Malpaux B, Robinson J E, Wayne N L, Karsch F J
Department of Physiology, University of Michigan, Ann Arbor 48109-0404.
J Endocrinol. 1989 Jul;122(1):269-78. doi: 10.1677/joe.0.1220269.
Three experiments were conducted to evaluate the role that the increasing day lengths of late winter and spring play in timing the breeding season of the Suffolk ewe. In the first experiment, ewes were denied their normal complement of increasing day length by maintaining them on the photoperiod experienced at the winter solstice. This prevented the breeding season from occurring the subsequent autumn. In the second experiment, ewes were exposed to increases in day length at different time-intervals after the winter solstice: the normal time, later than normal or earlier than normal. Once the summer solstice photoperiod was reached, it was maintained until the end of the study. When increasing photoperiod was provided early, the breeding season was advanced; when it was provided late, reproduction was delayed. In the third experiment, ewes were exposed to a continuously increasing photoperiod matching the maximal rate of rise in natural conditions; this treatment was begun on the spring equinox and continued until mid-autumn. The steadily increasing photoperiod did not alter the time of reproductive onset in the autumn. These findings support the following conclusions for timing of the breeding season of the Suffolk ewe. (1) The lengthening photoperiod between the winter and summer solstices is required for the occurrence of the breeding season in the autumn. (2) The time of initial exposure to this lengthening photoperiod provides an important cue for determining when the reproductive period occurs. (3) The time of onset of the breeding season does not depend upon the decreasing photoperiod after the summer solstice, nor does it require the photoperiod to stop increasing as the summer solstice approaches. These findings have been incorporated into a conceptual model for temporal regulation of the annual reproductive cycle of the ewe. An important component of this model is a critical role for increasing photoperiod to initiate a process in the late winter-spring which ultimately leads to an obligatory reproductive onset in the autumn.
进行了三项实验，以评估冬末和春季白昼时长增加在确定萨福克母羊繁殖季节时间方面所起的作用。在第一项实验中，通过让母羊维持在冬至时经历的光周期，剥夺了它们正常增加的白昼时长。这使得随后的秋季没有出现繁殖季节。在第二项实验中，在冬至后的不同时间间隔让母羊接触白昼时长的增加：正常时间、晚于正常时间或早于正常时间。一旦达到夏至光周期，就维持到研究结束。当提前提供增加的光周期时，繁殖季节提前；当延迟提供时，繁殖则推迟。在第三项实验中，让母羊接触持续增加的光周期，其速率与自然条件下的最大上升速率相匹配；这种处理从春分开始，持续到中秋。持续增加的光周期并没有改变秋季生殖开始的时间。这些发现支持了关于萨福克母羊繁殖季节时间安排的以下结论。（1）冬至和夏至之间白昼时长的延长是秋季繁殖季节出现所必需的。（2）首次接触这种延长的光周期的时间为确定生殖期何时发生提供了重要线索。（3）繁殖季节开始的时间不取决于夏至后白昼时长的缩短，也不要求随着夏至临近光周期停止增加。这些发现已被纳入一个关于母羊年度生殖周期时间调节的概念模型。该模型的一个重要组成部分是增加的光周期在冬末至春季启动一个过程中起关键作用，这最终导致秋季必然出现生殖开始。
