Central projections of persistent larval sensory neurons prefigure adult sensory pathways in the CNS of Drosophila

We have used a GAL4 enhancer-trap line driving the expression of a lacZ construct to examine the reorganisation of an identified group of proprioceptive sensory neurons during metamorphosis in Drosophila. The results show that whilst most larval sensory neurons degenerate during the first 24 hours of metamorphosis a segmentally repeated array of 6 neurons per segment persists into the adult stages to become functional adult neurons. These sensory neurons retain their axonal projections in the central nervous system intact and unchanged throughout. The adult sensory neuron axons enter the central nervous system at around 44 hours after puparium formation. Most of these axons grow along the pathways defined by the persistent larval sensory axons. The ordering of the adult sensory projections is, therefore, established upon the larval pattern of projections. The possibility that the larval neurons act as guidance cues for organising the ordered arrays of sensory neurons is discussed.