The Pax-homeobox gene eyegone is involved in the subdivision of the thorax of Drosophila

The eyegone (eyg) gene is known to be involved in the development of the eye structures of Drosophila. We show that eyg and its related gene, twin of eyegone (toe), are also expressed in part of the anterior compartment of the adult mesothorax (notum). We report experiments concerning the role of these genes in the notum. In the absence of eyg function the anterior-central region does not develop, whereas ectopic activity of either eyg or toe induces the formation of the anterior-central pattern in the posterior or lateral region of the notum. These results demonstrate that eyg and toe play a role in the genetic subdivision of the notum, although the experiments indicate that eyg exerts the principal function. However, by itself the Eyg product cannot induce the formation of notum patterns; its thoracic function requires co-expression with the Iroquois (Iro) genes. We show that the restriction of eyg activity to the anterior-central region of the wing disc is achieved by the antagonistic regulatory activities of the Iro and pnr genes, which promote eyg expression, and those of the Hh and Dpp pathways, which act as repressors. We argue that eyg is a subordinate gene of the Iro genes, and that pnr mediates their thoracic patterning function. The activity of eyg gives rise to a new notum subdivision that acts upon the pre-extant one generated by the Iro genes and pnr. As a result the notum becomes subdivided into four distinct genetic domains.