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.