Discussion
We compared early spring proteomic profiles of neuroendocrine tissues (hypothalamus and pituitary gland) derived from two populations of free-living dark-eyed juncos that differ in timing of reproduction and whether or not they migrate. Feathers hydrogen isotope values were significantly higher in resident juncos, indicating a lower breeding latitude than migrants (Fudickar et al., 2016b; Singh et al., 2019). The discrete non-overlapping feather δ2Hfvalues provide strong evidence that the populations do not have overlapping breeding ranges. The hydrogen isotope values measured in claw tissue that were grown on their wintering location did not differ between two populations, suggesting that both groups overwintered at the capture site (Lymburner, Kelly, Hobson, & MacDougall-Shackleton, 2016; Kimmitt, Hardman, Stricker, & Ketterson, 2019). Together, feather and claw δ2H data indicate that migrant and resident juncos in the current study were sympatric during winter and originated from different breeding latitudes. Resident juncos (earlier breeding) exhibited larger cloacal protuberances and higher levels of circulating testosterone than migrants; whereas, migrant juncos had greater fat deposits than resident juncos, supporting our hypothesis that the two seasonally sympatric junco subpopulations differ in indices of reproductive and migratory preparedness in early spring, despite exposure to the same environmental cues at their overwintering location.
Prior research on this system using transcriptomics and quantification of specific target genes has shown variation in gene expression in tissues related to the phase lag in the timing of reproduction in migrants as compared to residents, including muscle, blood, and brain (Fudickar et al., 2016a; Bauer et al., 2018; Singh et al., under preparation). But to our knowledge, prior research has not quantified neuroendocrine proteins as a functional correlate for reproductive timing and migratory preparedness.
Results reported here provide a comparative proteome for the neuroendocrine system for birds sampled in the wild in early spring as residents prepare to breed while migrants prepare to migrate. We found a number of proteins thought to play critical roles in GnRH1 transcription, translation, post-translational modification, stability, and synthesis and release in hypothalamic and pituitary gonadotrophs to be more abundant in early recrudescing resident juncos. In contrast, we found increased levels of different proteins involved in cholesterol homeostasis, positive regulation of angiogenesis, oxidative reductive processes, neuron and dendrite morphogenesis, and proteins that possibly foster hyperphagia, fat gain and migratory locomotor activity to be more abundant in migrants. These results provide important insight into how differences in neuroendocrine sensitivity to day length give rise to population-level variation in timing in songbirds occupying the same environment while some are preparing to breed, while others are preparing to migrate. To aid in interpretation of the role played by the differentially identified proteins, we turned to the literature in other better-studied taxa, particularly mammals.