Segmental elements of the vertebrate body plan, such as vertebrae, ribs and most skeletal muscles, derive from embryonic somites. Somitogenesis is a spatially periodic process that is prefigured in the presomitic mesoderm (PSM) by coordinated oscillations of gene expression that set the rhythm of the “segmentation clock”. The position in the PSM where these oscillations arrest is termed the determination front and is the site where segment boundary is positioned. Although the Delta/Notch pathway has recently been shown to synchronize oscillating cells, little is known about the molecular mechanisms that define the determination front and link it to the segmentation clock. The Wnt pathway has been hypothesized to provide such a link in mouse and chick, but its early effects on mesoderm specification and tail formation have made studying its role in later somite patterning difficult. Here we propose to investigate the role of Wnt signaling during somitogenesis in the zebrafish embryo, which offers unique accessibility to both genetics and imaging. In order to uncouple Wnt function in early mesoderm specification and somite patterning, we will use inducible transgenic zebrafish lines to transiently modulate Wnt signaling later in development. Our preliminary results clearly show that this transient disruption of Wnt specifically affects somite patterning. Thus we will combine this method with analysis of determination markers, boundary formation and oscillating gene expression to further explore the role of Wnt. We will also use a new quantitative time-lapse method to measure potential changes in oscillator period. Finally, we will test genetic interactions between Wnt and the Notch pathway by using mutant lines available in the Oates lab. Our project will thus take advantage of the zebrafish model to investigate the spatial and temporal functions of Wnt signaling in somitogenesis, providing a combined analysis at tissue, cellular and molecular levels.