The role of eyg Pax gene in the development of the head vertex in Drosophila.

The Drosophila head vertex is composed of three ocelli, stereotypic bristle patterns and characteristic cuticles. It is derived from the fusion of two eye-antenna discs. The head vertex primordium is located at the anterior-dorsal region of the eye disc. The orthodenticle (otd) homeobox gene is expressed in the primordium and is functionally required for its development and patterning. Here we show that the Pax gene eye gone (eyg) is expressed adjacent to the otd expression domain in the eye disc. otd is required and sufficient to repress eyg transcription, thereby preventing eyg from expressing in the head vertex primordium. In otd mutant, eyg expression is derepressed in the head vertex primordium and is a major negative effector to block head vertex development. Therefore, otd not only needs to induce downstream effector genes to execute the development and patterning of the head vertex development, but also needs to actively repress the negative regulator eyg. In addition, eyg is required for the development of the lateral bristles in the head vertex. So eyg plays both positive and negative roles in head vertex development.