月的1年中将家禽转移到16小时光照：8小时黑暗（16L：8D）中持续17-26周时，确实对性腺的生长-退化周期产生了一年中的影响。最终实验研究了麻雀对各种明暗（LD）周期的响应。在第一组中 在以下纬度下，麻雀暴露于光周期的时间为31周，该光周期接近于从日出到日落之间的时间：9L：15D（接近12月的最短日照长度），12L：12D（3月和9月的春分点） ）15L：9D（接近6月的最长一天）。他们在12L和15L的光周期中经历了睾丸的生长和消退，并蜕皮，而在9L的光周期中则没有。在第二组中，将麻雀暴露于光周期17周，光照时间延长到每日光敏节律的不同持续时间（例如2L：22D，6L：18D，10L：14D，14L：10D，18L：6D和22L：2D） ）。有趣的是，在2L和10L光周期下，睾丸反应缓慢而缓慢。6L：18D为非感应式。另一方面14L 18L和22L光周期产生了睾丸生长，随后出现了退化响应，这是一天光刺激的典型表现。结论亚热带麻雀的光周期反应类似于其生活在高纬度地区的报道。这可能表明，对鸟类的光周期控制机制的保护是长期发展的，这是在时间变化的环境中的生理策略，可确保在一年中最合适的时间繁殖。