The Hox gene Abdominal-B antagonizes appendage development in the genital disc of Drosophila

In Drosophila, the Hox gene Abdominal-B is required to specify the posterio r abdomen and the genitalia. Homologues of Abdominal-B in other species are also needed to determine the posterior part of the body. We have studied t he function of Abdominal-B in the formation of Drosophila genitalia, and sh ow here that absence of Abdominal-B in the genital disc of Drosophila trans forms male and female genitalia into leg or, less frequently, into antenna. These transformations are accompanied by the ectopic expression of genes s uch as Distal-less or dachshund, which are normally required in these appen dages, The extent of wild-type and ectopic Distal-less expression depends o n the antagonistic activities of the Abdominal-B gene, as a repressor, and of the decapentaplegic and wingless genes as activators. Absence of Abdomin al-B also changes the expression of Homothorax, a Hox gene co-factor, Our r esults suggest that Abdominal-B forms genitalia by modifying an underlying positional information and repressing appendage development. We propose tha t the genital primordia should be subdivided into two regions, one of them competent to be transformed into an appendage in the absence of Abdominal-B .