SPECIFICATION OF MESODERMAL PATTERN IN XENOPUS-LAEVIS BY INTERACTIONSBETWEEN BRACHYURY, NOGGIN AND XWNT-8

We demonstrate that the nuclear, sequence-specific DNA-binding protein Xbra causes dorsal mesodermal differentiation of animal cap ectoderm when co-expressed with the secreted proteins noggin and Xwnt-8. None o f these molecules causes dorsal mesoderm formation when expressed alon e. Co-expression of Xenopus Brachyury (Xbra) mRNA with noggin mRNA in animal caps specifies the main dorsal tissues, namely muscle, notochor d and neural tissue. Co-expression of Xbra with Xwnt-8, in contrast, c onverts animal caps to muscle masses. We have previously shown that ex pression of Xbra alone in animal caps is sufficient to specify ventral mesoderm which expresses Xhox3 and low levels of muscle-specific acti n. We now conclude that the putative transcription factor Xbra defines a cell state in the vertebrate embryo which can respond to diffusible dorsal signals such as noggin and Xwnt-8, resulting in dorsal mesoder mal differentiation. In the absence of such dorsal signals Xbra causes ventral mesodermal differentiation. This state is only partially main tained after the mid-blastula transition as it permits the dorsal resp onse to zygotically expressed noggin but it does not allow a dorsal re sponse to zygotically expressed Xwnt-8, which elicits only ventral mes odermal differentiation.