Abstract
The genital disc consists of three primordia: the female genital, the male genital and the anal primordia. In individuals of either sex, only one of the two genital primordia grows, whereas the anal primordium develops in both sexes. It is proposed here that the genital disc, which is of ventral origin, is organized in a manner similar to the antennal and leg discs: the expression domains of decapentaplegic (dpp) and wingless (wg) are mostly complementary and abut engrailed (en) expression. An analysis was made of the roles of the genes hedgehog (hh), patched (ptc), dpp and wg in the development of the three primordia that form the genital disc. The morphogenetic alterations produced by ectopic expression of hh mimics lack of ptc function. Both genetic conditions cause derepression of dpp and wg. Ectopic expression of either of these genes causes non-autonomous duplications and/or reductions of genital and anal structures. Some of these alterations are explained by the mutual repression of wg and dpp. In the development of the genital disc, the functional relationships between these genes seem to be analogous to those described for leg and antennal discs: dpp and wg are induced in the anterior compartment by Hh protein blocking the repressive effect of ptc, and the mutual repression of dpp and wg restrict each other to their domains. It may be concluded that dpp and wg act as general organizers for development of the genital disc.
