title: Adoption of conserved developmental genes in development and origin of the medusa body plan.
abstract: BACKGROUND
  The metagenesis of sessile polyps into pelagic medusae in cnidarians represents one of the most ancient complex life cycles in animals. Interestingly, scyphozoans and hydrozoans generate medusae by apparently fundamentally different processes. It is therefore unclear whether medusa formation has evolved independently in different medusozoans. To this end, a thorough understanding of the correspondence of polyp and medusa is required.
  
  
  RESULTS
  We monitored the expression patterns of conserved developmental genes in developing medusae of Clytia hemisphaerica (Hydrozoa) and Aurelia aurita (Scyphozoa) and found that developing medusae and polyps share similarities in their morphology and developmental gene expression. Unexpectedly, however, polyp tentacle marker genes were consistently expressed in the developing medusa bell, suggesting that the bell of medusae corresponds to modified and fused polyp tentacle anlagen.
  
  
  CONCLUSIONS
  Our data represent the first comparative gene expression analysis of developing medusae in two representatives of Scyphozoa and Hydrozoa. The results challenge prevailing views about polyp medusa body plan homology. We propose that the evolution of a new life stage may be facilitated by the adoption of existing developmental genes.
authors.affiliation: Department for Molecular Evolution and Development, Centre for Organismal Systems Biology, University of Vienna, Althanstraße 14, Wien, Vienna 1090 Austria.
authors.forename: Johanna E M
authors.initials: JE
authors.lastname: Kraus
authors.affiliation: Department for Molecular Evolution and Development, Centre for Organismal Systems Biology, University of Vienna, Althanstraße 14, Wien, Vienna 1090 Austria ; Present address: Computational Biology Unit, University of Bergen, Thormohlensgate 55, 5008 Bergen, Norway.
authors.forename: David
authors.initials: D
authors.lastname: Fredman
authors.affiliation: Zoologisches Institut, Christian-Albrechts Universität zu Kiel, Am Botanischen Garten 1-9, Kiel, 24118 Germany.
authors.forename: Wei
authors.initials: W
authors.lastname: Wang
authors.affiliation: Marine Genomics Unit, Okinawa Institute of Science and Technology, 1919-1 Tancha, Onna-son, Kunigami-gun, Okinawa 904-0495 Japan.
authors.forename: Konstantin
authors.initials: K
authors.lastname: Khalturin
authors.affiliation: Department for Molecular Evolution and Development, Centre for Organismal Systems Biology, University of Vienna, Althanstraße 14, Wien, Vienna 1090 Austria.
authors.forename: Ulrich
authors.initials: U
authors.lastname: Technau
keywords: Aurelia aurita; Body plan evolution; Clytia hemisphaerica; Life cycle; Polyp-medusa metagenesis