Fig. 6. A model for induction of the L5 primordium along the omb/brk border. The expression domain of the two Dpp signaling target genes omb (blue) and sal (green) is defined by their sensitivity to activation by Dpp and repression by Brk (red domains), in extreme anterior (A) and posterior (P) regions of the wing disc. L5 forms along the posterior border between omb and brk domains (solid blue line) following activation of the L5 organizer gene ab. brk-expressing cells on one side of the border produce a short range signal (Y) to which they cannot respond. This signal diffuses to the neighboring cells and, in combination with Omb, activates expression of ab along the border (yellow stripe). The green dotted line, which marks the posterior extent of strong sal expression, corresponds to a cryptic paravein border located midway between L4 and L5 (Sturtevant et al., 1997). This model is similar to that suggested for L2 formation along the border of the high level sal expression domain (Lunde et al., 1998). In the case of L2, sal-expressing cells produce a short range signal (X), to which they cannot respond due to repression by Sal, and this signal induces the expression of the L2 organizer gene kni along the border of cells just anterior to the sal domain (yellow stripe). The blue dotted line, which is indicated within the domain of strong omb expression along its anterior border and adjacent to cells expressing high levels of brk, may correspond to a cryptic paravein border located between L2 and the wing margin.