title: On a possible evolutionary link of the stomochord of hemichordates to pharyngeal organs of chordates.
abstract: As a group closely related to chordates, hemichordate acorn worms are in a key phylogenic position for addressing hypotheses of chordate origins. The stomochord of acorn worms is an anterior outgrowth of the pharynx endoderm into the proboscis. In 1886 Bateson proposed homology of this organ to the chordate notochord, crowning this animal group "hemichordates." Although this proposal has been debated for over a century, the question still remains unresolved. Here we review recent progress related to this question. First, the developmental mode of the stomochord completely differs from that of the notochord. Second, comparison of expression profiles of genes including Brachyury, a key regulator of notochord formation in chordates, does not support the stomochord/notochord homology. Third, FoxE that is expressed in the stomochord-forming region in acorn worm juveniles is expressed in the club-shaped gland and in the endostyle of amphioxus, in the endostyle of ascidians, and in the thyroid gland of vertebrates. Based on these findings, together with the anterior endodermal location of the stomochord, we propose that the stomochord has evolutionary relatedness to chordate organs deriving from the anterior pharynx rather than to the notochord.
authors.affiliation: Marine Genomics Unit, Okinawa Institute of Science and Technology Graduate University, Onna, Okinawa, Japan.
authors.forename: Noriyuki
authors.initials: N
authors.lastname: Satoh
authors.forename: Kunifumi
authors.initials: K
authors.lastname: Tagawa
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Lowe
authors.forename: Jr-Kai
authors.initials: JK
authors.lastname: Yu
authors.forename: Takeshi
authors.initials: T
authors.lastname: Kawashima
authors.forename: Hiroki
authors.initials: H
authors.lastname: Takahashi
authors.forename: Michio
authors.initials: M
authors.lastname: Ogasawara
authors.forename: Marc
authors.initials: M
authors.lastname: Kirschner
authors.forename: Kanako
authors.initials: K
authors.lastname: Hisata
authors.forename: Yi-Hsien
authors.initials: YH
authors.lastname: Su
authors.forename: John
authors.initials: J
authors.lastname: Gerhart
mesh_terms: D000818:Animals; D005075:Biological Evolution; D043344:Chordata / Q000033:anatomy & histology* / Q000145:classification / Q000235:genetics*; D004707:Endoderm / Q000378:metabolism; D005326:Fetal Proteins / Q000378:metabolism; D051858:Forkhead Transcription Factors / Q000378:metabolism; D005753:Gastric Mucosa / Q000378:metabolism; D009672:Notochord / Q000254:growth & development* / Q000378:metabolism; D010614:Pharynx / Q000254:growth & development* / Q000378:metabolism; D020825:T-Box Domain Proteins / Q000378:metabolism; D000098908:Brachyury Protein
chemical_list: D005326:Fetal Proteins; D051858:Forkhead Transcription Factors; D020825:T-Box Domain Proteins; D000098908:Brachyury Protein
keywords: chordates; hemichordates; notochord; pharyngeal organs; stomochord