Adaptation of mammalian myosin II sequences to body mass

The speed of muscle contraction is related to body size; muscles in larger species contract at a slower rate. We investigated the evolution of twelve myosin II isoforms across mammals to identify any adapted to increasing body mass. β-myosin head domain had the greatest rate of sequence divergence (0.05% per Myr) and it was the only domain where sequence divergence correlated with body mass (0.091% divergence per log mass unit). β-myosin is abundant in cardiac ventricle and slow skeletal muscle. Our analysis suggests that during evolution, β-myosin sequences have adapted to enable slower heart beating and contraction of slow skeletal muscle as body mass increased. Additionally, for eight of the twelve myosins, the ratio of divergence in the heads:tails was significantly different. For β-myosin the ratio was 3:1 while for the extraocluar, non-muscle A and embryonic myosin the ratio was u003c1:2. Our data provide new insights into the evolution of myosin function and indicate distinct evolutionary pressures on head and tail domains in individual isoforms.