PRESYNAPTIC DEVELOPMENT AT THE DROSOPHILA NEUROMUSCULAR-JUNCTION - ASSEMBLY AND LOCALIZATION OF PRESYNAPTIC ACTIVE ZONES

We describe the extent to which presynaptic structures at the embryoni c neuromuscular junction of Drosophila can form in mutants where devel opment of postsynaptic somatic muscles is affected. Although twist mut ant embryos lack mesoderm, motor axons still grow out of the CNS and f orm morphologically normal presynaptic active zones, independent of th eir target cells. In myoblast city mutant embryos, myoblasts do not fu se but form fully differentiated mononucleate muscles, which make func tional neuromuscular synapses with correctly localized presynaptic act ive tones. Myoblasts also fail to fuse but still attract appropriate i nnervation in mef2 mutant embryos. However, these myoblasts fail to di fferentiate into muscles and presynaptic active zones fail to localize at neuromuscular contacts. Thus, the process of synapse formation can be genetically separated from the process of target recognition, reve aling that localization of presynaptic active zones requires mef2-depe ndent muscle differentiation.