THE NOTCH LIGAND, X-DELTA-2, MEDIATES SEGMENTATION OF THE PARAXIAL MESODERM IN XENOPUS EMBRYOS

Segmentation of the vertebrate embryo begins when the paraxial mesoder m is subdivided into somites, through a process that remains poorly un derstood, To study this process, we have characterized X-Delta-2, whic h encodes the second Xenopus homolog of Drosophila Delta, Strikingly, X-Delta-2 is expressed within the presomitic mesoderm in a set of stri pes that corresponds to prospective semitic boundaries, suggesting tha t Notch signaling within this region establishes a segmental prepatter n prior to somitogenesis, To test this idea, we introduced antimorphic forms of X-Delta-2 and Xenopus Suppressor of Hairless (X-Su(H)) into embryos, and assayed the effects of these antimorphs on somite formati on, In embryos expressing these antimorphs, the paraxial mesoderm diff erentiated normally into semitic tissue, but failed to segment properl y, Both antimorphs also disrupted the segmental expression of X-Delta- 2 and Hairy2A, a basic helix-loop-helix (bHLH) gene, within the presom itic mesoderm, These observations suggest that X-Delta-2, via X-Notch- l, plays a role in segmentation, by mediating cell-cell interactions t hat underlie the formation of a segmental prepattern prior to somitoge nesis.