Fig. 1. Activation of Notch signalling is sufficient to induce the accumulation of Atonal at high levels. (A,B) In N^54/9 mutant clones (which lack green GFP staining), Atonal (red) expression is maintained at low levels and is not upregulated as it is in neighbouring wild-type tissue. The broken line indicates the border of the clone. (C,C') Clones of ectopic expression of Ser (green) do not induce ectopic neural differentiation. Photoreceptor differentiation is visualised by Elav expression (red); white arrows indicate the approximate position of the morphogenetic furrow. (D-I) Clones of ectopic expression of Dl (green). The expression of Atonal and Elav are shown in red and blue, respectively, in D,G. (D-F) Clones of Dl-expressing cells within 12-15 cell diameters of the furrow induce the expression of Atonal at high levels autonomously as well as in the cells surrounding the clone (e.g. white arrow in D). In clones that are partially within the competent zone (arrowhead in D,E) Atonal is only activated in the cells nearest to the morphogenetic furrow. Occasionally, we observe Elav-positive cells anterior to normal Elav expression (arrowhead in F). (G-I) Large clones of Dl-expressing cells that cross the morphogenetic furrow causes its anterior displacement. The expression of Atonal behind the morphogenetic furrow is disorganised, and the number of Atonal-expressing cells isolated seems increased. We also observe Elav-expressing cells in advance of its endogenous expression (arrowhead in I), and this expression is disorganised. White arrows indicate the approximate position of the morphogenetic furrow. Here, and in all figures, anterior is towards the left.