Fig. 1.

Two alternative evolutionary scenarios for Brachyury in relation to gene duplication along with diversification of five deuterostome lineages. a In the first scenario, Brachyury duplicated in a common ancestor of chordates, with an original gene copy retaining the primary expression-function (PEF) while the duplicate developed the secondary expression-function (SEF). The cephalochordate lineage retains this feature while the urochordate and vertebrate lineages lost one of duplicates in their last common ancestor. Brachyury evolution in vertebrates is complex, due to the 2R-GWGD that occurred in this lineage. b In the second scenario, chordates maintained a single-copy of Brachyury, just as non-chordate deuterostomes did. Duplication occurred only in the lineage leading to cephalochordates, but not in lineages leading to urochordates and vertebrates