Pectoralis muscle fiber ultrastructure and molecular regulators of hypertrophy change seasonally, but not with thermal acclimation in house sparrows (Passer domesticus).

Increasing thermal variability, including sudden cold snaps, is a predicted consequence of climate change that may require endothermic homeotherms to make rapid physiological adjustments to maintain thermal homeostasis. Birds exhibit pronounced seasonal phenotypic changes, yet the cellular mechanisms underlying these responses remain poorly understood. Because shivering thermogenesis is central to avian heat production, skeletal muscle ultrastructure changes seasonally to support thermogenic performance. We acclimated summer-phenotype house sparrows (Passer domesticus) to 25 °C (control/constant warm), 3 °C (stable cold), and mean 3 °C (fluctuating cold) conditions for six weeks and quantified pectoralis muscle fiber diameter, number of nuclei per mm of fiber, and myonuclear domain (MND). We also measured expression of the muscle-associated microRNA miR-1 and its target gene Itm2a, which regulate muscle hypertrophy in mammals. Thermal acclimation did not affect muscle ultrastructure or gene expression, suggesting that summer-phenotype sparrows are well equipped to tolerate variable thermal conditions. In contrast, comparisons with winter-phenotype birds from a previous study revealed clear seasonal differences. Summer-phenotype birds had larger diameter muscle fibers, fewer myonuclei per mm of fiber, and larger MNDs than winter-phenotype birds across all acclimation treatments. Similarly, miR-1 expression was lower and Itm2a expression higher in summer- than winter-phenotype birds, consistent with their roles in regulating muscle hypertrophy. These findings indicate that seasonal shifts, rather than short-term thermal variability, drive changes in pectoralis muscle ultrastructure and associated molecular regulators, highlighting seasonal remodelling as an important mechanism supporting thermogenic physiology in house sparrows.