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 endode rm and mesoderm formation. This finding and the localization of VegT mRNA i n the vegetal hemisphere of the oocyte are consistent with several models o f germ layer patterning. Specific models have been proposed in which (1) co mbinations of cytoplasmic determinants, (2) inductive signals, or (3) intra cellular 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 d eterminant. We further show that, at the right dose, VegT can induce mesode rmal marker expression cell autonomously and that it induces mesoderm at co ncentrations below those that induce endoderm, These results are consistent with a dual mechanism of mesoderm establishment in which both VegT-initiat ed inductive signals and an intracellular VegT morphogen gradient play a pa rt. (C) 2000 Wiley-Liss, Inc.