Even-skipped determines the dorsal growth of motor axons in Drosophila

Axon pathfinding and target choice are governed by cell type-specific respo nses to external cues. Here, we show that in the Drosophila embryo, motorne urons with targets in the dorsal muscle field express the homeobox gene eve n-skipped and that this expression is necessary and sufficient to direct mo tor axons into the dorsal muscle field. Previously, it was shown that motor neurons projecting to ventral targets express the LIM homeobox gene islet, which is sufficient to direct axons to the ventral muscle field. Thus, even -skipped complements the function of islet, and together these two genes co nstitute a bimodal switch regulating axonal growth and directing motor axon s to ventral or to dorsal regions of the muscle field.