Fig. 5

Evolutionary scheme of vertebrate skeletal muscles with reference to the embryonic expression of Lbx genes. In the hagfish (a), rectus muscle (m. rect) and anterior oblique muscle (m. decussatus) are the hypobranchial muscle counterpart and derived from Lbx-A-positive ventral cells of the somites. In the lamprey (b), Lbx-A is required in the formation of body wall muscles (bwm, light green) and hypobranchial muscle (HBM, dark green). In gnathostomes (c), Lbx1 (blue) and Lbx2 (green) regulate distinct domains of hypobranchial muscles. Lbx1 functions in the tongue (cmd, m. coracomandibularis in the shark) and appendicular muscles (pfm, pectoral fin muscles) both of which are lacking in cyclostomes. Pharyngeal muscles derived from head mesoderm are shown in orange