Résumé
We have used differential display to identify genes inducible by activin and isolated a novel member of the T-box gene family that includes the Xenopus genes Xbrachyury and Eomesodermin. Here we show that this novel gene is unique within the T-box family because it is maternally expressed at a high level. Furthermore, it belongs to a rare class of maternal mRNAs in Xenopus that are localised to the vegetal hemisphere of the egg and we have therefore named it Antipodean. We show here that low amounts of Antipodean injected into ectoderm (animal cap cells) strongly induce pan mesodermal genes such as Xbrachyury and ventral mesodermal genes such as Xwnt-8. Overexpression of Antipodean generates mesoderm of ventral character, and induces muscle only weakly. This property is consistent with the observed late zygotic Antipodean mRNA expression in the posterior paraxial mesoderm and ventral blastopore, and its exclusion from the most dorsal mesodermal structure, the notochord. Antipodean is induced by several molecules of the TGF-β class, but in contrast to Xbrachyury, not by bFGF. This result suggests that the expression of these T-box genes may be under the control of different regulatory pathways. Finally, we demonstrate that Antipodean and Eomesoder-min induce each other and both are able to induce Xbrachyury. The early zygotic expression of Antipodean is not induced by Xbrachyury, though later it is to some extent. Considering its maternal content, Antipodean could initiate a cascade of T-box gene activations. The expression of these genes may, in turn, sustain each other’s expression to define and maintain the mesoderm identity in Xenopus.