Distribution of dorsal-forming activity in precleavage embryos of the Japanese newt, Cynops pyrrhogaster: Effects of deletion of vegetal cytoplasm, UV irradiation, and lithium treatment

Two types of axis-deficient embryos developed after deletion of the vegetal cytoplasm: wasp-shaped embryos and permanent-blastula-type embryos. In sit u hybridization revealed that neither type of axis-deficient embryo express ed goosecoid or pax-6, brachyury was expressed in the constricted waist reg ion of the wasp shaped embryos but was not expressed in the permanent-blast ula-type embryos. Further, we examined the effect of UV irradiation on Japa nese newt embryos. Surprisingly, UV-irradiated Japanese newt eggs formed hy perdorsalized embryos. These embryos gastrulated in an irregular circular f ashion with goosecoid expression in the circular equatorial region. At tail bud stage, these embryos formed a proboscis which is very reminiscent of th at formed in hyperdorsalized Xenopus embryos. Transplantation of the margin al region of the UV irradiated embryos revealed that the entire marginal zo ne had organizer activity. Thus we conclude that UV hyperdorsalizes Japanes e newt embryos, finally, lithium treatment of normal embryos at the 32-cell stage also resulted in hyperdorsalization. Lithium treatment of vegetally deleted embryos had two distinct results. Lithium treatment of permanent-bl astula-type embryos did not result in the formation of dorsal axial structu res, while the same treatment reinduced gastrulation and dorsal axis format ion in the wasp-shaped embryos. Based on these results, we propose a model for early axis specification in Japanese newt embryos. The model presented here is fundamentally identical to the Xenopus model, with some important m odifications. The vegetally located determinants required for dorsal develo pment (dorsal determinants, DDs) are distributed over a wider region at fer tilization in Japanese newt embryos than in Xenopus embryos. The marginal r egion of the Japanese newt embryo at the beginning of development overlaps with the held of the DDs. Gastrulation is very likely to be a dorsal margin al-specific property, while self-constriction is most probably a ventral ma rginal-specific property in Japanese newt embryos, (C) 2000 Academic Press.