Establishing primordia in the Drosophila eye-antennal imaginal disc: the roles of decapentaplegic, wingless and hedgehog.

The eye-antennal imaginal discs of Drosophila melanogaster form the head capsule of the adult fly. Unlike the limb primordia, each eye-antennal disc gives rise to morphologically and functionally distinct structures. As a result, these discs provide an excellent model system for determining how the fates of primordia are specified during development. In this study, we investigated how the adjacent primordia of the compound eye and dorsal head vertex are specified. We show that the genes wingless (wg) and orthodenticle (otd) are expressed throughout the entire second instar eye-antennal disc, conferring a default fate of dorsal vertex cuticle. Activation of decapentaplegic (dpp) expression in the posterior eye disc eliminates wg and otd expression, thereby permitting eye differentiation. We also demonstrate that otd is activated by wg in the vertex primordium. Finally, we show that early activation of dpp depends on hedgehog (hh) expression in the eye anlage prior to morphogenetic furrow formation.