Abstract
The presence of a single EGF receptor in Drosophila is contrasted by multiple ligands activating it. This work explores the role of two ligands, Spitz and Vein, in the embryonic ventral ectoderm. Spitz is a potent ligand, whereas Vein is an intrinsically weak activating ligand. We show that secreted Spitz emanating from the midline, triggers expression of vein in the ventral-most cell rows, by inducing expression of the ETS domain transcription factor Pointed P1. In the absence of Vein, lateral cell fates are not induced when Spitz levels are compromised. The positive feedback loop of Vein generates a robust mechanism for patterning the ventral ectoderm.
