Somitogenesis has been linked both to a molecular clock that controls the oscillation of gene expression in the presomitic mesoderm (PSM) and to Notch pathway signaling. The oscillator, or clock, is thought to create a prepattern of stripes of gene expression that regulates the activity of the Notch pathway that subsequently directs somite border formation. Here, we report that the zebrafish gene after eight (aei) that is required for both somitogenesis and neurogenesis encodes the Notch ligand DeltaD. Additional analysis revealed that stripes of her1 expression oscillate within the PSM and that aei/DeltaDsignaling is required for this oscillation.aei/DeltaD expression does not oscillate, indicating that the activity of the Notch pathway upstream ofher1 may function within the oscillator itself. Moreover, we found that her1 stripes are expressed in the anlage of consecutive somites, indicating that its expression pattern is not pair-rule. Analysis of her1 expression inaei/DeltaD, fused somites (fss), and aei;fss embryos uncovered a wave-front activity that is capable of continually inducing her1 expression de novo in the anterior PSM in the absence of the oscillation of her1. The wave-front activity, in reference to the clock and wave-front model, is defined as such because it interacts with the oscillator-derived pattern in the anterior PSM and is required for somite morphogenesis. This wave-front activity is blocked in embryos mutant for fssbut not aei/DeltaD. Thus, our analysis indicates that the smooth sequence of formation, refinement, and fading ofher1 stripes in the PSM is governed by two separate activities.
A Nomenclature for Prospective Somites and Phases of Cyclic Gene Expression in the Presomitic Mesoderm
