Notch signaling directly controls cell proliferation in the Drosophila wing disc

Notch signaling is involved in cell differentiation and patterning during m orphogenesis, In the Drosophila wing, Notch activity regulates the expressi on of several genes at the dorsal/ventral boundary, and this is thought to elicit wing-cell proliferation. In this work, we show the eff ect of clones of cells expressing different forms of several members of the Notch signal ing pathway, which result in an alteration of Notch activity. The ectopic e xpression in clones of activated forms of Notch or of its ligands (Delta or Serrate) in the wing causes outgrowths associated with the appearance of e ctopic wing margins. These outgrowths consist of mutant territories and of surrounding wild-type cells. However, the ectopic expression of Delta, at l ow levels in ventral clones, causes large outgrowths that are associated ne ither with the generation of wing margin structures nor with the expression of genes characteristic of the dorsal/ventral boundary. These results sugg est that Notch activity is directly involved in cell proliferation, indepen dently of its role in the formation of the dorsal/ventral boundary. We prop ose that the nonautonomous effects (induction of extraproliferation and vei n differentiation in the surrounding wild-type cells) result from pattern a ccommodation to positional values caused by the ectopic expression of Notch .