INTRODUCTION 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 Martínez- Arias, 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 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 A10-11. Each segmental primordium has an anterior (A) and a posterior (P) compartment, the P cells of the three segments being contiguous at the lateral edges of the disc. We show that Hedgehog (Hh) expressed in the P compartment differentially signals A cells at the AP compartment border and A cells at the segmental border. As in the wing imaginal disc, cell lineage restriction of the AP compartment border is defined by Hh signalling. There is also a lineage restriction barrier at the segmental borders, even though the P compartment cells of the three segments converge in the lateral areas of the disc. Lineage restriction between segments A9 and A10-11 depends on factors other than the Hh, En and Hox genes. The segmental borders, however, can be permeable to some morphogenetic signals. Furthermore, cell ablation experiments show that the presence of all primordia (either the anal or the genital primordium) during development are required for normal development of genital disc. Collectively, these findings suggest that interaction between segmental primordia is required for the normal development of the genital disc. Key words: Drosophila genital disc patterning, Hh signalling, Dpp signalling, Wg signalling, Segmental boundaries, Cell lineage restriction SUMMARY Development of the Drosophila genital disc requires interactions between its segmental primordia Nicole Gorfinkiel 1 , Lucas Sánchez 2 and Isabel Guerrero 1, * 1 Centro de Biologia Molecular ‘Severo Ochoa’, C.S.I.C., Universidad Autonoma de Madrid, Cantoblanco, E-28049 Madrid, Spain 2 Centro de Investigaciones Biológicas, C.S.I.C., Velázquez 144, 28006 Madrid, Spain *Author for correspondence (e-mail: iguerrero@cbm.uam.es) Accepted 18 October 2002