Genetic regulation of CNC expression in the pharnygeal primordia ofDrosophila blastoderm embryos.

Genetic controls regulating the establishment of the pharyngeal primordia in the anterior region of the Drosophila embryo were investigated through the analysis of the expression of thecnc gene, which is continuously expressed specificially in three pharyngeal segments. The spatial regulation ofcnc gene transcription was analyzed by in situ hybridization of CNC transcript-specific probes to embryos mutant for other cephalic patterning genes. The anterior domain of CNC expression (corresponding to the labral segment primordium) was found to be activated bybicoid andtorso maternal pathways, independently of known zygotic gap genes, and sequentially constricted to its final size by repression from neighboring region-specific genes. Control of the posterior domain (corresponding to the intercalary and mandibular segment primordia) involved combinatorial regulation by zygotic gap genes: activation by thebtd gap gene and repression from theotd gap gene anteriorly and thesna gene ventrally. Surprisingly, the posterior domain was shifted relative to the segmentation plan in mutants of theems gap gene. These regulatory controls establishing the limits of CNC expression in the pharyngeal primordia suggest that one mechanism for patterning within the anterior terminal region may involve direct activation of region-specific gene(s) by maternal factors over a relatively broad domain followed by constriction of that domain by repression from adjacently activated zygotic genes.