Expression and interaction of muscle-related genes in the lamprey imply the evolutionary scenario for vertebrate skeletal muscle, in association with the acquisition of the neck and fins.

Gnathostomes (jawed vertebrates) possess skeletal muscles with unique functional and developmental features that are absent from cyclostomes-i.e., lamprey and hagfish. These gnathostome-specific traits include the epaxial and hypaxial division of myotomes, paired fin/limb muscles, shoulder girdle muscles, and the muscle associated with the tongue and the neck. Many of these muscles are derived from several rostral somites, specifically from their hypaxial myotomic domains. However, it has not been clarified how the complicated morphology of these muscles was acquired in the evolution of vertebrates. Here we describe the expression of lamprey homologs of transcription factor genes, including a myogenic regulatory factor of the Myod family (MRF), Pax3/7, Lbx, and Zic, which play important roles in the development of ep-/hypaxial somitic muscles in gnathostomes, and show that the ventral portion of lamprey somites is comparable to the ventral dermomyotome in gnathostomes. The supra- and infraoptic muscles, derived from the two anterior somites in the lamprey, are molecularly specified before their extensive invasion into the head region. Of these, the infraoptic myotomes are suggested to represent the cucullaris homologue in the lamprey based on their topographical position in the embryonic pattern. Slightly caudal myotomes in the lamprey give rise to the hypobranchial muscle, the developmental homologue of the gnathostome hypobranchial musculature. The dorsal moieties of the lamprey somites express a Zic gene, which in teleosts specifies the epaxial identities of the somites. These evidences suggest that, although the myotomes in the ancestral jawless vertebrates do not exhibit ep-/hypaxial distinction at the morphological level, their dorsoventral specification would have already been present at gene regulatory levels, prior to the cyclostome-gnathostome divergence, which may have functioned as the key innovation to establish the ep-/hypaxial distinction in gnathostomes.