ACCUMULATION AND PROLIFERATION OF ADULT LEG MUSCLE PRECURSORS IN MANDUCA ARE DEPENDENT ON INNVERVATION

During metamorphosis, the larval thoracic legs of the moth Manduca sex ta are replaced by new adult legs. The leg motoneurons do not die afte r the loss of the larval muscles, but persist to innervate the new adu lt leg muscles (Kent and Levine, 1988). The adult muscles form from my oblasts that originate in specific production sites within the legs an d migrate to the sites of muscle formation, where they accumulate, pro liferate, and fuse to form myofibers (Consoulas et al., 1996b). Throug hout adult leg muscle development, there is a close association betwee n nerves and the developing muscles, suggesting a role for the nervous system in myogenesis (Consoulas et al., 1996a), This prediction was c onfirmed and the role of the nervous system clarified in the present s tudy by cutting the larval leg nerves prior to metamorphosis. Although myoblasts were generated and migrated normally in the operated leg, t hey failed to accumulate in the appropriate regions. The myoblasts did not die, but failed to proliferate and remained in the denervated leg s as dispersed cells or as aggregates in inappropriate regions. In abo ut 26% of cases, this resulted in the formation of adult legs that lac ked muscles. In the remaining cases, however, delayed regeneration of the leg nerve occurred and small muscles appeared in the more proximal segments of the denervated legs. Each muscle fiber in these operated legs bore motor terminals belonging to axons of the leg nerves which h ad grown out from the proximal nerve stump and invaded the leg. Follow ing the delayed appearance of motor axons, myoblasts aggregated and un derwent proliferation and differentiation into muscle fibers. In a sec ond set of experiments, denervation was performed later, after myoblas ts had aggregated to establish anlagen. Myoblast proliferation was red uced but differentiation continued. These observations suggest that mo tor nerves are essential for both the accumulation of myoblasts into t he correct areas of muscle development and the appropriate level of pr oliferation. (C) 1997 John Wiley & Sons, Inc.