From mouse to frogs: Identification and functional analyses of genes required for induction and patterning of the mesoderm

Many genes have been identified in Xenopus embryos that are believed to be involved in the induction and patterning of the embryonic mesoderm, these genes frequently proving to have homologues in other vertebrate embryos. In some cases, however, the true importance of these genes in normal development has been questioned because mouse embryos in which the gene has been disrupted by homologous recombination develop apparently normally or with only minor defects. To overcome such ambiguities, and at the same time to take advantage of the ease of manipulation of the Xenopus embryo, we have studied the Xenopus cognates of two mouse genes, nodal (Xnr1-3) and Brachyury (Xbra). The three Xnr genes, members of the TGF beta group of signaling molecules, all play roles in inducing mesoderm patterning, while Xbra, a transcription factor, has a similar role, presumably by helping interpret the inductive signal, and here it is particularly intriguing that the type of mesoderm induced depends on the concentration of Xbra.