Fig. 4. Ectopic Wg signaling induces DRA fates in a fraction of ventral ommatidia. (A-D) Apc1 loss and ectopic Arm signaling induces expression of Hth (green) in some ventral ommatidia. Photoreceptors are marked with anti-Elav (blue). (A) In wild-type retinas, hth expression is observed in rare photoreceptors at the ventral rim (arrowheads). (B) In Apc1^Q8 mutant retinas, 30 hours APF, many photoreceptors near the ventral pole express hth. (C) In otherwise wild-type retinas, ectopic expression of the IRO-C protein Ara, under the control of a GMR promoter, induces the formation of an ectopic ventral rim area with one to three rows of ommatidia expressing hth. (D) In Apc1^Q8 flies, Hth is found throughout the entire retina when Ara is expressed under the control of a GMR promoter. (E-H) Expression of full-length Arm (E,F) and constitutively active Arm^S10 (G,H) under the control of elav and long GMR promoters, respectively, leads to the expansion of Hth expression (green) within the entire dorsal half of the retina, as well as in a fraction of ventral ommatidia. Photoreceptors are marked with anti-Elav (blue).