PEREYRA, M.E.*; SHARBAUGH, S.M.; HAHN, T.P.; U.C. Davis; U. of AK, Fairbanks; U.C. Davis: Interspecific variation in photo-induced hypothalamic GnRH plasticity in cardueline finches
In this study, we present data on photo-induced changes in hypothalamic GnRH immunoreactivity associated with the development of refractoriness in males of 3 closely related species of cardueline finch: common redpoll, pine siskin and white-winged crossbill. All are nomadic, mid- to high-latitude breeders with varying levels of reproductive flexibility. Redpolls are seasonal, pine siskins flexibly seasonal and white-winged crossbills temporally opportunistic breeders. Males were exposed to either long (20L:4D) or short day (5L:19D) for 4.5 months, plasma LH, testis size, and molt score measured and GnRH assessed via immunocytochemistry. Differences among the three species in GnRH cell numbers, size and staining, with respect to treatment, parallel patterns in breeding flexibility. Similar increases in plasma LH and testis size occurred in all three taxa upon initial exposure to long days, but responses to prolonged exposure were different. Significant reductions in GnRH cell numbers, area, perimeter length and staining intensity occurred in both redpolls and pine siskins on long, as compared with short days. These decreases, coupled with spontaneous testicular regression and onset of molt, were consistent with the development of photorefractoriness. In contrast, GnRH cell numbers and staining intensity did not differ between white-winged crossbills on long versus short days and only cell area was reduced in long-day birds. This, along with their limited gonadal regression and molt while on long days, suggests that the loss of photosensitivity and development of photorefractoriness occurs more slowly or not at all in this species. These results are consistent with the idea that interspecific differences in the duration and timing of reproductive seasons may result from underlying differences in the degree of neural plasticity in the GnRH system.
