In both sexes, the Drosophila genital disc comprises three segmental primordia: the female genital primordium derived from segment A8, the male genital primordium derived from segment A9 and the anal primordium derived from segments

The development of Drosophila imaginal discs to a large extent depends on the genetic interactions that take place at the border between anterior (A) and posterior (P) cells. These two populations of cells differ as the selector gene engrailed (en) is expressed in P cells but not in A cells. The limit between A and P cells is the compartment border and acts as a source of positional information (reviewed by Lawrence and Struhl, 1996). En activates the diffusible signal Hedgehog (Hh) in P cells, which signals to A cells and activates two different morphogens, Wingless (Wg) and Decapentaplegic (Dpp) (Basler and Struhl, 1994). Wg and Dpp act as general organisers of the imaginal discs, inducing growth and specifying different identities in a concentration-dependent manner (Zecca et al., 1996; Lecuit and Cohen, 1997). This model has arisen mainly from studies performed on the wing and leg discs. These are single discs derived from a cluster of cells of a unique segment in the embryo (Bate and MartínezArias, 1991). The genital disc, which gives rise to the terminalia of the adult fly (internal and external genitalia excluding the gonads and analia) (Bryant, 1978), is organised in a manner reminiscent of the leg disc (Gorfinkiel et al., 1999; Estrada and Sánchez-Herrero, 2001). However, there are several differences between the genital disc and other imaginal discs that make it an original experimental model (reviewed by Sánchez and Guerrero, 2001). For example, it is the only imaginal disc that shows clear sexual dimorphism. The genital disc is a compound disc formed by the fusion of three different primordia derived from the embryonic abdominal segments 8, 9 and 10-11 (Nöthiger et al., 1977; Schüpbach et al., 1978; Dübendorfer and Nöthiger, 1982; Epper and Nöthiger, 1982). Thus, a genital disc contains a female genital primordium (derived from segment A8), a male genital primordium (derived from segment A9) and an anal primordium (derived from segments A10-11). In females, the female primordium gives rise to almost all the internal and external genitalia. The male primordium was considered to be a ‘repressed primordium’ because its growth is limited and it does not form any adult structures (Epper and Nöthiger, 1982). However, it has recently been shown that the male primordium in the female genital disc gives rise to the parovaria (internal accessory female glands) (Keisman et al., 2001). The anal primordium in females gives rise to the female analia. The converse is true for males. In male genital discs, the male genital primordium gives rise to the majority of the internal and external male genital structures. The female genital 295 Development 130, 295-305 © 2003 The Company of Biologists Ltd doi:10.1242/dev.00214