NERVE-MUSCLE INTERACTIONS DURING FLIGHT-MUSCLE DEVELOPMENT IN DROSOPHILA

During Drosophila pupal metamorphosis, the motoneurons and muscles dif ferentiate synchronously, providing an opportunity for extensive inter cellular regulation during synapse formation. We examined the existenc e of such interactions by developmentally delaying or permanently elim inating synaptic partners during the formation of indirect flight musc les. When we experimentally delayed muscle development, we found that although adult-specific primary motoneuron branching still occurred, t he higher order (synaptic) branching was suspended until the delayed m uscle fibers reached a favourable developmental state. In reciprocal e xperiments we found that denervation caused a decrease in the myoblast pool. Furthermore, the formation of certain muscle fibers (dorsoventr al muscles) was specifically blocked. Exceptions were the adult muscle s that use larval muscle fibers as myoblast fusion targets (dorsal lon gitudinal muscles), However, when these muscles were experimentally co mpelled to develop without their larval precursors, they showed an abs olute dependence on the motoneurons for their formation. These data sh ow that the size of the myoblast pool and early events in fiber format ion depend on the presence of the nerve, and that, conversely, periphe ral arbor development and synaptogenesis is closely synchronized with the developmental state of the muscle.