Fig. 8. (A-D) Cells of Su(H) mutant proneural cluster express high levels of Dl. (A) Expression of Dl (red) and the SOP-E (blue) in a disc, bearing Su(H) mutant clones labelled by the absence of the green GFP marker. The arrow indicates the region shown in higher magnification in B-D. (B) Expression of Dl. (C) Expression of the SOP-E. (D) Pseudo-colour composite of the disc shown in A-C, showing expression of Dl in red, of the SOP-E in blue. The Su(H) mutant territory is revealed by the absence of GFP. The picture reveals that the cells that express the SOP-E and thus belong to a proneural cluster, also express high levels of Dl. (E-J) Expression of the Gbe+Su(H) construct [Gbe+Su(H)], Dl and Hnt in the notum of a wild-type wing imaginal disc of the late third larval instar stage. (E) Expression of Dl (red) and Gbe+Su(H) (green) occurs in similar domains. Hnt expression (also in red) reveals some of the SOPs present at this time. Arrow indicates the region of the pDC SOP. This region is shown at higher magnification in H-J. (F) Expression of Gbe+Su(H) in the notum of a wild-type disc. (G) Expression of Dl in the same disc as shown in F. The comparison of F and G further reveals the similarity of the expression pattern of Gbe+Su(H) and Dl. (H-J) Higher magnification of the region of the DC cluster, highlighted by the arrow in E. (H) Expression of Gbe+Su(H). (I) Expression of Dl and Hnt. At this focal plane expression of Hnt, but not Dl is visible. Dl is located on the apical side of the cell, which is out of focus. (J) Pseudo-colour image showing the expression of nuclear GFP in green to reveal all cells, of Hnt/Dl in red and the Gbe+Su(H) in blue. Comparison with I,J reveals that the expression of the Gbe+Su(H) is elevated in cells that are immediate neighbours of the SOP (arrowheads in H,J). The SOP itself does not express Gbe+Su(H). This suggests that the SOP sends a signal that activates the Notch pathway in its neighbours. (K-O) Su(H) mutant cells can activate the expression of Gbe+Su(H) in their wild-type neighbours. (K) Expression of Dl/hnt (red) and the Gbe+Su(H) (blue) in a wing disc, bearing Su(H) mutant clones, labelled by the absence of the green GFP marker. The arrow indicates the region shown at higher magnification in L-O. (L) Expression of Dl and Hnt. A group of cells express Dl in its membrane. One cell of this group expresses in addition Hnt, which is located in the nucleus (arrow). (M) Su(H) mutant area, revealed by the absence of GFP. Arrow indicates a single wild-type cell that is located in the Su(H) mutant territory. (N) Expression of Gbe+Su(H). The expression is lost in the mutant territory, indicating that it is dependent on the activity of Su(H). Arrowhead indicates the stripe of elevated expression of the Gbe+Su(H) in the wild-type cells at the clone boundary. (O) Pseudo-colour composite picture of the region shown in L-N. Expression of Dl and Hnt is shown in red, expression of Gbe+Su(H) is shown in blue. The clone is labelled by the absence of GFP. The arrow highlights the Hnt-expressing cell, which expresses GFP and is therefore Su(H) positive. Comparison with N reveals that this cell does not express the Gbe+Su(H), although it is surrounded by Su(H) mutant cells that strongly express Dl. At the left clone boundary, where the GFP positive wild-type cells are adjacent to the mutant cells (arrowhead) the expression of Gbe+Su(H) is elevated (revealed by the arrowheads in N,O). This suggests that the Su(H) mutant cells can activate the Notch pathway in their wild-type neighbours. Thus, Dl expressed in Su(H) mutant cells appears to be active. Nevertheless, the Notch pathway is not active in the wild-type SOP, suggesting that it is insensitive to Dl.