title: Evolutionary history of free-swimming and sessile lifestyles in urochordates as deduced from 18S rDNA molecular phylogeny.
abstract: Whether the ancestral chordates were free-swimming or sessile is a longstanding question that remains to be settled. Vertebrates and amphioxi are free-swimming, but the most basal chordate subphylum (the urochordates) includes both sessile and free-swimming species. Here, 1 report molecular phylogenetic analyses of 18S rDNA of urochordates to deduce which lifestyle is ancestral. This revealed a close relationship between salps and doliolids and paraphyly of the ascidians. An early divergence of larvaceans, which show a tadpole-like body plan throughout life, is also supported by the analyses. Based on this phylogeny, a free-swimming ancestor for chordates is more parsimonious than a sessile ancestor. The evolutionary history of various lifestyles of chordates from this ancestral form is proposed.
authors.affiliation: School of Animal and Microbial Sciences, University of Reading Whiteknights, England. hwada@seto.kyoto-u.ac.jp
authors.forename: H
authors.initials: H
authors.lastname: Wada
mesh_terms: D000818:Animals; D004275:DNA, Ribosomal / Q000235:genetics*; D019143:Evolution, Molecular; D008124:Locomotion / Q000235:genetics*; D010802:Phylogeny; D012337:RNA, Ribosomal, 18S / Q000235:genetics*; D014561:Urochordata / Q000235:genetics* / Q000502:physiology
chemical_list: D004275:DNA, Ribosomal; D012337:RNA, Ribosomal, 18S