Precision and plasticity during Drosophila neuromuscular development.

Synaptogenesis can be analyzed in a simple array of motoneurons and muscle fibers of the embryos and larvae of Drosophila melanogaster. Each abdominal hemisegment contains a stereotypic array of 30 muscle fibers. During middle to late embryogenesis, motoneurons exit the central nervous system to make precise synaptic connections with specific muscle fibers. Target recognition has been tested using both genetic and microsurgical manipulations, which indicate that motoneurons actively recognize specific muscle fibers. The molecular basis of target recognition has been examined by screens for mutations that disrupt both guidance events and correct innervation. In addition, the motoneurons and muscle fibers both express an array of putative cell adhesion molecules whose functions may contribute to normal connectivity. Postsynaptic specializations, including glutamate receptor distribution, depend on innervation and neural activity. The neuromuscular system is not "hardwired," as motoneurons are capable of altering both their branch arborizations and connectivity in response to local denervation and blockade of synaptic function. Collectively, these studies show that the Drosophila motor innervation is a powerful model system for testing at the cellular and molecular level the mechanisms that govern synaptic development.