The role of the Eph family in somitogenesis in the zebrafish

Somitogenesis involves the establishment of a segmental pattern in the presomitic mesoderm, before the somites themselves form. Here I describe the cloning of two transmembrane ligands of the Eph receptor tyrosine kinases from the zebrafish, ephrin-B2 and ephrin-B-L6. These, and the receptor EphA4, are shown to be segmentally expressed in the anterior presomitic mesoderm and forming somites. Ephrin-B2 is expressed in a posterior domain, and ephrin-B-L6 and EphA4 in a complementary anterior region of segments, showing that segments are patterned along the anterior-posterior axis prior to somite boundary formation. The overexpression of kinase-deleted receptors and soluble ligands resulted in a disruption of somitogenesis in zebrafish embryos. Somites failed to form or formed abnormally, in experimental embryos a segmental pattern was found to be correctly established, but her1 and delta D expression were incorrectly down-regulated in the anterior presomitic mesoderm. The expression of ephrin-B2, ephrin-B-L6, EphA4 and other somitic markers were investigated in five zebrafish mutants that show defects in somite formation. Infused somites embryos the somitic mesoderm was found to be posteriorized; whereas in beamter, deadly seven and after eight embryos markers of both anterior and posterior segment identity were found to be expressed throughout the somitic mesoderm. In whitetail embryos the establishment of a segmental pattern was found to be disturbed. This suggests that the Eph receptor and ephrins expressed in the presomitic mesoderm function downstream of the genes that establish a segmental pattern in this tissue. They function in conjunction with the Notch signalling pathway, translating the metameric pattern into the events of somite boundary formation. This process appears to require the patterning of segments along the anterior-posterior axis