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Yu, JK, Holland LZ, Jamrich M, Blitz IL, Holland ND.  2002.  AmphiFoxE4, an amphioxus winged helix/forkhead gene encoding a protein closely related to vertebrate thyroid transcription factor-2: expression during pharyngeal development. Evolution & Development. 4:9-15.   10.1046/j.1525-142x.2002.01057.x   AbstractWebsite

The full-length sequence and developmental expression of amphioxus AmphiFoxE4 are described. Transcripts of the gene are first detected in the pharyngeal endoderm, where the club-shaped gland is forming and subsequently in the definitive gland itself. AmphiFoxE4 is closely related to vertebrate genes encoding the thyroid-specific transcription factor-2 (TTF2), which plays an early developmental role in the morphogenesis of the thyroid gland and a later role in hormone-mediated control of thyroid function. In amphioxus, AmphiFoxE4 expression is not thyroid specific because the club-shaped gland, the only structure expressing the gene, is not homologous to the vertebrate thyroid; in-stead, the thyroid homologue of amphioxus is a specialized region of the pharyngeal endoderm called the endostyle. We propose that (a) the pharynx of an amphioxus-like ancestor of the vertebrates included a club-shaped gland that expressed FoxE4 as well as an endostyle that did not, and (b) the club-shaped gland soon disappeared in the vertebrate line of descent but (c) not before there was a homeogenetic transfer of FoxE4 expression from the club-shaped gland to the nearby endostyle. Such a transfer could have provided part of the genetic program enabling the endostyle to separate from the pharyngeal endoderm and migrate away as the rudiment of the thyroid gland.

Venkatesh, TV, Holland ND, Holland LZ, Su MT, Bodmer R.  1999.  Sequence and developmental expression of amphioxus AmphiNk2-1: insights into the evolutionary origin of the vertebrate thyroid gland and forebrain. Development Genes and Evolution. 209:254-259. AbstractWebsite

We characterized an amphioxus NK-2 homeobox gene (AmphiNk2-1), a homologue of vertebrate Nkx2-1, which is involved in the development of the central nervous system and thyroid gland. At the early neurula stage of amphioxus, AmphiNk2-1 expression is first detected medially in the neural plate. By the mid-neurula stage, expression is localized ventrally in the nerve cord and also begins in the endoderm. During the late neurula stage, the ventral neural expression becomes transiently segmented posteriorly and is then down-regulated except in the cerebral vesicle at the anterior end of the central nervous system. Within the cerebral vesicle AmphiNk2-1 is expressed in a broad ventral domain, probably comprising both the floor plate and basal plate regions: this pattern is comparable to Nkx2-1 expression in the mouse diencephalon. In the anterior part of the gut, expression becomes intense in the endostyle (the right wall of the pharynx), which is the presumed homologue of the vertebrate thyroid gland. More posteriorly, there is transitory expression in the midgut and hindgut. In sum, the present results help to support homologies (1) between the amphioxus endostyle and the vertebrate thyroid gland and (2) between the amphioxus cerebral vesicle and the vertebrate diencephalic forebrain.