Meeting Abstract
Interspecific variation in muscle fiber size may result from several mechanisms. For example, muscle fibers may increase in size if a muscle is used more frequently, or with more force; this is known as a “training effect.” Alternatively, a species may exhibit larger muscle fibers in adulthood if during embryonic development, a larger number of myoblasts (single-cell precursors to muscle fibers) fuse in the formation of multi-nucleated fibers. Previous work has shown that Anolis lizard species vary dramatically in the size of the fibers that compose two muscles: the ceratohyoid (CH), which controls extension of the dewlap (a colorful throat fan used in social displays), and in the retractor penis magnus (RPM), which controls movement of the copulatory organs. In this study, we tested the hypothesis that variation in the CH and RPM fiber sizes results from the fusion of varying numbers of myoblasts. We measured the cross-sectional area of CH and RPM fibers in adult males from 9 species, and because each myoblast contains a single nucleus, we counted the number of nuclei in cross-sections of a fiber as a proxy for the number of myoblasts that fused to form that fiber during development. We found that within 6 of the 9 species, RPM cell size increased in correlation with the number of myonuclei, but CH cell size only increased with the number of nuclei within 4 species. Further, using phylogenetically-informed analyses, our results showed that the evolution of larger RPM fibers is associated with the fusion of larger numbers of myoblasts, suggesting that this is one mechanism of the evolution of larger muscles in anoles. However, this pattern was not seen in the evolution of larger CH fibers, indicating that other mechanisms of muscle growth also occur in this group.
