Notch signalling plays a critical role in patterning the ventral mesoderm during early embryogenesis in Drosophila melanogaster

Notch signaling is a critical regulator of multiple developmental processes through its ability to control gene expression, and thereby influence cell fate specification and cell proliferation through direct cell-cell communication. Although Notch signalling has been implicated in myogenesis during late embryogenesis, the role of Notch signalling in early mesoderm development in largely unexplored. Endocytosis of the ligand and receptor extracellular domain is observed in the mesoderm and mesectoderm and therefore indicative of signalling activity. Here we present evidence that critical mesodermal genes require and are responsive to Notch signalling activity. We also developed a novel light-inducible variant of NICD to allow precise control of ectopic activation of Notch signalling. Using a combination of high-resolution fluorescent mRNA in situ hybridizations and qPCR, we identified a set of mesodermal genes that are directly regulated by Notch signalling. We also show evidence that Notch signalling indirectly regulates the dorsal-ventral patterning program mediated by the Toll signalling pathway through the Dorsal/ Twist/ Snail network. Our findings in this paper show that Notch signalling regulates mesoderm patterning and boundary formation between it and the ectoderm by regulating the expression of critical genes and providing negative feedback on the upstream patterning network. ### Competing Interest Statement The authors have declared no competing interest.