Pre-existing neuronal pathways in the leg imaginal discs of Drosophila

Injection of a cell lineage tracer (HRP) into Drosophila embryos before cellularization provides a way of selec-tively labelling cells at later stages that have undergone only a few mitoses. All cells born and differentiating during embryogenesis become labelled, whereas further proliferation and growth during postembryonic develop-ment causes an almost complete dilution of the marker in the adult cell complement. Early born neurons visual-ized in this way are good candidates for executing a pioneering function during postembryonic differen-tiation of the adult nervous system. In all three pairs of leg imaginal discs, a stereotyped set of larval sense organs becomes selectively labelled. Their axons fasciculate with a larval nerve, which connects the leg disc with the central nervous system. Larval sense organs are not present in the other imaginal discs. Larval neurons are not present in the transformed antennal discs of Antp 73B flies. Nonetheless adult axons successfully navigate to the base of these discs as they differentiate to form ectopic legs. We conclude that embryonically formed larval nerves are not essential for the guidance of adult axons within the leg discs