Mode of action of VegT in mesoderm and endoderm formation

mRNA encoding the T-box transcription factor VegT is located throughout the vegetal pole of the Xenopus egg and is believed to play an important part in endoderm and mesoderm formation. We find that VegT generates endoderm bo th by cell-autonomous action ana by generating TGF-beta family signals, the latter being entirely responsible for its mesoderm-inducing activity Signa lling molecules induced cell-autonomously by VegT include derriere, Xnr4 an d activin B. Xnr1 and Xnr2 are also induced, but primarily in a non-autonom ous manner. All of these signalling molecules are found in the blastula and gastrula vegetal pole and induce both endoderm and mesoderm in the animal cap assay, and hence are good candidates both for the endogenous zygotic me soderm-inducing signal and for reinforcing the vegetal expression of endode rm markers.