Segregation of a single neural precursor from each proneural cluster in Drosophila relies on Notch-mediated lateral signalling. Studies concerning the spacing of precursors for the microchaetes of the peripheral nervous system suggested the existence of a regulatory loop between Notch and its ligand Delta within each cell that is under transcriptional control. Activation of Notch leads to repression of the achaete-scute genes which themselves regulate transcription of Delta, perhaps directly. Here we have tested a requirement for transcriptional regulation of Notch and/or Delta during neuroblast segregation in embryos, by providing Notch and Delta ubiquitously at uniform levels. Neuroblast segregation occurs normally under conditions of uniform Notch expression. Under conditions of uniform Delta expression, a single neuroblast segregates from each proneural group in 80% of the cases, more than one in the remaining 20%. Thus transcriptional regulation of Delta is largely dispensable. We discuss the possibility that segregation of single precursors in the central nervous system may rely on a heterogeneous distribution of neural potential between different cells of the proneural group. Notch signalling would enable all cells to mutually repress each other and only a cell with an elevated neural potential could overcome this repression.
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