Su(H)-independent activity of hairless during mechano-sensory organ formation in Drosophila.

Formation of mechano-sensory organs in Drosophila involves the selection of neural precursor cells (SOPs) mediated by the classical Notch pathway in the process of lateral inhibition. Here we show that the subsequent cell type specifications rely on distinct subsets of Notch signaling components. Whereas E(spl) bHLH genes implement SOP selection, they are not required for later decisions. Most remarkably, the Notch signal transducer Su(H) is essential to determine outer but not inner cell fates. In contrast, the Notch antagonist Hairless, thought to act upon Su(H), influences strongly the entire cell lineage demonstrating that it functions through targets other than Su(H) within the inner lineage. Thereby, Hairless and numb may have partly redundant activities. This suggests that Notch-dependent binary cell fate specifications involve different sets of mediators depending on the cell type considered.