Brachyury (T) expression in embryos of a larvacean urochordate, Oikopleura dioica, and the ancestral role of T.
The Brachyury, or T, gene is required for notochord development in animals occupying all three chordate subphyla and probably also had this role in the last common ancestor of the chordate lineages. In two chordate subphyla (vertebrates and cephalochordates), T is also expressed during gastrulation in involuting endodermal and mesodermal cells, and in vertebrates at least, this expression domain is required for proper development. In the basally diverging chordate subphylum Urochordata, animals in the class Ascidiacea do not employ T during gastrulation in endodermal or nonaxial mesodermal cells, and it has been suggested that nonnotochordal roles for T were acquired in the cephalochordate-vertebrate lineage after it split with Urochordata. To test this hypothesis, we cloned T from Oikopleura dioica, a member of the urochordate class Appendicularia (or Larvacea), which diverged basally in the subphylum. Investigation of the expression pattern in developing Oikopleura embryos showed early expression in presumptive notochord precursor cells, in the notochord, and in parts of the developing gut and cells of the endodermal strand. We conclude that the ancestral role of T likely included expression in the developing gut and became necessary in chordates for construction of the notochord.