ABSTRACT
XHNF1β is a homeobox-containing gene initially expressed at the blastula stage in the vegetal part of the Xenopus embryo. We investigated its early role by functional ablation, through mRNA injection of an XHNF1β/engrailed repressor fusion construct (XHNF1β/EngR). Dorsal injections of XHNF1β/EngR mRNA abolish dorsal mesoderm formation, leading to axial deficiencies; ventral injections disrupt ventral mesoderm formation without affecting axial development. XHNF1β/EngR phenotypic effects specifically depend on the DNA-binding activity of its homeodomain and are fully rescued by coinjection of XHNF1β mRNA. Vegetal injection of XHNF1β/EngR mRNA blocks the mesoderm-inducing ability of vegetal explants. Both B-Vg1 and VegT maternal determinants trigger XHNF1β expression in animal caps. XHNF1β/EngR mRNA blocks B-Vg1-mediated, but not by eFGF-mediated, mesoderm induction in animals caps. However, wild-type XHNF1β mRNA does not trigger Xbra expression in animal caps. We conclude that XHNF1β function is essential, though not sufficient, for mesoderm induction in the Xenopus embryo.
