Meeting Abstract

P2-11    Phylogeographic variation in the responses of gonadotropin-releasing hormone neurons to low temperature winter dormancy Procter, ND*; Iopu, F; Merlino, LJ; LeMaster, MP; Lutterschmidt, DI; University of California, Irvine; Portland State University; University of California, Irvine; Western Oregon University; Portland State University; University of California, Irvine nprocter@uci.edu

Differences in reproductive timing between sexes and among populations are well documented across vertebrates, yet the mechanisms responsible for these differences are poorly understood. We previously showed that environmental temperature modulates the neuroendocrine gonadotropin-releasing hormone (GnRH) system in garter snakes. In this study, we examined potential differences in the sensitivity of GnRH to environmental temperature in four species of garter snakes: red-sided (Thamnophis sirtalis parietalis) and western plains (Thamnophis radix) garter snakes from Manitoba, Canada and red-spotted (Thamnophis sirtalis concinnus) and northwestern (Thamnophis ordinoides) garter snakes from western Oregon, US. While T. s. parietalis and T. radix are sympatric species, very little is known about the seasonal biology of T. radix, or why it varies from that of T. s. parietalis. Similarly, little is known about potential differences between sympatric T. s. concinnus and T. ordinoides. Finally, we asked if different populations of the same species (T. sirtalis) respond differently to identical environmental cues. All animals were field-collected and hibernated in complete darkness at 4ºC. Blood samples were collected at 0, 3, 6, or 12 weeks in hibernation before snakes were euthanized and brains collected for immunohistochemistry. The total number of GnRH immunoreactive cells was counted manually using a ZEISS Axio Imager 2 microscope; the area of GnRH cell bodies was measured as a proxy for cell activity using ImageJ software. As predicted, we observed significant differences in the response of the GnRH system to prolonged low-temperature winter dormancy among the different garter snake species. Our results suggest that differences in the temperature response curves of GnRH neurons may contribute to phylogeographic differences in reproductive timing.