DORSAL DETERMINANTS IN THE XENOPUS EGG ARE FIRMLY ASSOCIATED WITH THEVEGETAL CORTEX AND BEHAVE LIKE ACTIVATORS OF THE WNT PATHWAY

The Xenopus egg contains maternal dorsal determinants that are specifi cally located at the vegetal cortex. To study physical and functional properties of the dorsal determinants, we took advantage of the animal -vegetal reversed embryo. The animal-vegetal reversed embryo is produc ed by inversion of the fertilized egg, which results in formation of e ctoderm and endoderm from the unpigmented and the pigmented halves, re spectively [Neff ef al. (1983). Dev. Biol. 97, 103-112; Black and Gerh art (1985). Dev. Biol. 108, 310-324]. We demonstrated by cytoplasmic t ransplantation that the dorsal activity was specifically localized to the unpigmented cortical cytoplasm of the inverted egg, which is segre gated into the future ectodermal lineage. This result suggests that th e dorsal determinants are associated with the unpigmented cortex and a re not dislodged by the inversion. In addition, we found that two vege tally localized transcripts, Xcat2 and Vg1 mRNAs, were present in the reversed animal pole of the inverted egg, suggesting their association with the unpigmented cortex. In order to compare the dorsal determina nt activity with known dorsalizing molecules, we examined the expressi on pattern of Xnr3 and Siamois in the reversed embryo because these tw o genes are activated by the Wnt-pathway activators (Xwnt-8, beta-cate nin, etc.) but not by other dorsalizing molecules (noggin, BVg1, etc.) . Animal cap of the reversed embryo, which received the unpigmented co rtex of the egg, expressed Xnr3 and Siamois. However, Mix. 1, a marker expressed in endoderm and mesoderm in the normal embryo in response t o mesodermal inducers, was not detected in the animal cap of the rever sed embryo. In addition, we found that beta-catenin protein accumulate d in nuclei of unpigmented animal pole cells of the reversed embryo. T hese results suggest that the maternal dorsal determinants behave more similarly to the Wnt-pathway activators than noggin or BVg1. (C) 1997 Academic Press.