ANIMAL AND VEGETAL POLE CELLS OF EARLY XENOPUS EMBRYOS RESPOND DIFFERENTLY TO MATERNAL DORSAL DETERMINANTS - IMPLICATIONS FOR THE PATTERNING OF THE ORGANIZER

The maternal dorsal determinants required for the specification of the dorsal territories of Xenopus early gastrulae are located at the vege tal pole of unfertilised eggs and are moved towards the prospective do rsal region of the fertilised egg during cortical rotation, While the molecular identity of the determinants is unknown, there are dorsal fa ctors in the vegetal cortical cytoplasm (VCC). Here, we show that the VCC factors, when injected into animal cells activate the zygotic gene s Siamois and Xnr3, suggesting that they act along the Wnt/beta-cateni n pathway, In addition, Siamois and Xnr3 are activated at the vegetal pole of UV-irradiated embryos, indicating that these two genes are tar gets of the VCC factors in all embryonic cells, However, the consequen ces of their activation in cells that occupy different positions along the animal-vegetal axis differ, Dorsal vegetal cells of normal embryo s or VCC-treated injected animal cells are able to dorsalise ventral m esoderm in conjugate experiments but UV-treated vegetal caps do not ha ve this property, This difference is unlikely to reflect different lev els of activation of FGF or activin-like signal transduction pathways but may reflect the activation of different targets of Siamois. Chordi n, a marker of the head and axial mesoderm, is activated by the VCC/Si amois pathway in animal cells but not in vegetal cells whereas cerberu s, a marker of the anterior mesendoderm which lacks dorsalising activi ty, can only be activated by the VCC/Siamois pathway in vegetal cells, We propose that the regionalisation of the organiser during gastrulat ion proceeds from the differential interpretation along the animal-veg etal axis of the activation of the VCC/beta-catenin/Siamois pathway.