title: β-Catenin-driven binary fate specification segregates germ layers in ascidian embryos.
abstract: β-catenin is a transcriptional cofactor mediating the "canonical" Wnt signaling pathway, which activates target genes in a complex with TCF (LEF) transcription factors [1]. In many metazoans, embryos are first subdivided during early cleavage stages into nuclear β-catenin-positive and -negative domains, with β-catenin specifying endoderm or mesendoderm fate. This process has been demonstrated in a wide range of phyla including cnidarians, nemerteans, and invertebrate deuterostomes (echinoderms, hemichordates, and ascidians), implying that β-catenin-dependent (mes)endoderm specification is evolutionarily ancient [2-10]. However, the mechanisms leading to the segregation of mesoderm and endoderm fates from a transient mesendodermal state are less well defined. We show that subdivision of the ascidian embryo into the three germ layers involves differential nuclear β-catenin activity coupled with the first two animal-vegetal (A-V)-oriented cell divisions. We reveal that each of these A-V divisions operates as a binary fate choice: the first between ectoderm and mesendoderm and the second between margin (notochord and neural) and endoderm, such that a β-catenin activation sequence of ON-to-ON specifies endoderm, OFF-to-OFF ectoderm, and ON-to-OFF margin.
authors: Université Pierre et Marie Curie and Centre National de la Recherche Scientifique, Laboratoire de Biologie du Développement de Villefranche-sur-Mer, Observatoire Océanologique, 06234 Villefranche-sur-mer, France. hudson@obs-vlfr.fr
authors: Clare
authors: C
authors: Hudson
authors: Narudo
authors: N
authors: Kawai
authors: Takefumi
authors: T
authors: Negishi
authors: Hitoyoshi
authors: H
authors: Yasuo
mesh_terms: D000818:Animals; D002938:Ciona intestinalis / Q000196:embryology / Q000235:genetics / Q000378:metabolism; D004625:Embryo, Nonmammalian / Q000196:embryology / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental; D005855:Germ Layers / Q000196:embryology / Q000378:metabolism; D016133:Polymerase Chain Reaction; D013045:Species Specificity; D014561:Urochordata / Q000196:embryology* / Q000235:genetics / Q000378:metabolism*; D051176:beta Catenin / Q000235:genetics / Q000378:metabolism*
chemical_list: D051176:beta Catenin