Seasonal differences in hypothalamic thyroid?stimulating hormone ?, gonadotropin?releasing hormone?I and deiodinase expression between migrant and resident subspecies of white?crowned sparrow ( <i>Zonotrichia leucophrys</i> )

Across taxa, the seasonal transition between non-breeding and breeding states is controlled by localised thyroid hormone signalling in deep brain via reciprocal switching of deiodinase enzyme expression from type 3 (DIO3) to 2 (DIO2). This switch considered be mediated increasing thyroid-stimulating ? (TSH?) release pars tuberalis, which occurs response a change photoperiod. Although well characterised handful model organisms laboratory settings, this pathway remains largely unexplored free-living animals under natural environmental conditions. In comparative gene study, we investigated hypothalamic two seasonally subspecies white-crowned sparrow (Zonotrichia leucophrys), across entirety their annual cycles. The migratory Gambel's (Z. l. gambelii) resident Nuttall's nuttalii) differ with respect timing reproduction, as life history stage strategies. DIO3 mRNA was elevated DIO2 reduced wintering period both subspecies, peaked prior onset reproduction. However, there differential peak expression. Intriguingly, modulation TSH? only observed migrants, where at start breeding, consistent observations other highly photoperiodic species. There no correlation TSH?, gonadotropin-releasing hormone-I or reproductive metrics residents. Based on these differences, discuss potential implications for our understanding how changes medial basal mediates initiation