The homeobox gene Siamois is a target of the Wnt dorsalisation pathway and triggers organiser activity in the absence of mesoderm.

Siamois, a Xenopus zygotic homeobox gene with strong dorsalising activity, is expressed in the dorsal-vegetal organiser known as the Nieuwkoop centre. We show that, in contrast to Spemann organiser genes such as goosecoid, chordin and noggin, Siamois gene expression is not induced following overexpression of mesoderm inducers in ectodermal (animal cap) cells. However, Siamois is induced by overexpressing a dorsalising Wnt molecule. Furthermore, like Wnt, Siamois can dorsalise ventral mesoderm and cooperate with Xbrachyury to generate dorsal mesoderm. These results suggest that Siamois is a mediator of the Wnt-signalling pathway and that the synergy between the Wnt and mesoderm induction pathways occurs downstream of the early target genes of these two pathways. Overexpression of Siamois in animal cap cells reveals that this gene can act in a non vegetal or mesodermal context. We show the following. (1) Animal cap cells overexpressing Siamois secrete a factor able to dorsalise ventral gastrula mesoderm in tissue combination experiments. (2) The Spemann organiser-specific genes goosecoid, Xnr-3 and chordin, but not Xlim.1, are activated in these caps while the ventralising gene Bmp-4 is repressed. However, the dorsalising activity of Siamois-expressing animal caps is significantly different from that of noggin- or chordin-expressing animal caps, suggesting the existence of other dorsalising signals in the embryo. (3) Ectodermal cells overexpressing Siamois secrete a neuralising signal and can differentiate into cement gland and, to a lesser extent, into neural tissue. Hence, in the absence of mesoderm induction, overexpression of Siamois is sufficient to confer organiser properties on embryonic cells.