Vg1 is an essential signaling molecule in Xenopus development

Xenopus Vg1, a transforming growth factor β (Tgfβ) family member, was one of the first maternally localized mRNAs identified in vertebrates. Its restriction to the vegetal pole of the egg made it the ideal candidate to be the mesoderm-inducing signal released by vegetal cells, but its function in vivo has never been resolved. We show that Vg1 is essential for Xenopus embryonic development, and is required for mesoderm induction and for the expression of several key Bmp antagonists. Although the original Vg1 transcript does not rescue Vg1-depleted embryos, we report that a second allele is effective. This work resolves the mystery of Vg1 function, and shows it to be an essential maternal regulator of embryonic patterning.