title: Hox expression in the direct-type developing sand dollar Peronella japonica.
abstract: BACKGROUND
  Echinoderms are a curious group of deuterostomes that forms a clade with hemichordates but has a pentameral body plan. Hox complex plays a pivotal role in axial patterning in bilaterians and often occurs in a cluster on the chromosome. In contrast to hemichordates with an organized Hox cluster, the sea urchin Strongylocentrotus purpuratus has a Hox cluster with an atypical organization. However, the current data on hox expression in sea urchin rudiments are fragmentary.
  
  
  RESULTS
  We report a comprehensive examination of hox expression in a sand dollar echinoid. Nine hox genes are expressed in the adult rudiment, which are classified into two groups, but hox11/13b belongs to both: one with linear expression in the coelomic mesoderm and another with radial expression around the adult mouth. The linear genes may endow the coelom/mesentery with axial information to direct postmetamorphic transformation of the digestive tract, whereas the radial genes developmentally correlate with the morphological novelties of echinoderms and/or sea urchins. Recruitment of the radial genes except hox11/13b appears to be accompanied by the loss of ancestral/axial roles.
  
  
  CONCLUSIONS
  This in toto co-option of the hox genes provides insight into the molecular mechanisms underlying the evolution of echinoderms from a bilateral ancestor.
authors.affiliation: Division of Life Science, Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa, Japan; Institute for Molecular Science of Medicine, Aichi Medical University, Nagakute, Japan.
authors.forename: Jun
authors.initials: J
authors.lastname: Tsuchimoto
authors.forename: Masaaki
authors.initials: M
authors.lastname: Yamaguchi
mesh_terms: D000818:Animals; D019521:Body Patterning / Q000235:genetics / Q000502:physiology; D018507:Gene Expression Regulation, Developmental; D005801:Genes, Homeobox / Q000235:genetics / Q000502:physiology*; D018398:Homeodomain Proteins / Q000235:genetics / Q000378:metabolism; D012617:Sea Urchins / Q000196:embryology* / Q000235:genetics*; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism
chemical_list: D018398:Homeodomain Proteins; D014157:Transcription Factors
keywords: Ambulacraria; body plan; co-option; echinoderm; sea urchin