EARLY DEVELOPMENTAL EXPRESSION AND EXPERIMENTAL AXIS DETERMINATION BYTHE CHICKEN VG1 GENE

Background: Genes of the transforming growth factor beta (TGF beta) su perfamily have been implicated in the earliest steps of developmental patterning in vertebrates. In Xenopus, the Vg1 gene is a candidate for the initiator of axis formation: its RNA and protein are broadly but appropriately localized at the start of development, and processed Vg1 protein is a powerful inducer of organized axial tissue in blastular animal caps in vitro and when locally produced in vivo after injection of Vg1 mRNA into blastomeres. Site-specific proteolytic processing oc curs ubiquitously for most TGF beta members, producing the active pept ide ligand, but is tightly restricted, by unknown mechanisms, for endo genous Vg protein in Xenopus and zebrafish embryos. This restriction m ay be involved in the spatial localization of activity required for an organizing role. Results: We have characterized an amniote (chick) or thologue of Vg1, cVg1, and examined its developmental expression. The early expression of cVg1 includes a phase broadly related to the known time and site of axis (primitive streak) initiation; the initial tran scription of cVg1 is centred in the posterior marginal zone (PMZ), a r egion of the blastoderm known to contain the axial organizing activity at this stage. We also observed later neural and paraxial mesodermal expression of cVg1, which has not been described previously for Vg hom ologues in other vertebrates. We have grafted transfected COS cells, p roducing processed cVg1 protein, to peripheral positions around the ch ick early blastoderm. Such grafts initiate formation of morphologicall y complete primitive streaks, simulating the properties of grafts from the PMZ. Conclusions: In vertebrate development, Vg genes may be requ ired for an evolutionarily conserved early step in positioning or indu ction of the axis.