title: Six3 demarcates the anterior-most developing brain region in bilaterian animals.
abstract: BACKGROUND
  The heads of annelids (earthworms, polychaetes, and others) and arthropods (insects, myriapods, spiders, and others) and the arthropod-related onychophorans (velvet worms) show similar brain architecture and for this reason have long been considered homologous. However, this view is challenged by the 'new phylogeny' placing arthropods and annelids into distinct superphyla, Ecdysozoa and Lophotrochozoa, together with many other phyla lacking elaborate heads or brains. To compare the organisation of annelid and arthropod heads and brains at the molecular level, we investigated head regionalisation genes in various groups. Regionalisation genes subdivide developing animals into molecular regions and can be used to align head regions between remote animal phyla.
  
  
  RESULTS
  We find that in the marine annelid Platynereis dumerilii, expression of the homeobox gene six3 defines the apical region of the larval body, peripherally overlapping the equatorial otx+ expression. The six3+ and otx+ regions thus define the developing head in anterior-to-posterior sequence. In another annelid, the earthworm Pristina, as well as in the onychophoran Euperipatoides, the centipede Strigamia and the insects Tribolium and Drosophila, a six3/optix+ region likewise demarcates the tip of the developing animal, followed by a more posterior otx/otd+ region. Identification of six3+ head neuroectoderm in Drosophila reveals that this region gives rise to median neurosecretory brain parts, as is also the case in annelids. In insects, onychophorans and Platynereis, the otx+ region instead harbours the eye anlagen, which thus occupy a more posterior position.
  
  
  CONCLUSIONS
  These observations indicate that the annelid, onychophoran and arthropod head develops from a conserved anterior-posterior sequence of six3+ and otx+ regions. The six3+ anterior pole of the arthropod head and brain accordingly lies in an anterior-median embryonic region and, in consequence, the optic lobes do not represent the tip of the neuraxis. These results support the hypothesis that the last common ancestor of annelids and arthropods already possessed neurosecretory centres in the most anterior region of the brain. In light of its broad evolutionary conservation in protostomes and, as previously shown, in deuterostomes, the six3-otx head patterning system may be universal to bilaterian animals.
authors.affiliation: Developmental Biology Unit, European Molecular Biology Laboratory, Meyerhofstrasse 1, 69012 Heidelberg, Germany.|University of Vienna, Department for Molecular Evolution and Development, Althanstrasse 14, A-1090 Vienna, Austria.
authors.forename: Patrick Rh
authors.initials: PR
authors.lastname: Steinmetz
authors.affiliation: Johannes Gutenberg-Universität Mainz, Institut für Genetik, J.-J.-Becher-Weg 32, 55128 Mainz, Germany.
authors.forename: Rolf
authors.initials: R
authors.lastname: Urbach
authors.affiliation: Johann-Friedrich-Blumenbach-Institute of Zoology, Anthropology and Developmental Biology, DFG Research Centre for Molecular Physiology of the Brain (CMPB), Georg August University, von-Liebig-Weg-11, 37077 Göttingen, Germany.|Vetmeduni Vienna, Institute of Population Genetics, Veterinärplatz 1, A-1210 Vienna, Austria.
authors.forename: Nico
authors.initials: N
authors.lastname: Posnien
authors.affiliation: University Museum of Zoology, Department of Zoology, Downing Street, Cambridge CB2 3EJ, UK.|Queen Mary University of London, School of Biological and Chemical Sciences, Mile End Road, London E1 4NS, UK.
authors.forename: Joakim
authors.initials: J
authors.lastname: Eriksson
authors.affiliation: Department of Embryology, State University of St. Petersburg, Universitetskaya nab. 7/9, 199034 St. Petersburg, Russia.
authors.forename: Roman P
authors.initials: RP
authors.lastname: Kostyuchenko
authors.affiliation: University Museum of Zoology, Department of Zoology, Downing Street, Cambridge CB2 3EJ, UK.
authors.forename: Carlo
authors.initials: C
authors.lastname: Brena
authors.affiliation: Developmental Biology Unit, European Molecular Biology Laboratory, Meyerhofstrasse 1, 69012 Heidelberg, Germany.
authors.forename: Keren
authors.initials: K
authors.lastname: Guy
authors.affiliation: University Museum of Zoology, Department of Zoology, Downing Street, Cambridge CB2 3EJ, UK.
authors.forename: Michael
authors.initials: M
authors.lastname: Akam
authors.affiliation: Johann-Friedrich-Blumenbach-Institute of Zoology, Anthropology and Developmental Biology, DFG Research Centre for Molecular Physiology of the Brain (CMPB), Georg August University, von-Liebig-Weg-11, 37077 Göttingen, Germany.
authors.forename: Gregor
authors.initials: G
authors.lastname: Bucher
authors.affiliation: Developmental Biology Unit, European Molecular Biology Laboratory, Meyerhofstrasse 1, 69012 Heidelberg, Germany.
authors.forename: Detlev
authors.initials: D
authors.lastname: Arendt