2 NOVEL CHICK T-BOX GENES RELATED TO MOUSE BRACHYURY ARE EXPRESSED INDIFFERENT, NONOVERLAPPING MESODERMAL DOMAINS DURING GASTRULATION

The mouse Brachyury (T) gene plays critical roles in the genesis of no rmal mesoderm during gastrulation and in the maintenance of a function ing notochord. Abrogation of Brachyury (T) expression within the chord amesoderm of homozygous null mutants nevertheless spares anterior axis formation. An intriguing possibility to explain the preservation of a nterior axis formation in these mutants would be the existence of othe r genes compensating for the loss of Brachyury. This compensation and the recent demonstration that Brachyury is the prototype for an evolut ionarily conserved family, prompted a search for other T-box genes par ticipating in axis formation. The chick Brachyury orthologue and two r elated chick T-box genes that are expressed at the onset of gastrulati on have been isolated. One of these novel genes (Ch-TbxT) becomes rest ricted to the axial mesoderm lineage and is a potential candidate for complementing or extending Brachyury function in the anterior axis (fo rmation of the head process, prechordal plate). The other gene (Ch-Tbx 6L), together with chick T, appears to mark primitive streak progenito rs before gastrulation. As cells leave the primitive streak, Ch-Tbx6L becomes restricted to the early paraxial mesoderm lineage and could pl ay a role in regulating somitogenesis.