XSOX17-ALPHA AND XSOX17-BETA MEDIATE ENDODERM FORMATION IN XENOPUS

We have isolated two Xenopus relatives of murine Sox17 expressed in ga strula presumptive endoderm. Xsox17 alpha and -beta expression can be induced in animal caps by activin, but not by FGF. Ectopic expression of these genes in animal caps induces the expression of endoderm marke rs; this induction is blocked by overexpression of a fusion of the Xso x17 beta HMG domain to the Drosophila Engrailed repressor domain, as i s induction of endoderm markers by activin and the expression of endod ermal markers in whole embryos and isolated vegetal poles. These exper iments, as well as the effects of the mRNAs on embryo phenotypes, sugg est that the Xsox17 genes mediate an activin-induced endoderm differen tiation pathway in animal caps and are involved in normal endoderm dif ferentiation in embryos.