Ambient Temperature Effects on the Spring and Autumn Somatic Growth Trajectory Show Plasticity in the Photoneuroendocrine Response Pathway in the Tundra Vole.

Abstract

Seasonal mammals register photoperiodic changes through the photoneuroendocrine system enabling them to time seasonal changes in growth, metabolism, and reproduction. To a varying extent, proximate environmental factors like ambient temperature (T_a) modulate timing of seasonal changes in physiology, conferring adaptive flexibility. While the molecular photoneuroendocrine pathway governing the seasonal responses is well defined, the mechanistic integration of nonphotoperiodic modulatory cues is poorly understood. Here, we explored the interaction between T_a and photoperiod in tundra voles, Microtus oeconomus, a boreal species in which the main impact of photoperiod is on postnatal somatic growth. We demonstrate that postweaning growth potential depends on both gestational and postweaning patterns of photoperiodic exposure, with the highest growth potential seen in voles experiencing short (8 h) gestational and long (16 h) postweaning photoperiods-corresponding to a spring growth program. Modulation by T_a was asymmetric: low T_a (10 °C) enhanced the growth potential of voles gestated on short photoperiods independent of postweaning photoperiod exposure, whereas in voles gestated on long photoperiods, showing a lower autumn-programmed growth potential, the effect of T_a was highly dependent on postweaning photoperiod. Analysis of the primary molecular elements involved in the expression of a neuroendocrine response to photoperiod, thyrotropin beta subunit (tshβ) in the pars tuberalis, somatostatin (srif) in the arcuate nucleus, and type 2/3 deiodinase (dio2/dio3) in the mediobasal hypothalamus identified dio2 as the most T_a-sensitive gene across the study, showing increased expression at higher T_a, while higher T_a reduced somatostatin expression. Contrastingly dio3 and tshβ were largely insensitive to T_a. Overall, these observations reveal a complex interplay between T_a and photoperiodic control of postnatal growth in M. oeconomus, and suggest that integration of T_a into the control of growth occurs downstream of the primary photoperiodic response cascade revealing potential adaptivity of small herbivores facing rising temperatures at high latitudes.