Conservation of the Developmental Role ofBrachyuryin Notochord Formation in a Urochordate, the AscidianHalocynthia roretzi

AbstractThe notochord is one of the characteristic features of the phylum Chordata. The vertebrateBrachyurygene is known to be essential for the terminal differentiation of chordamesoderm into notochord. In the ascidian, which belongs to the subphylum Urochordata, differentiation of notochord cells is induced at the late phase of the 32-cell stage through cellular interaction with adjacent endoderm cells as well as neighboring notochord cells. The ascidianBrachyurygene (As-T) is expressed exclusively in the notochord-lineage blastomeres, and the timing of gene expression at the 64-cell stage precisely coincides with that of the developmental fate restriction of the blastomeres. In addition, experimental studies have demonstrated a close relationship between the inductive events andAs-Texpression. In the present study, we show that overexpression ofAs-Tby microinjection of the synthesizedAs-TRNA results in the occurrence, without the induction, of notochord-specific features in the A-line presumptive notochord blastomeres. We also show that overexpression ofAs-TRNA leads to ectopic expression of notochord-specific features in non-notochord lineages, including those of spinal cord and endoderm. These results strongly suggest that the developmental role of theBrachyuryis conserved throughout chordates in notochord formation