Meeting Abstract
S10.11 Tuesday, Jan. 6 The evolution of photoperiod response systems and seasonal GnRH plasticity in birds MACDOUGALL-SHACKLETON, S.A.*; STEVENSON, T.J.; WATTS, H.E.; PEREYRA, M.E.; HAHN, T.P.; Univ. Western Ontario; Johns Hopkins Univ.; Univ. of California, Davis; Univ. of Tulsa; Univ. of California, Davis smacdou2@uwo.ca
Many birds use lengthening photoperiod as an initial predictive cue to time gonadal recrudescence in preparation for the onset of seasonal breeding. In addition, in many species long days result in an eventual decrease in sensitivity to their own stimulatory effects: photorefractoriness. Thus, photorefractoriness can terminate reproduction and lead to gonadal involution while days are still long. Photorefractoriness is often associated with down-regulation of the entire HPG axis, including a cessation of GnRH-I production and release. Opportunistically breeding birds, on the other hand, may never become photorefractory and have only minimal seasonal changes in GnRH-I. Here we review the distribution of different forms of photorefractoriness among birds and test hypotheses regarding the evolution of photorefractoriness and GnRH plasticity. Our conclusions include that spontaneous regression of the gonads while experiencing long days is an ancestral trait, and is associated with down regulation of the GnRH system. Both GnRH-I and GnRH-II exhibit seasonal plasticity. Only four species tested fail to spontaneously regress their gonads while on long days, and all of them are opportunistic breeders. Other aspects of the photoperiod response system appear more evolutionarily labile. For example, several species of cardueline finches retain responsiveness to very long days when putatively photorefractory, differing from other species in which photorefractoriness is absolute. Neural GnRH-I and GnRH-II have been found to respond rapidly to photic and non-photic cues in birds and other vertebrates. Thus, birds that appear to become photorefractory during their annual cycle may possess cryptic flexibility in their ability to respond to photoperiod and other cues.
