Fig. 4.

Constitutive Yki activation sustains morphogen-dependent growth and recruitment of pre-wing cells. (A and B) Constitutive Yki activity sustains FF growth in stalled discs. In contrast to wing discs of phm.lexA:VP16 LexOP.fh^RNAi but otherwise wild-type larva, which stall after ∼6 to 7 d (A), wing discs from phm.lexA:VP16 LexOP.fh^RNAi larvae that are homozygous for the wts^P2 mutation have constitutive Yki activity and show continuous proliferative growth of cells at the wing periphery (monitored by EdU incorporation, green, in both A and B), as well as a dramatic expansion in the wing proper, primarily in the D/V axis (5XQE.DsRed, red). As observed for blanket Dpp and Wg expression in stalled discs, growth is restricted to the wing periphery: Once cells initiate vg expression and enter the expanding wing primordium they arrest. (C–F) Continuous FF growth in stalled wts^P2 larvae depends on Wg produced by IR cells surrounding the wing primordium. (C) In wts^P2 stalled discs, wing growth by FF propagation should be limited only by the availability of Dpp and Wg but is observed far outside of the expected range of Wg produced by D/V border cells. However, IR cells (WgIR, light blue) provide a second, potential source of Wg. In stalled but otherwise wild-type discs, we envisage that pre-wing cells that are in position to receive this Wg as well as Dpp do not respond because they are located too far from the wing proper to receive FF signal. By contrast, in stalled wts^P2 discs, these pre-wing cells should now receive constitutive FF input in addition to Wg produced by IR cells and hence should continue to grow and be recruited into the wing, provided they also receive Dpp. As observed (D and F), it is specifically non-Vg-expressing cells in the immediate surround of the expanding wing primordium that are exposed to Wg from the IR (F, green, IR) and continue to proliferate (D). No wing growth is observed in wing discs from stalled, wts^P2 larvae that are also homozygous for wg^spd-flg, an enhancer mutant allele of wg that selectively eliminates Wg expression in IR cells (E), confirming that these cells are the source of Wg responsible for the continuous growth at the wing periphery in wts^P2 stalled discs. (G and H) Continuous wing growth in stalled wts^P2 discs depends on Dpp. No growth is observed in stalled, wts^P2 wing discs that are transheterozygous for two dpp^disk enhancer mutant alleles, dpp^d8 and dpp^d10, that abolish Dpp expression in the wing disc (G); however, continuous growth can be reinitiated in such discs by heat-shock-induced excision of the stop cassette of an Act > stop > Gal4 transgene to generate clones of UAS.dpp^GFP cells that ectopically express a GFP-tagged form of Dpp (H, blue; SI Appendix).