Animal and vegetal pole cells of early Xenopus embryos respond differently to maternal dorsal determinants: implications for the patterning of the organiser

The maternal dorsal determinants required for the specification of the dorsal territories of Xenopus early gastrulae are located at the vegetal pole of unfertilised eggs and are moved towards the prospective dorsal region of the fertilised egg during cortical rotation. While the molecular identity of the determinants is unknown, there are dorsal factors in the vegetal cortical cytoplasm (VCC). Here, we show that the VCC factors, when injected into animal cells activate the zygotic genes Siamois and Xnr3, suggesting that they act along the Wnt/β-catenin pathways. In addition, Siamois and Xnr3 are activated and at the vegetal pole of UV-irradiated embryos, indicating that these two genes are targets of the VCC factors in all embyronic cells. However, the consequences of their activation in cells that occupy different positions along the animal-vegetal axis differ. Dorsal vegetal cells of normal embryos or VCC-treated injected animal cells are able to dorsalise ventral mesoderm in conjugate experiments but UV-treated vegetal caps do not have this property. We propose that the regionalisation of the organiser during gastrulation proceeds from the differential interpretation along the animal-vegetal axis of the activation of the VCC/β-catenin/Siamois pathway. (author)