The chordate ancestor possessed a single copy of the gene for notochord acquisition.

BACKGROUND

The T-box family transcription-factor gene, , has two expression domains with discrete functions during animal embryogenesis. The primary domain, associated with the blastopore, is shared by most metazoans, while the secondary domain, involved in the notochord, is specific to chordates. In most animals is present in a single copy, but in cephalochordates, the most basal of the chordates, the gene is present in two copies, suggesting allotment of the two domains to each of the duplicates.

RESULTS

In order to clarify whether duplication occurred in the common ancestor of chordates after which one of duplicates was lost in the urochordate and vertebrate lineages, we estimated phylogenetic relationships of genes and examined the synteny of a -containing genomic region of deuterostomes with decoded genomes The monophyletic origin of tandemly arranged genes of cephalochordates indicates that the tandem duplication occurred in the cephalochordate lineage, but not in the chordate ancestor.

CONCLUSIONS

Our results thus suggest that, in the common ancestor of chordates, a single copy of acquired two expression domains and that the duplication was not involved in the acquisition of the notochord. However, in relation to regulatory mechanisms, both possibilities-namely a single copy with two domains and two copies with different domains-should be considered in future studies of .