The ascidian genome contains another T-domain gene that is expressed in differentiating muscle and the tip of the tail of the embryo.

The vertebrate Brachyury (T) gene is transiently expressed in nascent and migrating mesoderm, in the differentiating notochord, and in the tail bud, reflecting its independent functions. In contrast, the expression of an ascidian Brachyury gene (As-T) is restricted to differentiating notochord. The present study revealed that the genome of Halocynthia roretzi contains another T-domain gene (As-T2) which encodes a divergent T-domain protein. The transient expression of As-T2 was detected in the endoderm- and muscle-lineage blastomeres of the early embryo and the transcript was retained by involuting and differentiating muscle cells until it became undetectable by the mid-tailbud stage. In addition, As-T2 was expressed transiently in cells that form the tip of the newly forming tail. Interestingly, the combined pattern of spatial expression of As-T and As-T2 appears to correspond to that of a single vertebrate Brachyury gene.