Pattern of Brachyury gene expression in starfish embryos resembles that ofhemichordate embryos but not of sea urchin embryos

Echinoderms, hemichordates and chordates are deuterostomes and share a numb er of developmental features. The Brachyury gene is responsible for formati on of the notochord, the most defining feature of chordates, and thus may b e a key to understanding the origin and evolution of the chordates. Previou s studies have shown that the ascidian Brachyury (As-T and Ci-Bra) is expre ssed in the notochord and that a sea urchin Brachyury (HpTa) is expressed i n the secondary mesenchyme founder cells. A recent study by Tagawa et al. ( 1998), however, revealed that a hemichordate Brachyury (PfBra) is expressed in a novel pattern in an archenteron invagination region and a stomodaeum invagination region in the gastrula. The present study demonstrated that th e expression pattern of Brachyury (ApBra) of starfish embryos resembles tha t of PfBra in hemichordate embryos but not of HpTa in sea urchin embryos. N amely, ApBra is expressed in an archenteron invagination region and a stomo daeum invagination region. (C) 1999 Elsevier Science Ireland Ltd. All right s reserved.