CENTRAL PROJECTIONS OF SENSORY NEURONS IN THE DROSOPHILA EMBRYO CORRELATE WITH SENSORY MODALITY, SOMA POSITION, AND PRONEURAL GENE-FUNCTION

The peripheral nervous system (PNS) of the Drosophila embryo is especi ally suited for investigating the specification of neuronal identity: the PNS consists of a relatively simple but diverse set of individuall y identified sensory neurons; mutants, including embryonic lethals, ca n be readily generated and analyzed; and axon growth can potentially b e followed from the earliest stages. We have developed a staining meth od to reveal the central projections of the full set of sensory neuron s in the preterminal abdominal segments of the embryo. The sensory neu rons exhibit modality-specific axonal projections in the CNS. The axon s of external sense (es) organ neurons, primarily tactile in function, are restricted to a particular region within each neuromere and exhib it a somatotopic mapping within the CNS. The axons of stretch-receptiv e chordotonal (ch) organs project into a discrete longitudinal fascicl e. Sensory neurons with multiple-branched dendrites (md neurons) proje ct into a separate fascicle, A small number of md neurons have distinc tive dorsal-projecting axonal processes in the CNS. A classification o f sensory neurons based on their axon morphology correlates closely wi th the identity of the proneural gene responsible for their generation , suggesting that proneural genes play a central role in determining n euronal identity in the PNS of the embryo.