Patterning of the Drosophila L2 vein is driven by regulatory interactions between region-specific transcription factors expressed in response to Dpp signalling.

Pattern formation relies on the generation of transcriptional landscapes regulated by signalling pathways. A paradigm of epithelial patterning is the distribution of vein territories in the Drosophila wing disc. In this tissue, Decapentaplegic signalling regulates its target genes at different distances from the source of the ligand. The transformation of signalling into coherent territories of gene expression requires regulatory cross-interactions between these target genes. Here, we analyse the mechanisms generating the domain of knirps expression in the presumptive L2 vein of the wing imaginal disc. We find that knirps is regulated by four Decapentaplegic target genes encoding the transcription factors aristaless, spalt major, spalt-related and optix The expression of optix is activated by Dpp and repressed by the Spalt proteins, becoming restricted to the most anterior region of the wing blade. In turn, the expression of knirps is activated by Aristaless and repressed by Optix and the Spalt proteins. In this manner, the expression of knirps becomes restricted to those cells where Spalt levels are sufficient to repress optix, but not sufficient to repress knirps.