Fringe-dependent separation of dorsal and ventral cells in the Drosophila wing

The separation of cells into populations that do not intermix, termed compa rtments, is a fundamental organizing principle during development(1-5). Dor sal-ventral compartmentalization of the Drosophila wing is regulated downst ream of the apterous (np) gene, which encodes a transcription factor that s pecifies dorsal wing fate(6-8). fringe (fng) is normally expressed by dorsa l cells downstream of ap(9); here we show that fng plays a key role in dors al-ventral compartmentalization. Loss of fng function causes dorsal cells t o violate the compartment boundary, and ectopic expression of the Fng prote in causes ventral cells to violate the compartment boundary. Fng modulates signalling through the Notch receptor(10,11). Notch and its ligands are ess ential for formation of the dorsal-ventral compartment border, and repositi oning the stripe of Notch activation that is normally established there app ears to reposition the compartment border. However, activation of Notch doe s not itself confer either dorsal or ventral cell location, and fng can inf luence compartmentalization even within regions of ubiquitous Notch activat ion. Our results indicate that the primary mechanism by which fng establish es a compartment border is by positioning a stripe of Notch activation, but also that fng may exert additional influences on compartmentalization.