title: The chordate ancestor possessed a single copy of theBrachyurygene for notochord acquisition.
abstract: BACKGROUND
  The T-box family transcription-factor gene,Brachyury, 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, Brachyuryis 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 whetherBrachyuryduplication occurred in the common ancestor of chordates after which one of duplicates was lost in the urochordate and vertebrate lineages, we estimated phylogenetic relationships ofBrachyurygenes and examined the synteny of aBrachyury-containing genomic region of deuterostomes with decoded genomes.The monophyletic origin of tandemly arrangedBrachyurygenes 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 ofBrachyuryacquired 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 ofBrachyury.
authors.affiliation: Marine Genomics Unit, Okinawa Institute of Science and Technology Graduate University, Onna, Okinawa, 904-0495 Japan.
authors.forename: Jun
authors.identifier: 0000-0003-4954-6373
authors.initials: J
authors.lastname: Inoue
authors.affiliation: Marine Genomics Unit, Okinawa Institute of Science and Technology Graduate University, Onna, Okinawa, 904-0495 Japan.
authors.forename: Yuuri
authors.initials: Y
authors.lastname: Yasuoka
authors.affiliation: Developmental Biology, National Institute for Basic Biology, Nishigonaka 38, Myodaiji, Okazaki, Aichi 444-8585 Japan.
authors.forename: Hiroki
authors.initials: H
authors.lastname: Takahashi
authors.affiliation: Marine Genomics Unit, Okinawa Institute of Science and Technology Graduate University, Onna, Okinawa, 904-0495 Japan.
authors.forename: Noriyuki
authors.initials: N
authors.lastname: Satoh
keywords: Blastopore; Brachyury; Chordate evolution; Gene duplication; Notochord; Primary and secondary expression domains