Fig. 5. The effects of Ras^ACT in scrib mutant eye clones are mediated through the MAPK cascade, but cannot be mimicked by enhancing cell cycle progression and preventing apoptosis. (A and B) Expression of Raf^ACT in scrib¹ eye clones mimics the effects of Ras^ACT. Elav staining shows a block to photoreceptor differentiation in third instar larval eye discs (A1), and the scrib^– tissue, marked by the expression of GFP, greatly overproliferates (A2). As a consequence, many larvae fail to pupate and overgrow (B, wild-type larvae on the left) like homozygous scrib mutants. (C) Expressing Ras^ACT in control wild-type eye clones, marked by GFP expression (C2 and 3), induces cyclin E (C1 and 3), and is pupal lethal. The position of the MF is indicated by a bar. (D–F) Blocking cyclin E activity by expressing Dacapo (Dap) in Ras^ACT-expressing control clones can rescue the pupal lethality associated with the constitutive expression of Ras^ACT, although the resulting adult eyes are severely disorganized (E). Expressing Dap in Ras^ACT-expressing scrib¹ clones can also rescue the overproliferation phenotype, although Elav staining (D) shows that Ras^ACT is still not effective at inducing photoreceptor differentiation of scrib^– tissue, and the developing adult flies fail to eclose and have severely disorganized eyes (F). (G and H) Mutating E2F1 (e2f1⁹¹) in Ras^ACT-expressing control eye clones rescues the pupal lethality associated with the constitutive expression of Ras^ACT in wild-type clones (G), as does mutating E2F1 (e2f1⁹¹) in Ras^ACT-expressing scrib¹ mutant clones (H). (I and J) Co-expression of cyclin E with the apoptosis inhibitor, p35, in scrib¹ eye clones (I) results in overgrowth but fails to reproduce the effects of Ras^ACT in scrib¹ clones. Co-expression of E2F1, DP and p35 in scrib¹ eye clones (J) results in overgrowth; however, the overgrowth is still not as aggressive as that produced by Ras^ACT or Raf^ACT in scrib¹ clones.