Abstract
The peripheral nervous system (PNS) of the Drosophila embryo is especially suited for investigating the specification of neuronal identity: the PNS consists of a relatively simple but diverse set of individually identified sensory neurons; mutants, including embryonic lethals, can be readily generated and analyzed; and axon growth can potentially be followed from the earliest stages. We have developed a staining method to reveal the central projections of the full set of sensory neurons in the preterminal abdominal segments of the embryo. The sensory neurons exhibit modality-specific axonal projections in the CNS. The axons of external sense (es) organ neurons, primarily tactile in function, are restricted to a particular region within each neuromere and exhibit a somatotopic mapping within the CNS. The axons of stretch-receptive chordotonal (ch) organs project into a discrete longitudinal fascicle. Sensory neurons with multiple-branched dendrites (md neurons) project into a separate fascicle. A small number of md neurons have distinctive dorsal-projecting axonal processes in the CNS. A classification of sensory neurons based on their axon morphology correlates closely with the identity of the proneural gene responsible for their generation, suggesting that proneural genes play a central role in determining neuronal identity in the PNS of the embryo.