title: Complementary striped expression patterns of NK homeobox genes during segment formation in the annelid Platynereis.
abstract: NK genes are related pan-metazoan homeobox genes. In the fruitfly, NK genes are clustered and involved in patterning various mesodermal derivatives during embryogenesis. It was therefore suggested that the NK cluster emerged in evolution as an ancestral mesodermal patterning cluster. To test this hypothesis, we cloned and analysed the expression patterns of the homologues of NK cluster genes Msx, NK4, NK3, Lbx, Tlx, NK1 and NK5 in the marine annelid Platynereis dumerilii, a representative of trochozoans, the third great branch of bilaterian animals alongside deuterostomes and ecdysozoans. We found that most of these genes are involved, as they are in the fly, in the specification of distinct mesodermal derivatives, notably subsets of muscle precursors. The expression of the homologue of NK4/tinman in the pulsatile dorsal vessel of Platynereis strongly supports the hypothesis that the vertebrate heart derived from a dorsal vessel relocated to a ventral position by D/V axis inversion in a chordate ancestor. Additionally and more surprisingly, NK4, Lbx, Msx, Tlx and NK1 orthologues are expressed in complementary sets of stripes in the ectoderm and/or mesoderm of forming segments, suggesting an involvement in the segment formation process. A potentially ancient role of the NK cluster genes in segment formation, unsuspected from vertebrate and fruitfly studies so far, now deserves to be investigated in other bilaterian species, especially non-insect arthropods and onychophorans.
authors.affiliation: Centre de Génétique Moléculaire, CNRS UPR 2167, 1 avenue de la terrasse, 91198 Gif-sur-Yvette, France.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Saudemont
authors.forename: Nicolas
authors.initials: N
authors.lastname: Dray
authors.forename: Bruno
authors.initials: B
authors.lastname: Hudry
authors.forename: Martine
authors.initials: M
authors.lastname: Le Gouar
authors.forename: Michel
authors.initials: M
authors.lastname: Vervoort
authors.forename: Guillaume
authors.initials: G
authors.lastname: Balavoine
mesh_terms: D000818:Animals; D000845:Annelida / Q000196:embryology*; D001483:Base Sequence; D016000:Cluster Analysis; D017931:DNA Primers / Q000235:genetics; D018076:DNA, Complementary / Q000235:genetics; D018507:Gene Expression Regulation, Developmental; D006321:Heart / Q000196:embryology*; D018398:Homeodomain Proteins / Q000235:genetics / Q000378:metabolism*; D017403:In Situ Hybridization; D008969:Molecular Sequence Data; D024510:Muscle Development / Q000502:physiology*; D010802:Phylogeny; D017422:Sequence Analysis, DNA; D019170:Somites / Q000196:embryology* / Q000378:metabolism
chemical_list: D017931:DNA Primers; D018076:DNA, Complementary; D018398:Homeodomain Proteins