Genetic analysis of vein function in the Drosophila embryonic nervous system.

The Drosophila epidermal growth factor receptor (EGFR) may be activated by two ligands expressed in the embryonic nervous system, Spitz and Vein. Previous studies have established Spitz as an essential activator of EGFR signaling in nervous system development. Here, we report the pattern of expression of vein mRNA in the nervous system and characterize the contribution of vein to cell lineage and axonogenesis. The number of midline glia (MG) precursors is reduced in vein mutants before the onset of embryonic apoptosis. In contrast to spitz, mis-expression of vein does not suppress apoptosis in the MG. These data indicate that early midline EGFR signaling, requiring vein and spitz, establishes MG precursor number, whereas later EGFR signals, requiring spitz, suppress apoptosis in the MG. vein mutants show early irregularities during axon tract establishment, which resolve later to variable defasciculation and thinner intersegmental axon tracts. vein and spitz phenotypes act additively in the regulation of MG cell number, but show synergism in a midline neuronal cell number phenotype and in axon tract architecture. vein appears to act downstream of spitz to briefly amplify local EGFR activation.