Hox genes differentially regulate Serrate to generate segment-specific structures.

Diversification of Drosophila segmental morphologies requires the functions of Hox transcription factors. However, there is little information describing pathways through which Hox activities effect the discrete cellular changes that diversify segmental architecture. We have identified the Drosophila signaling protein Serrate as the product of a Hox downstream gene that acts in many segments as a component of such pathways. In the embryonic epidermis, Serrate is required for morphogenesis of normal abdominal denticle belts and maxillary mouth hooks, both Hox-dependent structures. The Hox genes Ultrabithorax and abdominal-A are required to activate an early stripe of Serrate transcription in abdominal segments. In the abdominal epidermis, Serrate promotes denticle diversity by precisely localizing a single cell stripe of rhomboid expression, which generates a source of EGF signal that is not produced in thoracic epidermis. In the head, Deformed is required to activate Serrate transcription in the maxillary segment, where Serrate is required for normal mouth hook morphogenesis. However, Serrate does not require rhomboid function in the maxillary segment, suggesting that the Hox-Serrate pathway to segment-specific morphogenesis can be linked to more than one downstream function.