Expression pattern of the Brachyury gene in the arrow worm paraspadella gotoi (chaetognatha).

Arrow worms (the phylum Chaetognatha), which are among the major marine planktonic animals, are direct developers and exhibit features characteristic of both deuterostomes and protostomes. In particular, the embryonic development of arrow worms appears to be of the deuterostome type. Brachyury functions critically in the formation of the notochord in chordates, whereas the gene is expressed in both the blastopore and stomodeum invagination regions in embryos of hemichordates and echinoderms. Here we analyzed the expression of Brachyury (Pg-Bra) in the arrow worm Paraspadella gotoi and showed that Pg-Bra is expressed in the blastopore region and the stomodeum region in the embryo and then around the mouth opening region at the time of hatching. The expression of Pg-Bra in the embryo resembles that of Brachyury in embryos of hemichordates and echinoderms, whereas that in the mouth opening region in the hatchling appears to be novel.