title: Evolution of the Role of RA and FGF Signals in the Control of Somitogenesis in Chordates.
abstract: During vertebrate development, the paraxial mesoderm becomes segmented, forming somites that will give rise to dermis, axial skeleton and skeletal muscles. Although recently challenged, the "clock and wavefront" model for somitogenesis explains how interactions between several cell-cell communication pathways, including the FGF, RA, Wnt and Notch signals, control the formation of these bilateral symmetric blocks. In the cephalochordate amphioxus, which belongs to the chordate phylum together with tunicates and vertebrates, the dorsal paraxial mesendoderm also periodically forms somites, although this process is asymmetric and extends along the whole body. It has been previously shown that the formation of the most anterior somites in amphioxus is dependent upon FGF signalling. However, the signals controlling somitogenesis during posterior elongation in amphioxus are still unknown. Here we show that, contrary to vertebrates, RA and FGF signals act independently during posterior elongation and that they are not mandatory for posterior somites to form. Moreover, we show that RA is not able to buffer the left/right asymmetry machinery that is controlled through the asymmetric expression of Nodal pathway actors. Our results give new insights into the evolution of the somitogenesis process in chordates. They suggest that RA and FGF pathways have acquired specific functions in the control of somitogenesis in vertebrates. We propose that the "clock and wavefront" system was selected specifically in vertebrates in parallel to the development of more complex somite-derived structures but that it was not required for somitogenesis in the ancestor of chordates.
authors.affiliation: UPMC Univ Paris 06, UMR 7232, BIOM, Observatoire Océanologique de Banyuls sur Mer, F-66650, Banyuls/Mer, France.
authors.forename: Stéphanie
authors.initials: S
authors.lastname: Bertrand
authors.affiliation: UPMC Univ Paris 06, UMR 7232, BIOM, Observatoire Océanologique de Banyuls sur Mer, F-66650, Banyuls/Mer, France.
authors.forename: Daniel
authors.initials: D
authors.lastname: Aldea
authors.affiliation: UPMC Univ Paris 06, UMR 7232, BIOM, Observatoire Océanologique de Banyuls sur Mer, F-66650, Banyuls/Mer, France.
authors.forename: Silvan
authors.initials: S
authors.lastname: Oulion
authors.affiliation: UPMC Univ Paris 06, UMR 7232, BIOM, Observatoire Océanologique de Banyuls sur Mer, F-66650, Banyuls/Mer, France.
authors.forename: Lucie
authors.initials: L
authors.lastname: Subirana
authors.affiliation: Departamento de Química Orgánica, Facultade de Química, CINBIO, Universidade de Vigo, and Instituto de Investigación Biomédica de Vigo (IBIV), Vigo, Spain.
authors.forename: Angel R
authors.initials: AR
authors.lastname: de Lera
authors.affiliation: UPMC Univ Paris 06, UMR 7232, BIOM, Observatoire Océanologique de Banyuls sur Mer, F-66650, Banyuls/Mer, France.
authors.forename: Ildiko
authors.initials: I
authors.lastname: Somorjai
authors.affiliation: UPMC Univ Paris 06, UMR 7232, BIOM, Observatoire Océanologique de Banyuls sur Mer, F-66650, Banyuls/Mer, France.
authors.forename: Hector
authors.initials: H
authors.lastname: Escriva
mesh_terms: D000818:Animals; D004815:Epidermal Growth Factor / Q000378:metabolism*; D064175:Lancelets / Q000196:embryology*; D051880:Receptors, Notch / Q000378:metabolism; D019170:Somites / Q000196:embryology*; D014212:Tretinoin / Q000494:pharmacology*; D060449:Wnt Signaling Pathway / Q000187:drug effects / Q000502:physiology*
chemical_list: D051880:Receptors, Notch; D014212:Tretinoin; D004815:Epidermal Growth Factor