The Notch locus and the genetic circuitry involved in early Drosophila neurogenesis.

The genetic and molecular analysis of the Notch locus, which codes for a transmembrane protein sharing homology with the mammalian epidermal growth factor, suggests that the Notch protein is involved in a cell interaction mechanism essential for the differentiation of the embryonic nervous system of Drosophila. Taking advantage of the negative complementation between two Notch mutations that affect the extracellular domain of the protein, we have tried to dissect the genetic circuitry in which Notch is integrated by searching for genes whose products may interact with the Notch protein. This genetic screen has led to the identification of a surprisingly restricted set of interacting loci, including Delta and mastermind. Like Notch, both of these genes belong to a group of loci, the neurogenic loci, which have been previously identified by virtue of their similar mutant phenotype affecting early neurogenesis. We extend these studies by systematically exploring interactions between specific mutations in the Notch molecule and the other neurogenic genes. Furthermore, we show that the molecular lesions of two Notch alleles (nd and nd2), which interact dramatically with mastermind mutations, as well as with a mutation affecting the transducin homologous product of the neurogenic locus Enhancer of split, involve changes in the intracellular domain of the protein.