Lamprey contractile protein genes mark different populations of skeletal muscles during development.

Agnathan lampreys retain ancestral characteristics of vertebrates in the morphology of skeletal muscles derived from two mesodermal regions: trunk myotomes and unsegmented head mesoderm. During lamprey development, some populations of myoblasts migrate via pathways that differ from those of gnathostomes. To investigate the evolution of skeletal muscle differentiation in vertebrates, we characterize multiple contractile protein genes expressed in the muscle cells of the Japanese lamprey, Lethenteron japonicum. Lamprey actin gene LjMA2, and myosin heavy chain (MyHC) genes LjMyHC1 and LjMyHC2 are all expressed in the developing skeletal muscle cells of early embryos. However, LjMyHC1 and LjMyHC2 are expressed only in cells originating from myotomes, while LjMA2 is expressed in both myotomal and head musculature. Thus, in lampreys, myotomes and head mesoderm differ in the use of genes encoding contractile protein isoforms. Phylogenetic tree analyses including lamprey MyHCs suggest that the variety of muscle MyHC isoforms in different skeletal muscles may correspond to the morphological complexity of skeletal muscles of different vertebrate species. Another lamprey actin gene LjMA1 is likely to be the first smooth muscle actin gene isolated from non-tetrapods. We conclude that, in vertebrate evolution, the different regulatory systems for striated and smooth muscle-specific genes may have been established before the agnathan/gnathostome divergence.