Central projections of Drosophila sensory neurons in the transition from embryo to larva

Sensory axons of different sensory modalities project into typical domains within insect ganglia. Tactile and gustatory axons project into a ventral l ayer of neuropil and proprioceptive afferents, including chordotonal axone, into an intermediate or dorsal layer. Here, we describe the central projec tions of sensory neurons in the first instar Drosophila larva, relating the m to the projection of the same sensory afferents in the embryo and to sens ory afferents of similar type in other insects. Several neurons show marked morphologic changes in their axon terminals in the transition between the embryo and larva. During a short morphogenetic period late in embryogenesis , the axon terminals of the dorsal bipolar dendrite stretch receptor change their shape and their distribution within the neuromere. In the larva, ext ernal sense organ neurons (es) project their axons into a ventral layer of neuropil. Chordotonal sensory neurons (ch) project into a slightly more dor sal region that is comparable to their projection in adults. The multiple d endrite (md) neurons show two distinctive classes of projection. One group of md neurons projects into the ventral-most neuropil region, the same regi on into which es neurons project. Members of this group are related by line age to es neurons or share a requirement for expression of the same proneur al gene during development. Other md neurons project into a more dorsal reg ion. Sensory receptors projecting into dorsal neuropil possibly provide pro prioceptive feedback from the periphery to central motorneurons and are can didates for future genetic and cellular analysis of simple neural circuitry . J. Comp. Neurol. 425:34-44, 2000. (C) 2000 Wiley-Liss, Inc.