Ectopic orthodenticle expression alters segment polarity gene expression but not head segment identity in the Drosophila embryo.

The cephalic gap genes specify anterior head development in the Drosophila embryo. However, the mechanisms of action of these genes remain poorly understood. Here, we focused on the cephalic gap gene orthodenticle (otd), which establishes a specific region of the anterior head. It has been proposed that otd acts in a combinatorial fashion with the cephalic gap genes empty spiracles (ems) and buttonhead (btd) to assign segmental identities in this region. To test this model, we used a heat-inducible transgene to generate pulses of ubiquitous otd expression during embryonic development. Ectopic otd expression caused significant defects in head formation, including the duplication of sensory structures derived from otd-dependent segments. However, these defects do not appear to result from the transformation of head segment identities predicted by the combinatorial model. Instead, they correlate with specific regulatory effects of otd on the expression of the segment polarity genes engrailed (en) and wingless (wg). Ectopic otd expression also caused the loss of head structures derived from the maxillary segment, which lies posterior to the otd domain. We show that this effect is associated with otd repression of the homeotic selector gene Deformed (Dfd).