NOTCH SIGNALING REGULATES VEINLET EXPRESSION AND ESTABLISHES BOUNDARIES BETWEEN VEINS AND INTERVEINS IN THE DROSOPHILA WING

The veins in the Drosophila wing have a characteristic width, which is regulated by the activity of the Notch pathway. The expression of the Notch-ligand Delta is restricted to the developing veins, and coincid es with places where Notch transcription is lower, We find that this a symmetrical distribution of ligand and receptor leads to activation of Notch on both sides of each vein within a territory of Delta-expressi ng cells, and to the establishment of boundary cells that separate the vein from adjacent interveins, In these cells, the expression of the Enhancer of split gene m beta is activated and the transcription of th e vein-promoting gene veinlet is repressed, thus restricting vein diff erentiation. We propose that the establishment of vein thickness utili ses a combination of mechanisms that include: (1) independent regulati on of Notch and Delta expression in intervein and vein territories, (2 ) Notch activation by Delta in cells where Notch and Delta expression overlaps, (3) positive feedback on Notch transcription in cells where Notch has been activated and (4) repression of veinlet transcription b y E(spl)m beta and maintenance of Delta expression by veinlet/torpedo activity.