Spatial discontinuity of Optomotor-blind expression in the Drosophila wing imaginal disc disrupts epithelial architecture and promotes cell sorting

We show that the transcription factor Omb forms, in fact, a symmetrical gradient on both sides of the A/P compartment boundary. Disruptions of the Omb gradient lead to a re-organization of the epithelial cytoskeleton and to a retraction of cells toward the basal membrane suggesting that the Omb gradient is required for correct epithelial morphology. Moreover, by analysing the shape of omb gain- and loss-of-function clones, we find that Omb promotes cell sorting along the A/P axis in a concentration-dependent manner.Our findings show that Omb distribution in the wing imaginal disc is described by a gradient rather than a step function. Graded Omb expression is necessary for normal cell morphogenesis and cell affinity and sharp spatial discontinuities must be avoided to allow normal wing development.The concept of Dpp as morphogen in early wing development owes much to the observation of nested target gene expression domains, initially described for spalt (sal) and omb [1,2] and subsequently for vestigial (vg) and the vg quadrant enhancer [3]. Dpp spreads from its expression domain along the A/P compartment boundary to receiving cells forming a gradient which directs patterning and growth of the wing pouch [4-6]. Dpp signaling represses the transcriptional repressor gene brinker (brk), which is thereby expressed in a gradient inverse to the Dpp signaling activity [7]. The relationship between target gene expression and Brk level is not simply reciprocal. For instance, high level sal expression in the central wing pouch requires direct Dpp signaling in addition to repression of brk, i. e. the sal expression domain is specified by opposing gradients [8-10]. Moreover, different target genes appear to be repressed by Brk through different mechanisms [11]. Irrespective of the mechanistic details, the nested expression pattern of sal and omb forms the basis of the threshold model of Dpp (or rather Brk) target gene regulation [12,13].Apart from setting up gene expression pat