Fig. 7. Temporal progression of Dll and dac expression during leg development. (A-C) Expression of brk, Dll and dac during leg development. The discs are not shown to scale. (A) At 60 hours AEL Dll (green) is present in the center of the disc where there is no detectable Brk (red). In the ventral and lateral regions of the disc (arrow), there is a sharp border that separates Dll and brk expression. Wg (blue) is highest in the ventral region of the disc but is also observed in Dll-expressing cells. (B) At about 72 hours AEL, dac (blue) is broadly expressed in the dorsal part of the leg disc where there is no brk (arrowhead) and in the lateral and ventral regions where there are low levels of Brk (arrow). In the cells where dac is activated, Dll is also expressed at low levels. The ' and '' panels show subsets of these expression patterns as indicated. (C) At about 108 hours AEL, the expression of Dll and dac overlaps with brk. At this time these genes are refractory to Brk repression. (D) In the early second instar (left), Dll is activated in the center of the disc where there is no Brk and high Wg levels. In a late second instar disc (middle), dac is activated: (1) in the dorsal region of the disc where there is no Brk and low levels of Wg signaling (region 1); and (2) in lateral and ventral cells, where there is high Wg signaling and low Brk (region 2). In late third instar (right), Dll and dac expression is maintained and become refractory to Brk repression. The bottom set of schematics illustrate the unique Brk:Wg ratios required to activate Dll and dac. For dac activation, two ratios are suggested.