Fig. 5. Expression of da and ato, but not hh and dpp, can rescue the block in MF progression caused by ectopic activation of Wg signaling. Wandering third instar larval eye discs stained for ß-gal (all samples have a P[dpp-lacZ] transgene) and Elav. (A) WT; (B) P[dpp-Gal4], P[UAS-wg]; (C) P[dpp-Gal4], P[UAS-wg], P[UAS-TCF^DN]; (D-F) P[dpp-Gal4], P[UAS-wg], P[UAS-da]; (G-I) P[dpp-Gal4], P[UAS-wg], P[UAS-hh]; (J) P[dpp-Gal4], P[UAS-wg], P[UAS-ato]; (K) P[dpp-Gal4], P[UAS-arm^act]; (L) P[dpp-Gal4], P[UAS-arm^act], P[UAS-da]. (A) At this stage, the MF is normally more than halfway across the eye primordia. (B) Ectopic expression of wg at the posterior edge of the eye completely blocks MF progression and differentiation of Elav-positive cells. (C) Coexpression of TCF^DN with wg significantly rescues MF progression. (D-F) Coexpression of da with wg always resulted in some MF progression, with approximately 40% of the eyes similar to D, 40% similar to E and the remainder rescued to the extent shown in F. Coexpression of dpp with wg resulted in no rescue (data not shown). (G-I) Coexpression of hh with wg did not rescue MF progression from the posterior edge, but did result in usually one (occasionally more) MF being initiated in the interior of the eye primordia and progressing concentrically. These ectopic MFs had progressed to different extents at the time of fixation, with some recent (G), some several rows of photoreceptors (H) and some progressed to the periphery of the eye (I; note that dpp-lacZ is at the periphery, indicating the leading edge of the MF). Coexpression of ato with wg resulted in considerable rescue of MF progression, though less than seen with da coexpression (the eye in J is one of the best rescues). Expression of an activated form of arm (arm^act) blocked MF progression, though not as well as wg (note some Elav-positive cells in K). Coexpression of da with arm^act resulted in significant rescue of MF progression (L). All crosses were maintained at 29°C before fixation.