MOLECULAR-IDENTIFICATION OF SPADETAIL - REGULATION OF ZEBRAFISH TRUNKAND TAIL MESODERM FORMATION BY T-BOX GENES

Inhibition of fibroblast growth factor (FGF) signaling prevents trunk and tail formation in Xenopus and zebrafish embryos. While the T-box t ranscription factor Brachyury (called No Tail in zebrafish) is a key m ediator of FGF signaling in the notochord and tail, the pathways activ ated by FGF in non-notochordal trunk mesoderm have been uncertain. Pre vious studies have shown that the spadetail gene is required for non-n otochordal trunk mesoderm formation; spadetail mutant embryos have maj or trunk mesoderm deficiencies, but relatively normal tail and notocho rd development. We demonstrate encodes a T-box transcription factor wi th homologues in Xenopus and chick. Spadetail is likely to be a key me diator of FGF signaling in trunk non-notochordal mesoderm, since spade tail expression is regulated by FGF signaling. Trunk and tail developm ent are therefore dependent upon the complementary actions of two T-bo x genes, spadetail and no fail. We show that the regulatory hierarchy among spadetail, no tail and a third T-box gene, tbx6, are substantial ly different during trunk and tail mesoderm formation, and propose a g enetic model that accounts for the regional phenotypes of spadetail an d no fail mutants.