In long‐distance migrants, preparation for migration is typically associated with increases in fat and body mass, and with an enlargement of pectoralis muscle mass that likely improves flight performance. Although changes in muscle mass or size have been well described in migratory birds, potential changes in muscle ultrastructure during this transition still deserves scrutiny. Using outdoor captive snow buntings (Plectrophenax nivalis n = 15) measured during their transition into a spring migratory phenotype as a model system, we studied changes in pectoralis muscle ultrastructure and predicted that muscle fiber diameter could increase in parallel with the gain in body mass. We also expected that larger fibers could either recruit satellite cells to support cellular maintenance and protein turnover, increase myonuclear domain (cytoplasm per nuclei) with a potential increase in protein turnover load per myonucleus, or existing myonuclei could undergo endoreduplication. Buntings increased body mass by 46% within a month, largely due to a > 6‐fold increase in body fat. However, this increase in body mass was also associated with a 36% increase in muscle fiber diameter. Both pectoralis muscle mass (r2 = 0.57–0.77) and fiber diameter (r2 = 0.32) correlated with total body mass, without any change in the number of nuclei per fiber. Consequently, variation in myonuclear domain (i.e. the amount of cytoplasm per nucleus), was also positively associated with body mass (r2 = 0.51). Therefore, buntings preparing for migration may experience an increase in muscle contraction force due to larger muscle fibers, but this is also coupled with increases in myonuclear domain, which may force these cells to increase protein production to safeguard satellite cells.
The life-history theory of evolution posits that organisms are maximized for reproductive success through the processes of natural selection. Though believed to impact nearly every aspect of an organism, the life-history theory of evolution shines little attention on the cellular aspects of organismal physiology. Tropical bird species, compared with temperate birds, are typically placed on the 'slow' pace-of-life spectrum, and demonstrate a 'slow' pace of life across every level of organization. Previously, we demonstrated that phylogenetically paired tropical birds have significantly smaller muscle fiber diameters than their temperate counterparts. No other muscle-related properties have been addressed and correlated to life-histories in the literature. Myonuclear domain (MND), the cytoplasmic volume that each myonucleus serves, is an under-appreciated skeletal muscle property that has been largely ignored in bird muscle literature. Here, we measured MND in muscle cross-sections in phylogenetically paired species of tropical and temperate birds. We found that tropical bird species had significantly higher numbers of nuclei per mm of fiber as compared with temperate bird species. This may be due to the fact that tropical birds could have a higher population of slow-oxidative fiber and/or due to developing in warmer climates, as compared with temperate birds. In both tropical and temperate birds, we found significantly positive associations between muscle fiber diameter and MND and between muscle cross-sectional areas and number of nuclei per mm of fiber in tropical birds and temperate birds, as previously noted in other bird species.
With rapid climate change, heat wave episodes have become more intense and more frequent. This poses a significant threat to animals, and forces them to manage these physiologically challenging conditions by adapting and/or moving. As an invasive species with a large niche breadth, House sparrows (Passer domesticus) exhibit high phenotypic flexibility that caters to seasonal changes in function and metabolism. For example, their pectoral muscle complex exhibits size and mass plasticity with winter and summer acclimation. Here, we investigated the effects of acute whole‐organism heat stress to 43°C on cellular‐level changes in House sparrow pectoralis muscle myonuclear domain (MND), the volumetric portion each nucleus is responsible for, that have gone overlooked in the current literature. House sparrows were separated into a control group, a heat‐shocked group subjected to thermal stress at 43°C for 24 h, and a recovery group that was returned to room temperature for 24 h after experiencing the same temperature treatment. Here, we found that heat‐shocked and recovery groups demonstrated a decrease in number of nuclei per millimeter of fiber and increase in MND, when compared with the control. We also found a significant positive correlation between fiber diameter and MND in the recovery group, suggesting the possibility that nuclei number constrains the extent of muscle fiber size. Together, these results show that acute heat shock alters House sparrow pectoralis muscle cellular physiology in a rigid way that could prove detrimental to long‐term muscle integrity and performance.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.