Fig. 2. EGFR is required for Delta expression in pupal cone cells. (A-C) Delta expression in EGFR^ts1-mutant clones subjected to a nonpermissive temperature during pupal development. (A) GFP expression marks wild-type cells (green); non-GFP cells are mutant for EGFR^ts1. (B) The same disc as in A was co-stained for Delta expression (red). (C) The merged panel shows that the Delta protein is restricted to wild-type cells. (D-F) Expression of the cone-cell marker, Cut, in EGFR^ts1 clones transferred to non-permissive conditions during the mid-pupal stages. (D) GFP marks wild-type tissue; non-GFP cells are mutant for EGFR. (E) The same disc as in D was co-stained for Cut (red). (F) The merged panel shows that the loss of EGFR function in pupal stages does not compromise cone-cell fate. (G-J) Expression of a dominant-negative version of EGFR (EGFR^DN) in cone cells blocks Delta expression. (G,H) Wild-type (control) pupal eye discs stained for Delta showed its expression in cone cells (red, G); the spa-Gal4 driver was also expressed in cone cells (red, H). spa-Gal4, UAS-EGFR^DN pupal eye discs stained for Delta showed loss of Delta expression in the cone cells (I), whereas spa-Gal4, UAS-EGFR^DN pupal eye discs stained for Cut showed that a loss of EGFR function during pupal stages does not disrupt cone-cell fate specification (J). (K-N) Ectopic activation of EGFR in cone cells promotes Delta expression. MAPK activation in wild-type third instar eye disc (K) is seen in cells at the furrow followed by low levels of activation in the differentiated R cells behind the furrow (K). (L) In spa-Gal4, UASEGFR^act third instar eye disc, MAPK is activated at high levels at later stages in the developing cone cells, which express spa-Gal4. (M) Wild-type expression of Delta (green) in the third instar eye disc is limited to R cells and is not expressed in cone cells (red). (N) In spa-Gal4, UAS-EGFR^act third instar eye disc, ectopic activation of Delta (green) is seen in cone cells (red, arrow).