Development of adult thoracic leg muscles during metamorphosis of the hawk moth Manduca sexta.

During metamorphosis, the larval thoracic legs of the hawk moth Manduca sexta are replaced by a new set of adult legs. The larval leg motoneurons persist to innervate new adult muscles, and the motor terminals remain within the developing adult legs. Here we describe the fate of the larval leg muscles and the origin of new muscles within the adult legs. During the larval instars, large and small nuclei proliferate within leg muscle fibers. Near the end of the larval stage a subset of the small nuclei undergo a wave of proliferation, as indicated by the incorporation of 5-bromodeoxyuridine, whereas other nuclei die. However, none of the larval leg muscles fibers persist to serve as templates for adult muscle formation, and there was no evidence for persistence of larval myonuclei. Migrating myoblasts that are born within aggregate to form adult muscle anlagen at specific production sites within the developing imaginal legs. Intense nuclear proliferation occurs within the anlagen during the early pupal stage, followed by muscle fiber formation and striation. We conclude that adult leg muscles form mainly, if not exclusively, from migrating myoblasts that without the involvement of larval elements.