Figure 2. Gene silencing in the PE is sufficient to induce the PE–to-DP transformation.

A) pnr-Gal4 drives expression in ∼early-mid L2 discs. Top: XY and ZX views of one disc; bottom: ZX of another. Gal4 is expressed in a broad domain within the dorsal PE. B) pnr-Gal4 UAS-ykiRNAi disc stained for Dlg (for cell shape) and Elav (for neurons). Right: confocal XY image of L3 disc with ZX and ZY projections; left: high magnification ZY of the same disc. PE-restricted expression of yki-RNAi transforms the dorsal region of the PE into DP/retina. High magnification panel shows that the transition from transformed to non-transformed region occurs over a few cell diameters. C, D) L3 discs stained in red for (top to bottom) dpp (dpp-lacZ) which marks the transition zone between eye progenitor cells and developing neurons, the RDFs tsh (tsh-lacZ) and Ey which mark eye progenitor cells, and the pan-neural membrane marker 22C10 which highlights differentiating neurons and their axons. Where shown, the membrane marker E-cadherin is in green. C) wt L3 discs show normal expression of dpp-lacZ in the transition zone where morphogenesis of the ommatidial array begins (visible as a depression in the DP of the wt discs, but not so marked in most transformed discs), tsh-lacZ and Ey in eye progenitor cells in the anterior portion of the disc (expression of Ey in the PE layer is also seen), and 22C10 stained neurons projects their axons posteriorly, along the basal side of the DP into the optic stalk and brain (not shown). Asterisk marks an axonal fascicle outside the eye disc (Bolwig's nerve). D) sd-RNAi L3 discs show mirror image duplications of each expression domain in two thicker, DP-like cell layers. The domains of dpp-lacZ expression are offset, indicating that, in this disc, neurogenesis in one layer lags behind the other layer. This was not always the case.