title: Evolution, divergence and loss of the Nodal signalling pathway: new data and a synthesis across the Bilateria.
abstract: Since the discovery that the TGF-β signalling molecule Nodal and its downstream effector Pitx have a parallel role in establishing asymmetry between molluscs and deuterostomes the debate over the degree to which this signalling pathway is conserved across the Bilateria as a whole has been ongoing. Further taxon sampling is critical to understand the evolution and divergence of this signalling pathway in animals. Using genome and transcriptome mining we confirmed the presence of nodal and Pitx in a range of additional animal taxa for which their presence has not yet been described. In situ hybridization was used to show the embryonic expression of these genes in brachiopods and planarians. We show that both nodal and Pitx genes are broadly conserved across the Spiralia, and nodal likely appeared in the Bilaterian stem lineage after the divergence of the Acoelomorpha. Furthermore, both nodal and Pitx mRNA appears to be expressed in an asymmetric fashion in the brachiopod Terebratalia transversa. No evidence for the presence of a Lefty ortholog could be found in the non-deuterostome genomic resources examined. Nodal expression is asymmetric in a number of spiralian lineages, indicating a possible ancestral role of the Nodal/Pitx cascade in the establishment of asymmetries across the Bilateria.
authors.affiliation: Laboratorio 421, Centro de Biología Molecular Severo Ochoa, C/ Nicolás Cabrera, 1, Universidad Autónoma de Madrid, E-28049 - Madrid, Spain. cgrande@cbm.csic.es.
authors.forename: Cristina
authors.initials: C
authors.lastname: Grande
authors.forename: José María
authors.initials: JM
authors.lastname: Martín-Durán
authors.forename: Nathan J
authors.initials: NJ
authors.lastname: Kenny
authors.forename: Marta
authors.initials: M
authors.lastname: Truchado-García
authors.forename: Andreas
authors.initials: A
authors.lastname: Hejnol
mesh_terms: D000818:Animals; D005075:Biological Evolution; D019521:Body Patterning / Q000235:genetics; D054262:Gastrulation / Q000502:physiology; D018507:Gene Expression Regulation, Developmental; D055457:Nodal Protein / Q000235:genetics* / Q000378:metabolism; D051761:Paired Box Transcription Factors / Q000235:genetics* / Q000378:metabolism; D010932:Planarians / Q000196:embryology* / Q000235:genetics*; D015398:Signal Transduction; D059467:Transcriptome / Q000235:genetics
chemical_list: D055457:Nodal Protein; D051761:Paired Box Transcription Factors; C101485:homeobox protein PITX1