title: Are Hox genes ancestrally involved in axial patterning? Evidence from the hydrozoan Clytia hemisphaerica (Cnidaria).
abstract: BACKGROUND
  The early evolution and diversification of Hox-related genes in eumetazoans has been the subject of conflicting hypotheses concerning the evolutionary conservation of their role in axial patterning and the pre-bilaterian origin of the Hox and ParaHox clusters. The diversification of Hox/ParaHox genes clearly predates the origin of bilaterians. However, the existence of a "Hox code" predating the cnidarian-bilaterian ancestor and supporting the deep homology of axes is more controversial. This assumption was mainly based on the interpretation of Hox expression data from the sea anemone, but growing evidence from other cnidarian taxa puts into question this hypothesis.
  
  
  RESULTS
  Hox, ParaHox and Hox-related genes have been investigated here by phylogenetic analysis and in situ hybridisation in Clytia hemisphaerica, an hydrozoan species with medusa and polyp stages alternating in the life cycle. Our phylogenetic analyses do not support an origin of ParaHox and Hox genes by duplication of an ancestral ProtoHox cluster, and reveal a diversification of the cnidarian HOX9-14 genes into three groups called A, B, C. Among the 7 examined genes, only those belonging to the HOX9-14 and the CDX groups exhibit a restricted expression along the oral-aboral axis during development and in the planula larva, while the others are expressed in very specialised areas at the medusa stage.
  
  
  CONCLUSIONS
  Cross species comparison reveals a strong variability of gene expression along the oral-aboral axis and during the life cycle among cnidarian lineages. The most parsimonious interpretation is that the Hox code, collinearity and conservative role along the antero-posterior axis are bilaterian innovations.
authors.affiliation: UPMC Univ Paris 06, UMR 7138 CNRS UPMC MNHN IRD, Paris, France.
authors.forename: Roxane
authors.initials: R
authors.lastname: Chiori
authors.forename: Muriel
authors.initials: M
authors.lastname: Jager
authors.forename: Elsa
authors.initials: E
authors.lastname: Denker
authors.forename: Patrick
authors.initials: P
authors.lastname: Wincker
authors.forename: Corinne
authors.initials: C
authors.lastname: Da Silva
authors.forename: Hervé
authors.initials: H
authors.lastname: Le Guyader
authors.forename: Michaël
authors.initials: M
authors.lastname: Manuel
authors.forename: Eric
authors.initials: E
authors.lastname: Quéinnec
mesh_terms: D000818:Animals; D019521:Body Patterning; D003063:Cnidaria / Q000502:physiology*; D004707:Endoderm / Q000378:metabolism; D020224:Expressed Sequence Tags; D005786:Gene Expression Regulation; D005801:Genes, Homeobox; D014644:Genetic Variation; D018398:Homeodomain Proteins / Q000378:metabolism*; D008954:Models, Biological; D010802:Phylogeny
chemical_list: D018398:Homeodomain Proteins