Meeting Abstract
Most studies of reproductive timing and the underlying mechanisms have used only male subjects, despite the critical role females play in determining breeding phenology. To better understand how animals shift into the reproductive state, more knowledge of hormonal mechanisms in the brain underlying the transition is necessary, especially in females. Closely related populations found in the same environment in early spring that differ in reproductive timing provide an opportunity to examine differences in these mechanisms. We studied a migrant and resident population of dark-eyed juncos (Junco hyemalis) that are found in overlapping ranges during the winter and early spring. Populations differ in reproductive timing, as residents initiate breeding before migrants depart, whereas migrants do not breed until they reach northern breeding grounds. To study differences in the hypothalamic mechanisms of reproduction, we caught 16 migrant and 13 resident free-living females between March 25 and April 11. We quantified expression of mRNA transcripts for genes related to reproduction in the hypothalamus. We found that resident females had higher abundance of gonadotropin-releasing hormone transcripts than migrant females, indicating that residents have greater hormonal top-down stimulation than migrants during this time. Additionally, we found higher transcript abundance of estrogen receptor and androgen receptor in migrant than resident females, suggesting that negative feedback might delay reproductive development in migrants. These differences in hypothalamic mechanisms could help to explain population differences in reproductive timing.
