Dorsal/Ventral asymmetric expression of nodal in the early sea urchin embryo relies on specific suppression in dorsal cells by the Hbox12 homeodomain-containing regulator

Dorsal/Ventral (DV) polarization of the sea urchin embryo is directed by a Nodal-expressing signalingcentre located on the ventral side. The initial breaking of the symmetry and positioning of theorganizer are unclear. We show that, in Paracentrotus lividus embryos, the Hbox12 homeodomaincontainingrepressor is expressed on the opposite side and precedes the onset of nodal transcription.Hbox12 misexpression provokes DV abnormalities, attenuating nodal and nodal-dependent genetranscription. Reciprocally, clonal hbox12 loss-of-function imposed by blastomere transplantation orgene transfer assays disrupts DV polarization and allows ectopic expression of nodal. Remarkably,the localized knock-down of nodal restores DV polarity of embryos lacking hbox12 function. Finally,we show that Hbox12 is a dorsal-specific negative modulator of the p38 MAPK activity, which isknown to be required for nodal expression. Altogether, our results indicate that hbox12 functions atthe top of the symmetry-breaking sequence of events within the rising DV gene regulatory network.