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

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Abstract
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.