The Notch pathway helps to pattern the tips of the Drosophila tracheal branches by selecting cell fates

The Drosophila tracheal system consists of a stereotyped network of epithel ial tubes formed by several tracheal cell types. By the end of embryogenesi s, when the general branching pattern is established, some specialised trac heal cells then mediate branch fusion while others extend fine terminal bra nches. Here evidence is presented that the Notch signalling pathway acts di rectly in the tracheal cells to distinguish individual fates within groups of equivalent cells. Notch helps to single out those tracheal cells that me diate branch fusion by blocking their neighbours from adopting the same fat e. This function of Notch would require the restricted activation of the pa thway in specific cells. In addition, and probably later, Notch also acts i n the selection of those tracheal cells that extend the terminal branches. Both the localised expression and the mutant phenotypes of Delta, a known l igand for Notch, suggest that Delta may activate Notch to specify cell fate s at the tips of the developing tracheal branches.