Role of the Abdominal-B gene in genitalia development. E. Sanchez-Herrero , B. Estrada. Centro de Biologia Molecular, Univ Autonoma, Madrid, Spain.

   The Abdominal-B (Abd-B) Hox gene is required to specify the posterior abdomen and the genitalia. We have studied the function of Abd-B in the formation of the genitalia. Absence of Abd-B in the genital disc transforms male and female genitalia into leg or, less frequently, into antenna. These transformations are accompanied by the ectopic expression of genes like Distal-less and dachsund, normally required in these appendages. The extent of wildtype and ectopic Distal-less expression depends on the antagonistic activities of the Abd-B gene, as a repressor, and of the decapentaplegic and wingless genes as activators. Absence of Abd-B also changes the expression of homothorax, a Hox gene co-factor, repressing or activating it depending of the region of the genital primordium. Therefore, male and female genital primordia seems to be subdivided into two regions, one of them competent to be transformed into an appendage in the absence of Abd-B. There are two Abd-B proteins, m (or I) and r (or II). The Abd-B m product is expressed in the female genitalia and the Abd-B r product in the male genitalia. When both proteins are absent in the female genitalia there is absence of abdominal-A product, ectopic expression of Distal-less and transformation into leg or antenna. When only the Abd-B m product is absent in the female genitalia, however, abdominal-A remains expressed, Distal-less is not ectopically activated and the genitalia is transformed into abdomen and not into leg. Moreover, Abd-B protein is still detected in this primordium when Abd-B m is absent. This suggest that transcription of Abd-B r is ectopically activated in the absence of Abd-B m product, the opposite to what happens in the embryo.