title: Evolutionary modification of mouth position in deuterostomes.
abstract: In chordates, the oral ectoderm is positioned at the anterior neural boundary and is characterized by pituitary homeobox (Pitx) and overlapping Dlx and Six3 expressions. Recent studies have shown that the ectoderm molecular map is also conserved in hemichordates and echinoderms. However, the mouth develops in a more posterior position in these animals, in a domain characterized by Nkx2.1 and Goosecoid expression, in a manner similar to that observed in protostomes. Furthermore, BMP signaling antagonizes mouth development in echinoderms and hemichordates, but seems to promote oral ectoderm specification in chordates. Conversely, Nodal signaling appears to be required for oral ectoderm specification in sea urchins but not in chordates. The Nodal/BMP antagonism at work during ectoderm patterning thus seems to constitute a conserved feature in deuterostomes, and mouth relocation may have been accompanied by a change in the influence of BMP/Nodal signals on oral ectoderm specification. We suggest that the mouth primordium was located at the anterior neural boundary, in early chordate evolution. In extant chordate embryos, subsequent mouth positioning differ between urochordates and vertebrates, presumably as a consequence of surrounding tissues remodelling. We illustrate these morphogenetic movements by means of morphological data obtained by the confocal imaging of ascidian tailbud embryos, and provide a table for determining the tailbud stages of this model organism.
authors.affiliation: Center for Integrative Genomics, Molecular & Cell Biology Department, University of California, Berkeley, CA 94720, USA. lionelchristiaen@berkeley.edu
authors.forename: Lionel
authors.initials: L
authors.lastname: Christiaen
authors.forename: Yan
authors.initials: Y
authors.lastname: Jaszczyszyn
authors.forename: Marina
authors.initials: M
authors.lastname: Kerfant
authors.forename: Shungo
authors.initials: S
authors.lastname: Kano
authors.forename: Violette
authors.initials: V
authors.lastname: Thermes
authors.forename: Jean-Stéphane
authors.initials: JS
authors.lastname: Joly
mesh_terms: D000818:Animals; D005075:Biological Evolution; D019521:Body Patterning; D019485:Bone Morphogenetic Proteins / Q000378:metabolism*; D043344:Chordata / Q000196:embryology / Q000378:metabolism; D004475:Ectoderm / Q000196:embryology* / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental; D009055:Mouth / Q000196:embryology* / Q000378:metabolism; D015398:Signal Transduction
chemical_list: D019485:Bone Morphogenetic Proteins