School of Biological Sciences, University of Southampton, Southampton SO16 7PX, U.K.,
From a screen of 1400 Gal4 enhancer-trap lines we have isolated lines in which reporter gene expression is restricted to identified subsets of sensory neurons. Using the Gal4 construct to drive expression of a UAS-linked lacZ we have used these lines to reveal and describe the central projections of sensory neurons in both larval and adult CNS (Smith and Shepherd, 1996. J. Comp. Neurol. 364: 311-323).
Here we present work using one of these lines which describes the anatomical reorganisation of an identified class of sensory neurons during metamorphosis. In the line used for this work, C161, reporter gene expression is restricted to proprioceptive neurons and reveals the central projections of these neurons in larvae and adult. The line can, therefore, be used to examine the reorganisation of these neurons through metamorphosis.
In larvae expression reveals a segmentally repeated array of sensory neurons which includes: all multidendritic and bipolar dendrite neurons plus a single component of the lateral pentascolopidial organ.
In adult thorax expression reveals leg hairplate neurons; femoral chordotonal organs; wing hinge stretch receptors and some campaniform sensilla. In the adult abdomen expression reveals a large scolopidial organ, and a segmentally repeated array on internal stretch receptors (3 per hemisegment). Details of the organisation of the central projections will be presented.
Reorganisation of the neurons during metamorphosis. During early metamorphosis (0-24hrs After Puparium Formation) most larval sensory neurons degenerate. This is supported by observations of degenerating neuron cell bodies peripherally and the loss of central axon projections. A small number of sensory neurons (3 per hemisegment), however, survive these early stages and can be seen to persist to adult stages. Peripherally these neurons can be seen in characteristic locations at all stages of metamorphosis including the adult. Of these three neurons, two can be identified unequivocally, these are the larval lateral and dorsal bipolar dendrite neurons, both of which are associated with persistent larval muscles. Evidence of the persistence of these neurons can also be seen in the CNS since the axons from these neurons can be seen at all stages. Thus between 26 and 40hrs APF it is possible to identify only three axon pathways in each hemineuromere. Importantly each axon represents one of the major axon pathways seen in the larval CNS. Between 40-48hrs APF the adult axons enter the CNS and grow along the pathways defined by the persisting larval axons. This is such that by the completion of metamorphosis it is possible to describe the adult projections in terms of these three larval axon pathways. The evidence, although circumstantial, suggests that the larval axons prefigure the adult sensory organisation and provide a guidance substrate for organising the assembly of the adult pattern of sensory projections. The implications of this will be discussed.