The CNS midline cells control the spitz class and Egfr signaling genes to establish the proper cell fate of the Drosophila ventral neuroectoderm

The spitz class genes, pointed (pnt), rhomboid (rho), single-minded (sim), spitz (spi) and Star (S), as well as the Drosophila epidermal growth factor receptor (Egfr) signaling genes, argos (aos), Egfr, orthodenticle (otd) an d vein (vn), are required for the proper establishment of ventral neuroecto dermal cell fate. The roles of the CNS midline cells, spitz class and Egfr signaling genes in cell fate determination of the ventral neuroectoderm wer e determined by analyzing the spatial and temporal expression patterns of e ach individual gene in spitz class and Egfr signaling mutants. This analysi s showed that the expression of all the spitz class and Egfr signaling gene s is affected by the sim gene, which indicates that sim acts upstream of al l the spitz class and Egfr signaling genes. It was shown that overexpressio n of sim in midline cells fails to induce the ectodermal fate in the spi an d Egfr mutants. On the other hand, overexpression of spi and Draf causes ec topic expression of the neuroectodermal markers in the sim mutant. Ectopic expression of sim in the en-positive cells induces the expression of downst ream genes such as otd, pnt rho, and vn, which clearly demonstrates that th e sim gene activates the EGFR signaling pathway and that CNS midline cells, specified by sim, provide sufficient positional information for the establ ishment of ventral neuroectodermal fate. These results reveal that the CNS midline cells are one of the key regulators for the proper patterning of th e ventral neuroectoderm by controlling EGFR activity through the regulation of the expression of spitz class genes and Egfr signaling genes.