title: Development of the annelid axochord: insights into notochord evolution.
abstract: The origin of chordates has been debated for more than a century, with one key issue being the emergence of the notochord. In vertebrates, the notochord develops by convergence and extension of the chordamesoderm, a population of midline cells of unique molecular identity. We identify a population of mesodermal cells in a developing invertebrate, the marine annelid Platynereis dumerilii, that converges and extends toward the midline and expresses a notochord-specific combination of genes. These cells differentiate into a longitudinal muscle, the axochord, that is positioned between central nervous system and axial blood vessel and secretes a strong collagenous extracellular matrix. Ancestral state reconstruction suggests that contractile mesodermal midline cells existed in bilaterian ancestors. We propose that these cells, via vacuolization and stiffening, gave rise to the chordate notochord.
authors.affiliation: Developmental Biology Unit, European Molecular Biology Laboratory (EMBL), D-69117 Heidelberg.
authors.forename: Antonella
authors.initials: A
authors.lastname: Lauri
authors.affiliation: Developmental Biology Unit, European Molecular Biology Laboratory (EMBL), D-69117 Heidelberg.
authors.forename: Thibaut
authors.initials: T
authors.lastname: Brunet
authors.affiliation: Developmental Biology Unit, European Molecular Biology Laboratory (EMBL), D-69117 Heidelberg. Janelia Farm Research Campus, 19700 Helix Drive, Ashburn, VA 20147, USA.
authors.forename: Mette
authors.initials: M
authors.lastname: Handberg-Thorsager
authors.affiliation: Developmental Biology Unit, European Molecular Biology Laboratory (EMBL), D-69117 Heidelberg.
authors.forename: Antje H L
authors.initials: AH
authors.lastname: Fischer
authors.affiliation: Developmental Biology Unit, European Molecular Biology Laboratory (EMBL), D-69117 Heidelberg.
authors.forename: Oleg
authors.initials: O
authors.lastname: Simakov
authors.affiliation: Developmental Biology Unit, European Molecular Biology Laboratory (EMBL), D-69117 Heidelberg.
authors.forename: Patrick R H
authors.initials: PR
authors.lastname: Steinmetz
authors.affiliation: Developmental Biology Unit, European Molecular Biology Laboratory (EMBL), D-69117 Heidelberg. Janelia Farm Research Campus, 19700 Helix Drive, Ashburn, VA 20147, USA.
authors.forename: Raju
authors.initials: R
authors.lastname: Tomer
authors.affiliation: Janelia Farm Research Campus, 19700 Helix Drive, Ashburn, VA 20147, USA.
authors.forename: Philipp J
authors.initials: PJ
authors.lastname: Keller
authors.affiliation: Developmental Biology Unit, European Molecular Biology Laboratory (EMBL), D-69117 Heidelberg. Centre for Organismal Studies, University of Heidelberg, Heidelberg, Germany. detlev.arendt@embl.de.
authors.forename: Detlev
authors.initials: D
authors.lastname: Arendt
mesh_terms: D000009:Abdominal Muscles / Q000166:cytology / Q000196:embryology; D000818:Animals; D005075:Biological Evolution; D018507:Gene Expression Regulation, Developmental; D008648:Mesoderm / Q000166:cytology / Q000196:embryology*; D009672:Notochord / Q000166:cytology / Q000196:embryology*; D010802:Phylogeny; D011077:Polychaeta / Q000145:classification* / Q000196:embryology* / Q000235:genetics