Logic of Wg and Dpp induction of distal and medial fates in the Drosophila leg.

Drosophila leg development requires the cooperation of two secreted signals, Decapentaplegic (Dpp) and Wingless (Wg), to form the proximodistal (PD) axis. Wg and Dpp are also required to pattern the dorsoventral (DV) axis of the leg. Here, we show that Distalless (Dll) and dachshund (dac), genes expressed at different positions along the PD axis, are activated by Wg signaling and repressed by Brinker (Brk), a transcriptional repressor in the Dpp pathway. The levels of both Brk and Wg determine which of these PD genes is activated. Surprisingly, Brk does not play a role in DV axis specification in the leg, suggesting that Dpp uses two distinct mechanisms for generating the PD and DV axes. Based on these results, we present a model for how Dpp and Wg, which are present as dorsal and ventral gradients, respectively, induce nearly circular domains of gene expression along the PD axis.