Evidence for Dual Mechanisms of Mesoderm Establishment in Xenopus Embryos

In Xenopus embryos, the maternally encoded transcription factor VegT (also known as Xombi, Antipodean, Brat, and Xtbx6) is essential for normal endoderm and mesoderm formation. This finding and the localization of VegT mRNA in the vegetal hemisphere of the oocyte are consistent with several models of germ layer patterning. Specific models have been proposed in which (1) combinations of cytoplasmic determinants, (2) inductive signals, or (3) intracellular concentration (morphogen effects) predominate. We test predictions of these models. We show that contrary to previous proposals, FGF does not suppress endoderm formation and so cannot be an anti-endoderm mesodermal determinant. We further show that, at the right dose, VegT can induce mesodermal marker expression cell autonomously and that it induces mesoderm at concentrations below those that induce endoderm. These results are consistent with a dual mechanism of mesoderm establishment in which both VegT-initiated inductive signals and an intracellular VegT morphogen gradient play a part.