Gastrocnemius Muscle Structural and Functional Changes Associated with Domestication in the Turkey

SIMPLE SUMMARY: Domestic turkeys have been bred to reach a body mass of up to three times that of wild turkeys. Most of this increase is from larger muscles, but it is unclear exactly how the components of each muscle have been altered and what that may mean for muscle function. In this study, we looked for potential differences between wild and domestic turkeys in (1) the size of the individual muscle fibers and (2) the collagenous skeleton that supports those fibers in the lateral gastrocnemius muscle, an ankle extensor important for walking. We also measured the force this muscle could produce. The domestic turkey muscle had a greater number of smaller muscle fibers than the wild turkey. The amount of collagen in the domestic turkey muscle was also lower than wild turkeys, likely contributing to meat tenderness. While the domestic turkey lateral gastrocnemius muscles could produce the same amount of force per a given area of muscle, they could only produce half as much force per unit body mass. Selection for increased muscle mass has altered the structure of the lateral gastrocnemius muscle; however, overall body mass likely contributes more to hind limb functional differences observed in the domestic turkey. ABSTRACT: Selection for increased muscle mass in domestic turkeys has resulted in muscles twice the size of those found in wild turkeys. This study characterizes muscle structural changes as well as functional differences in muscle performance associated with selection for increased muscle mass. We compared peak isometric force production, whole muscle and individual fiber cross-sectional area (CSA), connective tissue collagen concentration and structure of the lateral gastrocnemius (LG) muscle in wild and adult domestic turkeys. We also explored changes with age between juvenile and adult domestic turkeys. We found that the domestic turkey’s LG muscle can produce the same force per cross-sectional area as a wild turkey; however, due to scaling, domestic adults produce less force per unit body mass. Domestic turkey muscle fibers were slightly smaller in CSA (3802 ± 2223 μm(2)) than those of the wild turkey (4014 ± 1831 μm(2), p = 0.013), indicating that the absolutely larger domestic turkey muscles are a result of an increased number of smaller fibers. Collagen concentration in domestic turkey muscle (4.19 ± 1.58 μg hydroxyproline/mg muscle) was significantly lower than in the wild turkeys (6.23 ± 0.63 μg/mg, p = 0.0275), with visible differences in endomysium texture, observed via scanning electron microscopy. Selection for increased muscle mass has altered the structure of the LG muscle; however, scaling likely contributes more to hind limb functional differences observed in the domestic turkey.