The evolutionary relationship of avian and mammalian myosin heavy-chain genes.

Sequence comparisons of avian and mammalian skeletal and cardiac myosin heavy-chain isoforms are used to examine the evolutionary relationships of sarcomeric myosin multigene families. Mammalian fast-myosin heavy-chain isoforms from different species, with comparable developmental expression, are more similar to each other than they are to other fast isoforms within the same genome. In contrast, the developmentally regulated chicken fast isoforms are more similar to each other than they are to myosin heavy-chain isoforms in other species. Extensive regions of nucleotide identity among the chicken fast myosin heavy chains and in the mouse and rat alpha- and beta-cardiac myosin heavy-chain sequences suggest that gene-conversion-like mechanisms have played a major role in the concerted evolution of these gene families. We also conclude that the chicken fast myosin heavy-chain multigene family has undergone recent expansion subsequent to the divergence of birds and mammals and that both the developmental regulation and the specialization of myosin isoforms have likely developed independently in birds and mammals.