Direct control of antennal identity by the spineless-aristapedia gene of Drosophila

Summary Loss-of-function mutations in the spineless-aristapedia gene of Drosophila (ss^a mutants) cause transformations of the distal antenna to distal second leg, deletions or fusions of the tarsi from all three legs, a general reduction in bristle size, and sterility. Because ss^a mutants are pleiotropic, it has been suggested that ss⁺ has some rather general function and that the ss^a antennal transformation is an indirect consequence of perturbations in the expression of other genes that more directly control antennal or second leg identity. Here we test whether the ss^a transformation results from aberrant expression of Antennapedia (Antp), a homeotic gene thought to specify directly the identity of the second thoracic segment. We find that Antp^–ss^a mitotic recombination clones in the distal antenna behave identically to Antp⁺ss^a clones, and are transformed to second leg. This demonstrates that the ss^a antennal transformation is independent of Antp⁺, and suggests that ss⁺ may itself directly define distal antennal identity. The results also reveal that Antp⁺ is not required for the development of distal second leg structures, as these develop apparently normally in Antp^–ss^a antennal clones. Because Antp^– mutations cause deletions or transformations that are restricted to proximal structures, whereas ss^a alleles cause similar defects that are distally restricted, we suggest that ss⁺ and Antp⁺ may play similar, but complementary, roles in the distal and proximal portions of appendages, respectively.