Fasciclin III, a cell adhesion molecule of the immunoglobulin superfamily, is expressed by motor neuron RP3 and its synaptic targets (muscle cells 6 and 7) during embryonic neuromuscular development of Drosophila. We report here that RP3 often incorrectly innervates neighbouring non-target muscle cells when these cells misexpress fasciclin III, but still innervates normal targets in the fasciclin III null mutant. Fasciclin III manipulations do not influence target selections by other motor neurons, including fasciclin III-expressing RP1. We propose that fasciclin III acts as a synaptic target recognition molecule for motor neuron RP3, and also that its absence can be compensated for by other molecule(s).
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