EXPRESSION CLONING OF SIAMOIS, A XENOPUS HOMEOBOX GENE EXPRESSED IN DORSAL VEGETAL CELLS OF BLASTULAS AND ABLE TO INDUCE A COMPLETE SECONDARY AXIS

Using an expression cloning strategy that relies on a functional assay , we have cloned a novel Xenopus homeobox-containing gene, Siamois. Em bryos injected in a ventral-vegetal blastomere with as little as 5 pg of Siamois mRNA develop a complete secondary axis, but the progeny of the injected cells do not participate in the secondary axis formation. In normal development, Siamois mRNA is first detected shortly after t he midblastula transition, which is earlier than mRNAs for goosecoid o r Xbrachyury, and is present most abundantly in the dorsal endoderm of early gastrulae. The activation of this gene can be obtained cell aut onomously in dispersed embryo cells. These results indicate that Siamo is may play an important role in the formation of the Nieuwkoop center .