Patterning the early zebrafish by the opposing actions of bozozok and vox/vent

Fish and hog embryos are patterned along the dorsal-ventral axis during the gastrula stage by opposing gradients of Bmps and Bmp inhibitory proteins. Three transcriptional repressors with partially overlapping expression doma ins have been proposed to be important mediators of Bmp function in Xenopus . We find that two related factors are expressed in the early zebrafish emb ryo. Although these factors are considerably divergent from the related Xen opus genes, they are expressed in domains similar to those of their Xenopus relatives throughout embryogenesis. Both of the zebrafish genes, which we have named vex and vent, are potent ventralizing factors in both zebrafish and Xenopus embryos. Using mutants in the Bmp pathway, we find that there a re Bmp-dependent and Bmp-independent domains of vex expression, whereas ven t is mostly dependent upon Bmp signaling. We show that ectopic vex or vent negatively regulates expression of the early dorsal gene bozozok (boz) and that ectopic boz eliminates vex and vent expression. Moreover, the normal e xclusion of vex and vent from the organizer region is lost in boz mutant em bryos. Our results show that boz and vox/vent are mutually antagonistic and indicate that the early establishment of the size of the organizer domain is dependent on an interplay between these early expressed transcriptional repressors. (C) 2000 Academic Press.