Dorsoventral lineage restriction in wing imaginal discs requires Notch.

The formation of boundaries that prevent the intermixing of cells is an important developmental patterning mechanism. The compartmental lineage restrictions that appear in the developing imaginal discs of Drosophila are striking examples of such boundaries. However, little is known about the cellular mechanism underlying compartmental lineage restrictions. The dorsoventral (D/V) lineage restriction that arises late in the developing wing imaginal disc requires the dorsal expression of the transcription factor Apterous and it has been hypothesized that apterous (ap) maintains compartmentalization by directly regulating the expression of molecules that modify cell adhesion or affinity. However, ap expression also regulates signalling between dorsal and ventral compartments, resulting in high levels of Notch signalling at the D/V boundary. Here we show that the formation of Notch-dependent boundary cells is required for the D/V lineage restriction.

The function and regulation of cut expression on the wing margin of Drosophila: Notch, Wingless and a dominant negative role for Delta and Serrate.

We have investigated the role of the Notch and Wingless signaling pathways in the maintenance of wing margin identity through the study of cut, a homeobox-containing transcription factor and a late-arising margin-specific marker. By late third instar, a tripartite domain of gene expression can be identified about the dorsoventral compartment boundary, which marks the presumptive wing margin. A central domain of cut- and wingless-expressing cells are flanked on the dorsal and ventral side by domains of cells expressing elevated levels of the Notch ligands Delta and Serrate. We show first that cut acts to maintain margin wingless expression, providing a potential explanation of the cut mutant phenotype. Next, we examined the regulation of cut expression. Our results indicate that Notch, but not Wingless signaling, is autonomously required for cut expression. Rather, Wingless is required indirectly for cut expression; our results suggest this requirement is due to the regulation by wingless of Delta and Serrate expression in cells flanking the cut and wingless expression domains. Finally, we show that Delta and Serrate play a dual role in the regulation of cut and wingless expression. Normal, high levels of Delta and Serrate can trigger cut and wingless expression in adjacent cells lacking Delta and Serrate. However, high levels of Delta and Serrate also act in a dominant negative fashion, since cells expressing such levels cannot themselves express cut or wingless. We propose that the boundary of Notch ligand along the normal margin plays a similar role as part of a dynamic feedback loop that maintains the tripartite pattern of margin gene expression.

wingless refines its own expression domain on the Drosophila wing margin.

The imaginal discs of Drosophila, which give rise to the adult appendages, are patterned during a period of intense cell proliferation. The specification of differing regions occurs in some cases by subdividing the disc epithelium into lineage compartments. However, in most cases precise boundaries are formed between different cell types without early compartmentalization. One such boundary occurs between the wingless (wg)-expressing cells of the wing margin and the adjacent proneural cells, which give rise to margin sensory bristles. Here we show that this boundary arises in part by a mechanism of 'self-refinement', by which wingless protein (Wg) represses wg expression in adjacent cells. Cells unable to receive the Wg signal do not resolve the boundary between wg-expressing and proneural cells.