Characterization of a new mastermind allele identified from somatic mosaic screen

The Notch signaling pathway is highly conserved and regulates various fundamental development events. Activation of Notch signaling relies on production of the Notch intracellular domain (NICD), which assembles a transcription factor complex to turn on down-stream targets expression. The mastermind (mam) gene encodes an essential co-activator that permits NICD activity in the cell nucleus. During a somatic mosaic screen in Drosophila, an uncharacterized gene l(2)S9998 is identified as a positive regulator of the Notch signaling pathway. Genetic analysis demonstrates that l(2)S9998 functions at the level of transcriptional activation of Notch targets in the signal receiving cells. Whole genome sequencing reveals that l(2)S9998 is a novel allele of the mam gene, which is further confirmed by complementation tests. Along with three molecularly defined transposon insertions isolated from the screen, four mutants of mam are shown to modulate Notch signaling during fly wing development. Our analysis provides additional genetic resources for understanding mam function and Notch signaling regulation.