Pectoralis Muscle Fiber Diameter and Myonuclear Domain Remain Unchanged in Winter-Phenotype House Sparrows (<i>Passer domesticus</i>) Acclimated to Stable Warm, Stable Cold, and Fluctuating Cold Thermal Treatments

AbstractPhenotypic flexibility can allow for reversible changes in adult phenotypes according to prevailing environmental conditions, a concept referred to as environmental matching. Increasing thermal variability across seasons is one of the predicted effects of global climate change, and it includes sudden cold snaps, which will require rapid and flexible adjustments of physiology to match environmental conditions. Inability to achieve rapid physiological adjustment could lead to physiology-environment mismatches, which may be metabolically costly for organisms. Seasonal changes in organismal-level phenotypes are commonly measured in birds. Cell-level changes linked to these seasonal phenotypes, however, are less well understood. At the ultrastructural level, trade-offs may manifest in the size of functional tissues like the pectoralis muscle. Muscle fiber diameter can be linked to whole-animal metabolism and is optimally regulated. Fiber diameter changes may be closely tied to other ultrastructural changes in muscle tissue itself, such as myonuclear domain, which is important to muscle remodeling, as muscle hypertrophy or atrophy is often accompanied by changes in myonuclei content. Here, we acclimated winter-phenotype house sparrows (<i>Passer domesticus</i>) to three different 6-wk acclimation treatments: a control warm treatment, a stable cold treatment, and a fluctuating cold treatment. Following acclimation, we measured fiber diameter, number of nuclei per millimeter of fiber, and myonuclear domain from pectoralis muscle of birds from each treatment. We found no differences in pectoralis muscle ultrastructure for any variable across thermal acclimation treatments, highlighting that winter-phenotype house sparrows may be properly equipped to withstand fluctuating thermal conditions and/or that they do not rapidly change pectoralis structure within winter. Basal metabolic rate was negatively correlated with muscle fiber diameter across treatment groups, suggesting higher maintenance costs for smaller muscle fibers, likely because of higher surface area-to-volume ratios and attendant higher ion regulation costs, and providing the first linkage of whole-organism metabolic responses to muscle ultrastructure in birds. Muscle ultrastructural variables, however, were not significantly correlated with maximal thermogenic metabolic rates, suggesting little effect of muscle ultrastructure, independent of muscle size, on regulating maximal thermogenic capacity in birds.