The Iroquois homeobox genes function as dorsal selectors in the Drosophilahead

The Iroquois complex (Iro-C) genes are expressed in the dorsal compartment of the Drosophila eye/antenna imaginal disc, Previous work has shown that t he Iro-C homeoproteins are essential for establishing a dorsoventral patter n organizing center necessary for eye development. Here we show that, in ad dition, the Iro-C products are required for the specification of dorsal hea d structures. In mosaic animals, the removal of the Iro-C transforms the do rsal head capsule into ventral structures, namely, ptilinum, prefrons and s uborbital bristles. Moreover, the Iro-C- cells can give rise to an ectopic antenna and maxillary palpus, the main derivatives of the antenna part of t he imaginal disc. These transformations are cell-autonomous, which indicate s that the descendants of a dorsal Iro-C- cell can give rise to essentially all the ventral derivatives of the eye/antenna disc. These results support a role of the Iro-C as a dorsal selector in the eye and head capsule. More over, they reinforce the idea that developmental cues inherited from the di stinct embryonic segments from which the eye/antenna disc originates play a minimal role in the patterning of this disc.