Cell fate at the dorsal margin of the zebrafish gastrula

To understand how experimental manipulations affect development, it is important to first characterize the normal development of cells in situ. To this end, I have constructed a detailed fate map of the dorsal marginal region in the zebrafish gastrula. I find that trunk tissue precursors are arranged in coherent fate map domains and are tissue-restricted, meaning that their progeny inhabit a single organ or tissue type. A domain of tail precursors, the forerunner cells, is segregated in the gastrula. These cells have unique morphogenesis, express specific genes, and form Kupffer's vesicle in the tailbud during segmentation. I have used the fate map to determine how genetic mutations affect cell fate. In floating head(flh) mutants, no notochord develops and somites fuse beneath the neural tube. In no tail (ntl) mutants, notochord differentiation is blocked, and the tail fails to develop. Cells from the notochord domain develop as muscle in flh mutants. Furthermore, the region of strongest flh expression coincides with the notochord domain at the early gastrula stage, suggesting that flh functions as an early switch promoting notochord over muscle fate. Gastrula notochord precursors in ntl mutants develop as axial mesenchymal cells, supporting the idea that the ntl mutation blocks notochord differentiation. Forerunner cells are present in ntl embryos, but Kupffer's vesicle never forms. Thus tail formation may depend on the proper development of Kupffer's vesicle. To understand how genes interact in notochord development, I have characterized the flh RNA expression pattern in flh and ntl mutants. Early flh expression is not affected in either mutant, implying that other genes set up the initial expression pattern. In flh mutants, flh expression is lost from involuted cells, indicating a spatial requirement for maintenance of flh expression. Differential regulation in anterior notochord precursors is also revealed by flh expression. The flh expression pattern in ntl mutants suggests that ntl may act later than flh for trunk notochord development, with subsequent cross-talk between the two genes. At later stages, the flh expression pattern is broad and diffuse, suggesting that the ntl mutation may be affecting cell movements. This thesis includes co-authored material.