Expression cloning of a Xenopus T-related gene (Xombi) involved in mesodermal patterning and blastopore lip formation.

We have used a functional assay to identify a putative T-box transcription factor (Xombi) that has the ability to induce sites of invagination in the ectoderm of Xenopus embryos that resemble the blastopore lip. Maternal Xombi transcript is first localized to the oocyte's vegetal cortex and cytoplasm, early sources of mesoderm and endoderm-inducing signals. Soon after zygotic transcription begins, there is a wave of Xombi expression, beginning in dorsal mesoderm and then extending to lateral and ventral mesoderm, that precedes and parallels blastopore lip formation at the border between the mesoderm and endoderm. Transcripts encoding brachyury, Xwnt8 and goosecoid colocalize with Xombi transcripts within the marginal zone; ectopic expression of Xombi induces expression of all three mesodermal genes. In ectodermal explants, Xombi expression is induced by the secreted mesoderm inducers activinA, activinB, Xnrl and eFGF, and by brachyury, another Xenopus T-box containing gene. The time course and location of Xombi expression, its biological activities and the partial dependence of Xombi expression and blastopore lip formation on fibroblast growth factor (FGF) signaling suggest that Xombi contributes to a traveling wave of morphogenesis and differentiation during Xenopus gastrulation.