CONTROL OF ANTENNAL VERSUS LEG DEVELOPMENT IN DROSOPHILA

During the evolution of insects from a millipede-like ancestor, the Ho x genes are thought to have promoted the diversification of originally identical body structures(1,2). In Drosophila melanogaster ter, anten nae and legs are homologous structures that differ from each other as a result of the Hox gene Antennapedia (Antp), which promotes leg ident ities by repressing unknown antennal-determining genes(3-7). Here we p resent four lines of evidence that identify extradenticle (exd) and ho mothorax (hth) as antennal-determining genes. First, removing the func tion of exd(8,9) or hth, which is required for the nuclear localizatio n of Exd protein(10), transforms the antenna into leg; such transforma tions occur without activation of Antp. Second, hth is expressed and E xd is nuclear in most antennal cells, whereas both are restricted to p roximal cells of the leg. Third, Antp is a repressor of hth. Fourth, e ctopic expression of Meisl, a murine hth homologue(10), can trigger an tennal development elsewhere in the fly. Taken together, these data in dicate that hth is an antennal selector gene, and that Antp promotes l eg development by repressing hth and consequently nuclear Exd.