Wellcome/CRC Institute, Tennis Court Road, Cambridge CB2 1QR, U.K.,
Pioneer neurons are the first to extend their growth cones and trace the axonal pathways that later neurons will follow. However, there is evidence both in favour and against unique pathfinding abilitites of pioneer neurons. We have readdressed this question in Drosophila by means of targeted cell ablation using the Gal4 system to drive the expression of the toxin ricin. First, we describe for the first time the temporal evolution of the pathways traced by the axons of the pioneer neurons. We show that the fasciculation partners of these neurons change with time. We analyse the consequences of ablating one, two or three pioneer neurons at a time. The effects of these ablations are synergistic, and include extension of follower axons, specific defasciculation defects and also loss of axonal tracts. We conclude that: (1) individual pioneer neurons are dispensable; (2) pioneer neurons function synergistically; (3) pioneer neurons constitute a group of cells which is required for the formation of the longitudinal tracts.