X-TWI IS EXPRESSED PRIOR TO GASTRULATION IN PRESUMPTIVE NEURECTODERMAL AND MESODERMAL CELLS IN DORSALIZED AND VENTRALIZED XENOPUS-LAEVIS EMBRYOS

Early X-twi expression has been now investigated from egg laying to th e early neurulation stages in Xenopus embryos, using both in situ hydr idization and the more sensitive techniques of RT-PCR. We show that in unfertilized eggs, a decreasing gradient of X-twi transcript distribu tion is observed from animal to vegetative caps. X-twi RNA can be weak ly detected at stages prior to gastrulation, and with increased intens ity from stage 8 onwards. At blastula, X-twi transcripts are located t owards the animal pole, and as gastrulation begins, they are detected in the developing axial mesoderm and then they accumulate in the senso rial layer of the neurectoderm, the mesodermal layer and in neural cre st cells up to late neurula stages. We show, in addition, that in lith ium-chloride- and UV-treated Xenopus embryos (that are respectively bo th ''anteriorized/dorsalized'' and in ''posteriorized/ventralized''), X-twi RNA is detected in cells in similar positions to those that expr ess X-twi in normal embryos. As a whole, our results show that X-twi i s expressed even when regionalization of the mesoderm is disturbed and raises the question of a putative function of X-twi prior to gastrula tion.