FROM MOUSE TO FROGS - IDENTIFICATION AND FUNCTIONAL ANALYSES OF GENESREQUIRED FOR INDUCTION AND PATTERNING OF THE MESODERM

Many genes have been identified in Xenopus embryos that are believed t o be involved in the induction and patterning of the embryonic mesoder m, these genes frequently proving to have homologues in other vertebra te embryos. In some cases, however, the true importance of these genes in normal development has been questioned because mouse embryos in wh ich the gene has been disrupted by homologous recombination develop ap parently normally or with only minor defects. To overcome such ambigui ties, and at the same time to take advantage of the ease of manipulati on 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, h as a similar role, presumably by helping interpret the inductive signa l, and here it is particularly intriguing that the type of mesoderm in duced depends on the concentration of Xbra.