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 transcrip tion factor (Xombi) that has the ability to induce sites of invaginati on 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 and then extending to lateral an d ventral that and parallels blastopore lip precedes border between th e mesoderm and endoderm. Transcripts encoding brachyury, Xwnt8 and goo secoid colocalize with Xombi transcripts within the marginal zone; ect opic expression of Xombi induces expression of all three mesodermal ge nes. In ectodermal explants, Xombi expression is induced by the secret ed mesoderm inducers activinA, activinB, Xnr1 and eFGF, and by brachyu ry, another Xenopus T-box containing gene. The time course and locatio n of Xombi expression, its biological activities and the partial depen dence of Xombi expression and blastopore lip formation on fibroblast g rowth factor (FGF) signaling suggest that Xombi contributes to a trave ling wave of morphogenesis and differentiation during Xenopus gastrula tion.