Fig. 5. Integration of prepatterning and lateral inhibition functions via Wnt and Notch signalling in Drosophila. In the epidermis of the imaginal disc of Drosophila, a combination of history and spatial patterning delimits clusters of cells (shown at the bottom) that can undergo neural specification through the expression of members of the Achaete-Scute complex (AS-C, blue). (A) AS-C expression is promoted by Wingless signalling and is antagonized by Notch (see text for details) in a manner that is independent of its transcriptional effector, Suppressor of Hairless [Su(H)], but depends on the activity of the ubiquitin ligase Deltex. (B) The binding of Wingless to its receptor complex triggers the activation of Armadillo/β-catenin and the suppression of the antagonistic activity of Notch, probably via Dishevelled. In consequence, all cells within the proneural cluster begin to express members of the AS-C. (C) One cell in the cluster (darker blue) begins to express higher levels of AS-C proteins than the other cells and, as a consequence, takes the lead in the process of `lateral inhibition', by which Notch signalling via Delta leads to the generation of free NICD and through Su(H) represses the expression of the AS-C genes.