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.