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 frho), 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) and vein (vn), are required for the proper establishment of ventral neuroectodermal cell fate. The roles of the CNS midline cells, spitz class and Egfr signaling genes in cell fate determination of the ventral neuroectoderm were determined by analyzing the spatial and temporal expression patterns of each individual gene in spitz class and Egfr signaling mutants. This analysis showed that the expression of all the spitz class and Egfrsignaling genes is affected by the sim gene, which indicates that sim acts upstream of all the spitz class and Egfr signaling genes. It was shown that overexpression of sim in midline cells fails to induce the ectodermal fate in the spi and Egfr mutants. On the other hand, overexpression of spi and Draf causes ectopic expression of the neuroectodermal markers in the sim mutant. Ectopic expression of sim in the en-positive cells induces the expression of downstream genes such as otd, pnt, rho, and vn, which clearly demonstrates that the sim gene activates the EGFR signaling pathway and that CNS midline cells, specified by sim, provide sufficient positional information for the establishment of ventral neuroectodermal fate. These results reveal that the CNS midline cells are one of the key regulators for the proper patterning of the ventral neuroectoderm by controlling EGFR activity through the regulation of the expression of spitz class genes and Egfr signaling genes.