T-box genes organize the dorsal ventral leg axis in Drosophila melanogaster

A key event in fly limb development is the binary subdivision of the cardinal axes of imaginal discs by selector genes.(1-3) As an example, all limbs are partitioned into anterior and posterior compartments through the expression of engrailed (en) in posterior cells of imaginal discs and en expression is both necessary and sufficient to direct posterior instead of default anterior development.(4-8) In the leg imaginal disc, posterior en-expressing cells secrete the ligand Hedgehog (Hh), which in turn activates ventral Wingless (Wg) expression and dorsal Decapentaplegic (Dpp) expression in domains that bisect the imaginal disc.(9,10) Dpp and Wg act in concert to specify distal fates by inducing the expression of downstream genes including the selector gene Distal-less,(11,12) which is both necessary and sufficient for the development of the distal part of the leg.(13) In contrast, the dorsal (D) versus ventral (V) decision is mediated by antagonistic signaling between Dpp and Wg, with Dpp specifying D and Wg specifying V.(10,14-22) Recently, we have shown that the ventral specific T-box transcription factors midline(mid) and H15 are downstream effectors of Wg acting as selector genes that specify ventral fate.(23).