Epidermal growth factor receptor signaling activates orthodenticle expression during Drosophila head development

The relation between signal transduction pathways and the genes that specif y regional identity remains poorly understood. We investigated the interact ion between the epidermal growth factor receptor (EGFR) pathway and the hom eobox gene orthodenticle (otd), which specifies cell fate during head devel opment. Previous studies of head formation in Drosophila melanogaster demon strated that reducing either EGFR signaling or otd expression in the imagin al primordium of the dorsal head capsule eliminates the ocelli and other do rsal head structures. Here, we show that blocking EGFR signaling reduces ot d expression and that activating EGFR signaling outside this primordium ind uces ectopic otd expression. We also demonstrate that loss of EGFR can be r escued by constitutive otd expression. Our results indicate that otd is a d ownstream target of the EGFR pathway during head development.