Molecular asymmetries in the animal-vegetal axis of the Xenopus oocyte are well known to regulate the formation of gametes and germ layers. Likewise, many transplantation and explant studies demonstrate that maternal dorsalizing activities are localized to the future dorsal side of the embryo after fertilization, but to date only a few of the molecules involved in this process have been shown to be asymmetrically distributed. In this report, we identify two new aspects of the maternal regulation of dorsal-ventral fate asymmetry in Xenopus blastomeres: cytoplasmic polyadenylation of dorsal maternal mRNAs and localized Wnt8b signaling. Previous studies demonstrated that there are maternal, dorsal axis-inducing RNAs localized to dorsal animal blastomeres that become activated between the 8- and 16-cell stage (Hainski and Moody [1992] Development 116:347-355; Hainski and Moody [1996] Dev. Genet. 19:210-221). We report herein that the activation of these axis-inducing dorsal mRNAs is regulated by cytoplasmic polyadenylation. We also show that maternal wnt8b mRNA is concentrated in ventral animal blastomeres. These ventral cells and exogenous Wnt8b both inhibit the dorsal fate of neighboring blastomeres in culture, indicating that a maternal Wnt signal also contributes to segregating dorsal and ventral fates.
