Evolutionary origins of vertebrate appendicular muscle

The evolution of terrestrial tetrapod species heralded a transition in loco motor strategies. While most fish species use the undulating contractions o f the axial musculature to generate propulsive force, tetrapods also rely o n the appendicular muscles of the limbs to generate movement(1,2). Despite the fossil record generating an understanding of the way in which the appen dicular skeleton has evolved to provide the scaffold for tetrapod limb musc ulature(3), there is, by contrast, almost no information as to how this mus culature arose. Here we examine fin muscle formation within two extant clas ses of fish. We find that in the teleost, zebrafish, fin muscles arise from migratory mesenchymal precursor cells that possess molecular and morphogen etic identity with the limb muscle precursors of tetrapod species. Chondric hthyan dogfish embryos, however, use the primitive mechanism of direct epit helial somitic extensions to derive the muscles of the fin. We conclude tha t the genetic mechanism controlling formation of tetrapod limb muscles evol ved before the Sarcopterygian radiation.