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 to identify any adapted to increasing body mass in mammals. We identified a correlation between body mass and sequence divergence for the motor domain of three adult myosin II isoforms ({beta}, 2A, 2B) suggesting that these isoforms have adapted to increasing body mass. In contrast the non-muscle and developmental isoforms show no correlation of sequence divergence with body mass, while the sarcomeric myosin 7b, extraocular and 2X isoforms showed a divergence intermediate between these two groups. The 2B and {beta}-myosin motor domain showed the greatest rate of sequence divergence (-0.84 and -0.69 % per ten-fold increase in mass respectively). {beta}-myosin is abundant in cardiac ventricle and slow skeletal muscle. We propose that {beta}-myosin has adapted to enable slower heart beating and contraction of slow skeletal muscle as body mass increased.