title: Mode of reduction in the number of pharyngeal segments within the sarcopterygians.
abstract: BACKGROUND
  Pharyngeal segmentation is a defining feature of vertebrate embryos and is apparent as a series of bulges found on the lateral surface of the embryonic head, the pharyngeal arches. The ancestral condition for gnathostomes is to have seven pharyngeal segments: jaw, hyoid, and five posterior branchial arches. However, within the sarcopterygians, the pharyngeal region has undergone extensive remodelling that resulted in a reduction in the number of pharyngeal segments, such that amniotes have only five pharyngeal arches. The aim of this study is to probe the developmental basis of this loss of pharyngeal segments.
  
  
  RESULTS
  We have therefore compared the development of the pharyngeal arches in an amniote, the chick, which has five segments, with those of a chondrichthyan, the catshark, which has seven segments. We have analysed the early phase of pharyngeal segmentation and we find that in both the most anterior segments form first with the posterior segments being added sequentially. We also documented the patterns of innervation of the pharynx in several vertebrates and note that the three most anterior segments receive distinct innervation: the first arch being innervated by the Vth nerve, the second by the VIIth and the third by the IXth. Finally, we have analysed Hox gene expression, and show that the anterior limit of Hoxa2 aligns with the second pouch and arch in both chick and catshark, while Hoxa3 is transiently associated with the third arch and pouch. Surprisingly, we have found that Hoxb1 expression is spatially and temporally dynamic and that it is always associated with the last most recently formed pouch and that this domains moves caudally as additional pouches are generated.
  
  
  CONCLUSIONS
  We propose that the first three pharyngeal segments are homologous, as is the posterior limit of the pharynx, and that the loss of segments occurred between these two points. We suggest that this loss results from a curtailment of the posterior expansion of the pharyngeal endoderm in amniotes at relatively earlier time point, and thus the generation of fewer segments.
authors.affiliation: Department of Developmental Neurobiology, Kings College London, London, SE1 1UL UK.
authors.forename: Victoria
authors.initials: V
authors.lastname: Shone
authors.affiliation: Laboratoire Évolution, génomes, comportement, écologie, CNRS université Paris-Sud UMR 9191, IRD UMR 247, Avenue de la Terrasse, bâtiment 13, boîte postale 1, 91198 Gif-sur-Yvette, France.
authors.forename: Silvan
authors.initials: S
authors.lastname: Oulion
authors.affiliation: Laboratoire Évolution, génomes, comportement, écologie, CNRS université Paris-Sud UMR 9191, IRD UMR 247, Avenue de la Terrasse, bâtiment 13, boîte postale 1, 91198 Gif-sur-Yvette, France.
authors.forename: Didier
authors.initials: D
authors.lastname: Casane
authors.affiliation: Laboratoire Évolution, génomes, comportement, écologie, CNRS université Paris-Sud UMR 9191, IRD UMR 247, Avenue de la Terrasse, bâtiment 13, boîte postale 1, 91198 Gif-sur-Yvette, France.
authors.forename: Patrick
authors.initials: P
authors.lastname: Laurenti
authors.affiliation: Department of Developmental Neurobiology, Kings College London, London, SE1 1UL UK.
authors.forename: Anthony
authors.identifier: 0000-0003-2011-7188
authors.initials: A
authors.lastname: Graham
keywords: Endoderm; Hoxb1; Pharyngeal; Sarcopterygians; Segmentation; Tetrapods