DeltaC and DeltaD ligands play different roles in the segmentation clock dynamics.

The vertebrate segmentation clock drives periodic somite segmentation during embryonic development. Her1 and Her7 clock proteins generate oscillatory expression of their own genes as well as that of deltaC in zebrafish. In turn, DeltaC and DeltaD ligands activate Notch signaling, which then activates transcription of clock genes in neighboring cells. While DeltaC and DeltaD proteins form homo- and heterodimers, only DeltaC-containing oscillatory dimers were expected to be functional. To investigate the contributions of DeltaC and DeltaD proteins on the transcription of her1 and her7 segmentation clock genes, we counted their transcripts by performing single molecule fluorescent in situ hybridization imaging in different genetic backgrounds of zebrafish embryos. Surprisingly, we found that DeltaD homodimers are also functional. We further found that Notch signaling promotes transcription of both deltaC and deltaD genes, thereby creating a previously unnoticed positive feedback loop. Our computational model highlighted the intriguing differential roles of DeltaC and DeltaD dimers on the clock synchronization and transcript numbers, respectively. We anticipate that a mechanistic understanding of the Notch signaling pathway will not only shed light on the mechanism driving robust somite segmentation but also inspire similar quantitative studies in other tissues and organs.