Mesoderm induction in Xenopus is a zygotic event regulated by maternal VegT via TGF beta growth factors

The maternal transcription factor VegT is important for establishing the pr imary germ layers in Xenopus. In previous work, we showed that the vegetal masses of embryos lacking maternal VegT do not produce mesoderm-inducing si gnals and that mesoderm formation in these embryos occurred ectopically, fr om the vegetal area rather than the equatorial zone of the blastula, Here w e have increased the efficiency of the depletion of maternal VegT mRNA and have studied the effects on mesoderm formation. We find that maternal VegT is required for the formation of 90% of mesodermal tissue, as measured by t he expression of mesodermal markers MyoD, cardiac actin, Xbra, Xwnt8 and al phaT4 globin, Furthermore, the transcription of FGFs and TGF beta s, Xnr1, Xnr2, Xnr4 and derriere does not occur in VegT-depleted embryos. We test wh ether these growth factors may be endogenous factors in mesoderm induction, by studying their ability to rescue the phenotype of VegT-depleted embryos , when their expression is restricted to the vegetal mass. We find that Xnr 1, Xnr2, Xnr4 and derriere mRNA all rescue mesoderm formation, as well as t he formation of blastopores and the wild-type body axis. Derriere rescues t runk and tail while nr1, nr2 and nr4 rescue head, trunk and tail. We conclu de that mesoderm induction in Xenopus depends on a maternal transcription f actor regulating these zygotic growth factors.