Fig. 3. The proneural function of N is reduced when dpp signalling pathway is blocked. (A) Clones of brk-expressing cells along the eye discs margins prevent the initialisation of the MF. (B-G) Clones co-expressing brk and Dl. (H-J) Clone of Dl-expressing cells. In all panels, green marks the clones, red indicates Atonal and blue indicates Elav. (B) Clones of brk- and Dl-expressing cells along the posterior eye margin show the same phenotype as clones of brk-expressing cells – preventing furrow initialisation, which leads to complete loss of neural differentiation (white arrow); this is a characteristic phenotype of the loss of function of the Dpp-signalling pathway. Clones ahead of the morphogenetic furrow only induce activation of Atonal in a thin band of cells immediately anterior to the morphogenetic furrow. Compare the clone indicated in B (arrowhead) with clones of Delta-expressing cells in H and Fig. 1; the distance from the furrow at which Atonal expression can occur is substantially reduced in the presence of brinker. (E-G) Several clones of Dl- and brk-expressing cells. Note the relatively poor ability of these cells to induce Atonal expression compared with clones expressing Dl alone (compare clones labelled with white arrows in E and F with clones of similar size and localisation labelled with white arrows in H,I). Surprisingly, despite the activation of Atonal expression, none of the double mutant clones analysed (n=30) express the neural marker Elav anteriorly to the endogenous Elav expression. Although the clones expressing Delta and brinker activate Atonal expression autonomously, there is a preferential activation of Atonal expression in neighbouring wild-type cells (when they lie near of the morphogenetic furrow – e.g. arrowhead in E); a higher magnification of this clone is shown in C,D. Red arrows indicate the approximate position of the morphogenetic furrow.
