PRE-MBT PATTERNING OF EARLY GENE-REGULATION IN XENOPUS - THE ROLE OF THE CORTICAL ROTATION AND MESODERM INDUCTION

Patterning events that occur before the mid-blastula transition (MBT) and that organize the spatial pattern of gene expression in the animal hemisphere have been analyzed in Xenopus embryos. We present evidence that genes that play a role in dorsoventral specification display dif ferent modes of activation. Using early blastomere explants (16-128-ce ll stage) cultured until gastrula stages, we demonstrate by RT-PCR ana lysis that the expression of goosecoid (gsc), wnt-8 and brachyury (bm) is dependent on mesoderm induction. In contrast, nodal-related 3 (nr3 ) and siamois (sin) are expressed in a manner that is independent of m esoderm induction, however their spatially correct activation does req uire cortical rotation. The pattern of sin and nr3 expression reveals that the animal half of the 16-cell embryo is already distinctly polar ized along the dorsoventral axis as a result of rearrangement of the e gg structure during cortical rotation. Similar to the antagonistic act ivity between the ventral and the dorsal mesoderm, the ventral animal blastomeres can attenuate the expression of nr3 and sin in dorsal anim al blastomeres. Our data suggest that no Nieuwkoop center activity at the blastula stage is required for the activation of nr3 and sin in vi vo. (C) 1998 Elsevier Science Ireland Ltd.