THE XENOPUS-LAEVIS HOMEOBOX GENE XGBX-2 IS AN EARLY MARKER OF ANTEROPOSTERIOR PATTERNING IN THE ECTODERM

In a search for homeobox genes expressed during early Xenopus developm ent, we have isolated a gene which appears to be the Xenopus cognate o f the mouse Gbx-2 gene. Expression of Xgbx-2 is first detectable by in situ hybridization at the midgastrula stage when it is predominantly expressed in the dorsolateral ectoderm, with a gap in expression at th e dorsal midline. By the end of gastrulation and during neurulation, X gbx-2 is expressed dorsolaterally in the neural ectoderm and laterally and ventrally in the epidermis with sharp anterior expression borders in both tissues. The anteriormost expression in the neural ectoderm p ersists throughout the early stages of development, and was mapped to the region of rhombomere I, with an anterior expression border in the region of the midbrain-hindbrain boundary. Thus Xgbx-2 is expressed an terior to the Hox genes. Xgbx-2 expression is induced by retinoic acid (RA) in animal caps, and RA treatment of whole embryos expands and en hances Xgbx-2 expression in the ectoderm. We suggest a role for Xgbx-2 in establishing the midbrain-hindbrain boundary, which appears to sep arate early neurectodermal regions expressing genes that are positivel y and negatively regulated by RA.