During gastrulation of Xenopus embryos the prospective mesoderm is induced initially with domains of dorsal and ventral fate, then further patterned to generate somitic mesoderm by signals from the gastrula organizer. Although Xwnt-8 is expressed in future ventrolateral mesoderm and induces prospective epidermis to differentiate in vitro as ventral mesoderm, no loss-of-function studies have demonstrated a requirement for Wnt signaling for the normal expression of mesodermal genes in the gastrula. We report development of a dominant-negative Wnt (dnXwnt-8) that inhibits embryonic responses to Wnt signaling in a cell-nonautonomous fashion. By expressing dnXwnt-8 in Xenopus embryos, we uncover a requirement of Wnt signaling for localized expression in prospective mesoderm of XMyoDa and Xenopus-posterior (Xpo). Because ectopic expression of functional Xwnt-8 in the dorsal marginal zone of the gastrula induces ectopic XMyoDa and Xpo, both gain-of-function and loss-of-function experiments support a model in which endogenous Xwnt-8 functions to induce expression of genes involved in specification of ventral and somitic mesoderm.

The Wnt/β-catenin pathway is a conserved cell-cell signalling mechanism in animals that regulates gene expression via TCF/LEF DNA-binding factors to coordinate many cellular processes. Vertebrates normally have four Tcf/Lef genes, which, through alternative splicing and alternative promoter use give rise to a variety of TCF/LEF isoforms. Recent evidence from several experimental systems suggests that this diversity of TCF/LEF factors is functionally important in vertebrates for mediating tissue- and stage-specific Wnt regulation in embryonic development, stem cell differentiation and associated diseases, such as cancer.