Biographical Information: During development cell interactions play an important role in determining properties such as cellular identity, and tissue patterning. The motor system comprises of two cell-types, motor neurons (ectodermal tissue) and muscle, (mesodermal tissue). Connectivity between motor neurons and their muscle targets is usually precise, and cell-cell interactions play a major role in generating this precision. It is known that formation of muscle fibers and the establishment of nerve branching involves interactions between the two cell types. In vertebrate muscle, anterograde signals (from nerve to muscle) are thought to control events such as clustering of receptors for neurotransmitter. Conversely, retrograde signals (from muscle to nerve) are believed to be important when synaptic connections are restructured. Our research efforts will be aimed at elucidating the genetic and molecular basis of such interactions in a model genetic organism, the fruit fly, Drosophila melanogaster. Using the indirect flight muscles of the adult, we have shown that the nerve initially regulates proliferation of myoblasts and is later involved in providing trophic support to the muscle fibers. The timing of nerve branching on the other hand, both in the central nervous system and in the periphery is influenced by retrograde signals from the muscle. We will investigate the molecular basis of the following processes: 1) Nerve-regulated molecules that promote myoblast proliferation and 2) Muscle-derived molecules that regulate nerve branching. An attractive feature of Drosophila is that it has two distinct life forms, the larva, and the adult. Many tissues that are generated in the embryonic stage are restructured to give rise to a more complex structure of the adult. Thus, two levels of complexity can be studied in the same organism, and this also allows an investigation of genes that might be expressed during both stages, and others that may be unique. The transition period of metamorphosis also provides an excellent opportunity to study plasticity of the nervous system, since this tissue is reorganized and serves to control two behaviorally distinct animals.