Fig. 6. Model of Fz and Stbm interactions in the pupal wing epithelium and border cell cluster. In the wing, Fz and Stbm mutually reinforce the localisation of each other in opposing junctions of neighbouring cells (rounded black arrows) and inhibit the localisation of each other in adjacent regions of the same cell (grey bars). Distally localised Fz within the same cell promotes the production of a single distal actin-rich trichome, via Dsh and RhoA function (Axelrod, 2001; Strutt, 2001; Strutt et al., 1997). In addition, proximally localised Stbm is thought to promote trichome formation at the opposite end by an uncharacterised mechanism (Adler et al., 2004). In the border cell cluster, Fz and Stbm are localised to the junctional regions in which the epithelial polar follicle cells and the partly epithelial border cells make contact. Because Fz-expressing polar follicle cells promote the migration of Stbm-expressing border cells in a contact-dependent manner, we infer that Fz in the junctions of polar cells promotes the localisation of Stbm to the junctions of border cells. In turn, this would lead to Fz localisation to the non-junctional (mesenchymal) migratory regions of the border cells. Fz in border cells locally modulates the formation of appropriate actin structures, probably via Dsh and RhoA, as in the wing. In addition, Stbm in the junctions of border cells promotes the formation of actin structures at a distance in the migratory region. In this way, Stbm localised to junctions and Fz in the migratory region both independently promote migration. Consistent with our mosaic analysis, this scheme predicts that (i) contact with an Fz-expressing polar cell promotes border cell migration (Fig. 4B,C), (ii) Fz-expressing polar cells are only able to promote the migration of border cells that express Stbm (Fig. 1D, Fig. 4D), and (iii) Stbm is required in the polar cells for efficient border cell migration (Fig. 4B), but the border cells do not need to touch the Stbm-expressing polar cell (Fig. 4D).