At the molecular, cellular and multicellular level, the spatial and temporal generation of biological organization is the product of the interaction of many independent components within biological networks. These networks constitute complex systems with properties greater than the sum of the constituent parts. During somitogenesis, a temporal patterning mechanism, the oscillator, gives rise to a emergent spatial pattern that presages the formation of morphological segments. This oscillator creates stripes/waves of gene expression that repeatedly travel through the field of cells about to undergo segmentation. Our first objective is to establish a more detailed chronology of the cyclical changes in gene expression and morphology that occur during zebrafish somitogenesis using vivo imaging, carefully staged embryos and fluorescent in situ hybridization. Recent work, including our own, has found that the Notch signaling pathway has successive functions in both the oscillator mechanism and in a later process that more directly regulates morphological segmentation. In the zebrafish, we have shown that the Notch ligands aei/deltaD, bea/deltaC and Notch target gene her1 each function within the oscillator mechanism, but each has a distinct mutant phenotype.
Our second aim i s to distinguish the functions of the two delta homologs by comparative phenotypic analysis of the two delta mutants, double mutant analysis, ectopic expression experiments and genetic mosaics. We hypothesize that aei/deltaD and beaJdeltaC have distinct signaling roles within the oscillator circuit and that the relative function and intensities of the delta signals vary at different positions within the segmenting tissue.
The third aim examines the function of her1 within the oscillator. We will determine if Her1 protein levels oscillate and if the stability of the protein determines the frequency of the oscillator.
The final aim i s to develop a real-time readout of the oscillator in order to visualize the oscillations in live embryos. Cumulatively, these experiments address how local signaling interactions between a cell and its immediate neighbors via the Notch pathway give rise to an emergent, higher-level of organization within the paraxial mesoderm. This analysis of the genetic network that governs somitogenesis is central to our understanding of vertebrate development and has broad implications concerning the complex properties of biological circuits.
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