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Seasonal Variation of Myostatin Gene Expression in Pectoralis Muscle of House Sparrows (Passer domesticus) Is Consistent with a Role in Regulating Thermogenic Capacity and Cold Tolerance

Winter acclimatization in small birds overwintering in cold climates, including house sparrows (Passer domesticus), is associated with improved cold tolerance, elevated summit metabolic rates (Msum = maximum cold‐induced metabolic rate), and increased pectoralis muscle mass compared to summer birds. Myostatin is a potent autocrine/paracrine inhibitor of skeletal muscle growth in mammals and birds and is a potential candidate for regulation of seasonal phenotypic flexibility in birds. As a first step toward examining such a role for myostatin in small birds, we measured summer and winter gene expression of myostatin and its potential metalloproteinase activators TLL‐1 and TLL‐2 in house sparrows from southeastern South Dakota. Gene expression of myostatin decreased significantly in winter, with summer values exceeding winter values by 1.52‐fold. Moreover, gene expression of TLL‐1 was also significantly reduced in winter, with summer values exceeding winter values by 1.55‐fold. These data are consistent with the hypothesis that the winter increases in pectoralis muscle mass, Msum, and cold tolerance in house sparrows are mediated by reduced levels of myostatin and its activator TLL‐1, and they suggest the possibility that myostatin may be a common mediator of phenotypic flexibility of muscle mass in birds.