Tbx6,aBrachyury-Related Gene Expressed by Ventral Mesendodermal Precursors in the Zebrafish Embryo

Abstract

Classical embryology experiments have indicated the existence of dorsal-type and ventral-type mesoderms that arise as a consequence of mesoderm induction during vertebrate development. Here we report that the zebrafishtbx6gene, a member of theBrachyury-related T-box family of genes, is exclusively expressed by ventral mesendoderm. Three observations link the expression oftbx6to ventral mesoderm specification. First, the gene is initially expressed at the onset of gastrulation within a ventrolateral subpopulation of cells that express the pan-mesodermal gene,no tail(Brachyury). Second, the mesoderm-inducing factors activin and bFGF activatetbx6expression in animal caps. Third, dorsalization of the mesendodermal precursor population following exposure of embryos to lithium ions causes down-regulation oftbx6transcription.tbx6is expressed transiently in the involuting derivatives of the ventral mesendoderm, which give rise to nonaxial mesodermal tissues; its expression is extinguished as tissue differentiation progresses. Transcription oftbx6commences about an hour after initiation of expression of the pan-mesendodermal geneno tailand the organizer genegoosecoid.The dependence oftbx6expression onno tailactivity was examined inno tailmutant embryos. The activation oftbx6transcription in ventral mesoderm does not depend onno tailgene activity. However,no tailappears to contribute to the maintenance of normal levels oftbx6transcription and may be required fortbx6transcription in the developing tail.