The role of the tracheae and musculature during pathfinding of Drosophila embryonic sensory axons.

Developing sensory axons were studied in Drosophila embryos which carried a mutation in the trachealess and/or the twist gene. In these embryos, the tracheae and/or somatic muscles, which represent part of the substrate on which sensory axons normally grow, are absent. The results demonstrate that in each of these three mutant backgrounds, the majority of sensory nerves form normally. This indicates that neither the tracheae nor the somatic musculature is absolutely required for pathfinding of the embryonic sensory axons. On the other hand, the incidence of misrouted axons is significantly increased, most strongly in the trh, twi double mutant. Furthermore, axonal elongation is considerably slowed down, and sensory neurons which fail to send out an axon are frequent. We take our results to indicate that peripheral axons in the Drosophila embryo may be guided by multiple cues which, acting together, ensure the high fidelity of axonal pathways observed in normal development. The removal of one of these cues by itself does not necessarily lead to the total disability of axons to reach their target, although it enhances the frequency of error in pathfinding.