title: AmphiNk2-tin, an amphioxus homeobox gene expressed in myocardial progenitors: insights into evolution of the vertebrate heart.
abstract: We isolated a full-length cDNA clone of amphioxus AmphiNk2-tin, an NK2 gene similar in sequence to vertebrate NK2 cardiac genes, suggesting a potentially similar function to Drosophila tinman and to vertebrate NK2 cardiac genes during heart development. During the neurula stage of amphioxus, AmphiNk2-tin is expressed first within the foregut endoderm, then transiently in muscle precursor cells in the somites, and finally in some mesoderm cells of the visceral peritoneum arranged in an approximately midventral row running beneath the midgut and hindgut. The peritoneal cells that express AmphiNk2-tin are evidently precursors of the myocardium of the heart, which subsequently becomes morphologically detectable ventral to the gut. The amphioxus heart is a rostrocaudally extended tube consisting entirely of myocardial cells (at both the larval and adult stages); there are no chambers, valves, endocardium, epicardium, or other differentiated features of vertebrate hearts. Phylogenetic analysis of the AmphiNk2-tin sequence documents its close relationship to vertebrate NK2 class cardiac genes, and ancillary evidence suggests a relationship with the Drosophila NK2 gene tinman. Apparently, an amphioxus-like heart, and the developmental program directing its development, was the foundation upon which the vertebrate heart evolved by progressive modular innovations at the genetic and morphological levels of organization.
authors.affiliation: Marine Biology Research Division, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093-0202, USA.
authors.forename: Nicholas D
authors.initials: ND
authors.lastname: Holland
authors.forename: Tyamagondlu V
authors.initials: TV
authors.lastname: Venkatesh
authors.forename: Linda Z
authors.initials: LZ
authors.lastname: Holland
authors.forename: David K
authors.initials: DK
authors.lastname: Jacobs
authors.forename: Rolf
authors.initials: R
authors.lastname: Bodmer
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D019070:Cell Lineage; D002816:Chordata, Nonvertebrate; D004330:Drosophila; D004625:Embryo, Nonmammalian; D004707:Endoderm / Q000378:metabolism; D019143:Evolution, Molecular; D018507:Gene Expression Regulation, Developmental; D005801:Genes, Homeobox / Q000235:genetics*; D006321:Heart / Q000196:embryology*; D018398:Homeodomain Proteins / Q000737:chemistry / Q000235:genetics*; D007814:Larva; D008969:Molecular Sequence Data; D018482:Muscle, Skeletal / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism*; D010802:Phylogeny
chemical_list: D018398:Homeodomain Proteins