title: Mechanisms of endoderm formation in a cartilaginous fish reveal ancestral and homoplastic traits in jawed vertebrates.
abstract: In order to gain insight into the impact of yolk increase on endoderm development, we have analyzed the mechanisms of endoderm formation in the catshark S. canicula, a species exhibiting telolecithal eggs and a distinct yolk sac. We show that in this species, endoderm markers are expressed in two distinct tissues, the deep mesenchyme, a mesenchymal population of deep blastomeres lying beneath the epithelial-like superficial layer, already specified at early blastula stages, and the involuting mesendoderm layer, which appears at the blastoderm posterior margin at the onset of gastrulation. Formation of the deep mesenchyme involves cell internalizations from the superficial layer prior to gastrulation, by a movement suggestive of ingressions. These cell movements were observed not only at the posterior margin, where massive internalizations take place prior to the start of involution, but also in the center of the blastoderm, where internalizations of single cells prevail. Like the adjacent involuting mesendoderm, the posterior deep mesenchyme expresses anterior mesendoderm markers under the control of Nodal/activin signaling. Comparisons across vertebrates support the conclusion that endoderm is specified in two distinct temporal phases in the catshark as in all major osteichthyan lineages, in line with an ancient origin of a biphasic mode of endoderm specification in gnathostomes. They also highlight unexpected similarities with amniotes, such as the occurrence of cell ingressions from the superficial layer prior to gastrulation. These similarities may correspond to homoplastic traits fixed separately in amniotes and chondrichthyans and related to the increase in egg yolk mass.
authors.affiliation: Sorbonne Universités, UPMC Univ Paris 06, CNRS, UMR 7150, 29688 Roscoff, France.
authors.forename: Benoit G
authors.initials: BG
authors.lastname: Godard
authors.affiliation: Université d'Orléans-CNRS, UMR 6218, 45070 Orléans, France Present address: CNRS UPR 3294, Institute of Neurobiology Alfred Fessard, 91198 Gif-sur-Yvette, France.
authors.forename: Marion
authors.initials: M
authors.lastname: Coolen
authors.affiliation: Plateforme d'Imagerie, Sorbonne Universités, UPMC Univ Paris 06, CNRS, FR 2424, Station Biologique, 29688 Roscoff, France.
authors.forename: Sophie
authors.initials: S
authors.lastname: Le Panse
authors.affiliation: Université d'Orléans-CNRS, UMR 6218, 45070 Orléans, France Present address: UMR 7355, Université d'Orleans-CNRS, 45071 Orléans, France.
authors.forename: Aurélie
authors.initials: A
authors.lastname: Gombault
authors.affiliation: Université d'Orléans-CNRS, UMR 6218, 45070 Orléans, France Present address: Instituto de Neurociencias, Consejo Superior de Investigaciones Científicas y Universidad Miguel Hernández, Campus San Juan de Alicante, 03550 Alicante, Spain.
authors.forename: Susana
authors.initials: S
authors.lastname: Ferreiro-Galve
authors.affiliation: Sorbonne Universités, UPMC Univ Paris 06, CNRS, UMR 7150, 29688 Roscoff, France.
authors.forename: Laurent
authors.initials: L
authors.lastname: Laguerre
authors.affiliation: Sorbonne Universités, UPMC Univ Paris 06, CNRS, UMR 7150, 29688 Roscoff, France.
authors.forename: Ronan
authors.initials: R
authors.lastname: Lagadec
authors.affiliation: CEA-Institut de Génomique-Genoscope, 2 rue Gaston-Crémieux, 91057 Evry, France.
authors.forename: Patrick
authors.initials: P
authors.lastname: Wincker
authors.affiliation: CEA-Institut de Génomique-Genoscope, 2 rue Gaston-Crémieux, 91057 Evry, France.
authors.forename: Julie
authors.initials: J
authors.lastname: Poulain
authors.affiliation: CEA-Institut de Génomique-Genoscope, 2 rue Gaston-Crémieux, 91057 Evry, France.
authors.forename: Corinne
authors.initials: C
authors.lastname: Da Silva
authors.affiliation: Genome Resource and Analysis Unit (GRAS), Center for Developmental Biology, RIKEN.2-2-3 Minatojima-minami, Chuo-KU, Kobe 650-0047, Japan.
authors.forename: Shigehiro
authors.initials: S
authors.lastname: Kuraku
authors.affiliation: ABiMS, Sorbonne Universités, UPMC Univ Paris 06, CNRS, FR 2424, 29688 Roscoff, France.
authors.forename: Wilfrid
authors.initials: W
authors.lastname: Carre
authors.affiliation: Sorbonne Universités, UPMC Univ Paris 06, CNRS, UMR 7150, 29688 Roscoff, France.
authors.forename: Agnès
authors.initials: A
authors.lastname: Boutet
authors.affiliation: Sorbonne Universités, UPMC Univ Paris 06, CNRS, UMR 7150, 29688 Roscoff, France smazan@sb-roscoff.fr.
authors.forename: Sylvie
authors.initials: S
authors.lastname: Mazan
keywords: Nodal signalling; chondrichthyan; endoderm; telolecithal egg