A maternal dorsoventral prepattern revealed by an asymmetric distribution of ventralizing molecules before fertilization in Xenopus laevis

Abstract

The establishment of the embryonic dorsoventral axis in Xenopus occurs whenthe radial symmetry around the egg’s animal-vegetal axis is broken to give rise tothe typical symmetry of Bilaterians. We have previously shown that theNotch1 protein is ventrally enriched during early embryogenesis in Xenopuslaevis and zebrafish and exerts ventralizing activity through β-Catenindestabilization and the positive regulation of ventral center genes in X. laevis.These findings led us to further investigate when these asymmetries arise. In thiswork, we show that the asymmetrical distribution of Notch1 protein and mRNAprecedes cortical rotation and even fertilization in X. laevis. Moreover, we foundthat in unfertilized eggs transcripts encoded by the ventralizing gene bmp4 arealso asymmetrically distributed in the animal hemisphere and notch1 transcriptsaccumulate consistently on the same side of the eccentric maturation point.Strikingly, a Notch1 asymmetry orthogonal to the animal-vegetal axis appearsduring X. laevis oogenesis. Thus, we show for the first time a maternal bias in thedistribution of molecules that are later involved in ventral patterning duringembryonic axialization, strongly supporting the hypothesis of a dorsoventralprepattern or intrinsic bilaterality of Xenopus eggs before fertilization.