Photoperiodic synchronization of the annual reproductive rhythm of the ewe.

Abstract

Many long lived species have endogenous circannual rhythms. The annual cycle of daylength synchronizes endogenous rhythms so that rhythms are appropriately phased with the seasons, occur synchronously among conspecifics and have a period of one year. The purpose of this thesis is to investigate the photoperiodic requirements for synchronization of the circannual reproductive rhythm of sheep. An experimental system was employed in which specific photoperiodic signals were replaced in a physiological and reversible manner to pinealectomized ewes that could not otherwise perceive photoperiod. Specific photoperiodic cues were restored via infusion of circadian patterns of melatonin, the pineal hormone that transduces photoperiodic information to the neuroendocrine axis. In this manner, the effects of specific photoperiodic signals on the circannual reproductive rhythm were monitored. The experimental findings demonstrate that the annual reproductive rhythm can be generated in the absence of the pineal gland. Pinealectomized ewes exhibited asynchronous circannual reproductive cycles, which were out of phase with the seasons and which usually had a period less than one year. In addition, the results of these studies demonstrate that the pineal gland mediates synchronization of the annual reproductive cycle via its circadian pattern of melatonin secretion. Significant new findings of these studies are that the circannual reproductive rhythm can be synchronized by exposure to information about just one discrete segment of the photoperiodic year and not all segments of the photoperiodic year provide equally effective synchronizing signals. Specifically, cues associated with the longer photoperiods perceived around the summer solstice are the most effective in synchronizing rhythms with the seasons and among ewes. Furthermore, these studies provide evidence that, in the sheep, the specific characteristics of the photoperiodic signal, rather than the stage of the reproductive rhythm when the signal is perceived, play the major role in synchronizing the rhythm with the environment. Findings were incorporated into a conceptual model to describe how discrete segments of the photoperiodic year synchronize circannual rhythms so they are appropriately phased with the seasons, occur synchronously among conspecifics and have a period of one year.