FGF8, Wnt8 and Myf5 are target genes of Tbx6 during anteroposterior specification in Xenopus embryo.

The T-box transcription factor Tbx6 is required for somite formation and loss-of-function or reduced activity of Tbx6 result in absence of posterior paraxial mesoderm or disorganized somites, but how it is involved in a regulatory hierarchy during Xenopus early development is not clear. We show here that Tbx6 is expressed in the lateral and ventral mesoderm of early gastrula, and it is necessary and sufficient to directly and indirectly regulate the expression of a subset of early mesodermal and endodermal genes. Ectopic expression of Tbx6 inhibits early neuroectodermal gene expression by strongly inducing the expression of posterior mesodermal genes, and expands the mesoderm territory at the expense of neuroectoderm. Conversely, overexpression of a dominant negative Tbx6 mutant in the ventral mesoderm inhibits the expression of several mesodermal genes and results in neural induction in a dose-dependent manner. Using a hormone-inducible form of Tbx6, we have identified FGF8, Xwnt8 and XMyf5 as immediate early responsive genes of Tbx6, and the induction of these genes by Tbx6 is independent of Xbra and VegT. These target genes act downstream and mediate the function of Tbx6 in anteroposterior specification. Our results therefore identify a regulatory cascade governed by Tbx6 in the specification of posterior mesoderm during Xenopus early development.