Coexpression of the homeobox genes Distal-less and homothorax determines Drosophila antennal identity

The Distal-less gene is known for its role in proximodistal patterning of D rosophila limbs. However, Distal-less has a second critical function during Drosophila limb development, that of distinguishing the antenna from the l eg. The antenna-specifying activity of Distal-less is genetically separable from the proximodistal patterning function in that certain Distal-less all elic combinations exhibit antenna-to-leg transformations without proximodis tal truncations. Here, me show that Distal-less acts in parallel with homot horax, a previously identified antennal selector gene, to induce antennal d ifferentiation. While mutations in either Distal-less or homothorax cause a ntenna-to-leg transformations, neither gene is required for the others expr ession, and both genes are required for antennal expression of spalt, Coexp ression of Distal-less and homothorax activates ectopic spalt expression an d can induce the formation of ectopic antennae at novel locations in the bo dy, including the head, the legs, the wings and the genital disc derivative s. Ectopic expression of homothorax alone is insufficient to induce antenna l differentiation from most limb fields, including that of the wing. Distal -less therefore is required for more than induction of a proximodistal axis upon which homothorax superimposes antennal identity, Based on their genet ic and biochemical properties, we propose that Homothorax and Extradenticle may serve as antenna-specific cofactors for Distal-less.