In the developing fly leg, events downstream of the Wnt ligand Wingless(Wg) and the BMP ligand Decapentaplegic (Dpp) determine dorsoventral fate choices. But how do distal cells, which are exposed to high levels of both Wg and Dpp, discriminate between the two when choosing dorsoventral fates? The answer, report William Brook and colleagues, lies with the T-box transcription factors H15 and Midline (Mid) (see ). The authors demonstrate that activation through ventral Wg and repression through dorsal Dpp determines the ventral expression of both factors. Whereas neither mid nor H15 alone are essential for leg development,loss-of-function mutations in both genes lead to the loss of Wg-dependent ventral leg structures. By contrast, the ectopic expression of midalone can induce ventral fates in dorsal cells; however, this effect is reduced by attenuated Wg signalling, indicating that Wg acts both upstream of and in parallel with mid. Together, these results suggest that mid and H15 act as selector genes for ventral leg fate.
