CONTROL OF DROSOPHILA HEAD SEGMENT IDENTITY BY THE BZIP HOMEOTIC GENECNC

Mutational analysis of cap'n'collar (cnc), a bZTP transcription factor closely related to the mammalian erythroid factor NF-E2 (p45), indica tes that it acts as a segment-specific selector gene controlling the i dentity of two cephalic segments. In the mandibular segment, cnc has a classical homeotic effect: mandibular structures are missing in cite mutant larvae and replaced with duplicate maxillary structures. We pro pose that cnc functions in combination with the homeotic gene Deformed to specify mandibular development. Labral structures are also missing in cnc mutant larvae, where a distinct labral primordia is not proper ly maintained in the developing foregut, as observed by the failure to maintain and elaborate patterns of labral-specific segment polarity g ene expression. Instead, the labral primordium fuses with the esophage al primordium to contribute to formation of the esophagus. The role of cite in labral development is reciprocal to the role of homeotic gene forkhead, which has an identical function in the maintenance of the e sophageal primordium. This role of homeotic selector genes for the seg ment-specific maintenance of segment polarity gene expression is a uni que feature of segmentation in the preoral head region of Drosophila.