Details of the evolutionary history from invertebratesto vertebrates, as deduced from the sequences of 18S rDNA.

Abstract

Almost the entire sequences of 18S rDNA were determined for two chaetognaths, five echinoderms, a hemichordate, and two urochordates (a larvacean and a salp). Phylogenetic comparisons ofthe sequences, together with those of other deuterostomes (an ascidian, a cephalochordate, and vertebrates) and protostomes (an arthropod and a mollusc), suggest the monophyly ofthe deuterostomes, with the exception of the chaetognaths. Chaetognaths may not be a group of deuterostomes. The deuterostome group closest to vertebrates was the group of cephalochordates. Ascidians, larvaceans, and salps seem to form a discrete group (urochordates), in which the early divergence of larvaceans is evident. These results support the hypothesis that chordates evolved from free-living ancestors.The evolutionary pathway from advanced invertebrates through primitive chordates to vertebrates has been a subject of extensive investigation and vigorous discussion for more than a century (1-5). Chordates are categorized as deuterostomes, which are characterized by several features that include, for example, radial cleavage, the fate of the blastopore that does not form a mouth, an enterocoelic coelom, and a tripartite body plan (4-7). Traditionally, the deuterostomes include pogonophorans, chaetognaths, echinoderms, hemichordates, and chordates (urochordates, cephalochordates, and vertebrates), but recent studies have forced the categorization of pogonophorans near the annelids (7, 8). The chaetognath (arrowworm) remains a mystery in terms of its ancestry.Recent advances in molecular biology have made it possible to answer some of the questions posed by evolutionary biologists. Comparisons based on molecular data, such as the amino acid sequences of certain proteins and the nucleotide sequences of certain RNAs and DNAs, provide powerful tools with which to examine phylogenetic relationships among animal groups since these molecular characteristics can be interpreted more objectively than others. Unfortunately, pioneer studies with sequences of 5S rRNA (9) and partial sequences of 18S rRNA (10) failed to affirm the monophyly ofdeuterostomes, probably because the radiation of bilaterally symmetrical animals occurred over a very short period of time. However, the monophyly of the deuterostomes was suggested by Lake (11) after application of his original method for the construction ofphylogenetic trees. A recent study by Stock and Whitt (12) provided evidence from sequences of 18S rRNAs that lampreys and hagfishes form a natural group, but conclusive support from molecularphylogenetic analysis for the monophyly of the deuterostomes has not yet been obtained.To further our knowledge of the phylogeny of deuterostomes, we determined almost the entire sequences of 18S rDNAs from two chaetognaths, five echinoderms, a hemichordate, and two urochordates (a larvacean and a salp) and, together with the sequences of other deuterostomes (an ascidian, a cephalochordate, and vertebrates) and protostomes (an arthropod and a mollusc) (13), we reexamined the ...

Additional molecular support for the new chordate phylogeny

“…The revision of the position of tunicates proposed by recent phylogenomic studies (Bourlat et al, 2006;Delsuc et al, 2006;Dunn et al, 2008) by concluding in favor of the monophyly of Olfactores, has not yet been considered as totally convincing, essentially because it is at odds with both the traditional view based on embryological and morphological characters (Rowe, 2004;Schaeffer, 1987), and with earlier molecular phylogenetic analyses based on rRNA Mallatt and Winchell, 2007;Swalla et al, 2000;Wada and Satoh, 1994;Winchell et al, 2002). The unexpected sister-group relationship between echinoderms and cephalochordates observed in one of these studies may also have suggested the possibility that the monophyly of Olfactores was due to an artefactual attraction of cephalochordates with echinoderms (Bourlat et al, 2006).…”

“…The sister-group relationship between tunicates and vertebrates (Olfactores) observed in phylogenomics is in conflict with most (if not all) analyses of rRNA which favor cephalochordates as the closest relatives of vertebrates (Euchordates) Mallatt and Winchell, 2007;Swalla et al, 2000;Wada and Satoh, 1994;Winchell et al, 2002). However, the statistical support for Euchordates in rRNA-based phylogenetic studies is moderate.…”

“…However, the statistical support for Euchordates in rRNA-based phylogenetic studies is moderate. Indeed, the first 18S rRNA study, based on a limited taxon sampling of deuterostomes, reported a bootstrap value of only 45% for Euchordates (Wada and Satoh, 1994). A subsequent 18S rRNA study considering only slowly evolving sequences for 16 deuterostomes found only a moderate bootstrap support of 71% for grouping cephalochordates and vertebrates .…”

Chordate evolution and the three-phylum system

“…On the other hand, recent studies of deuterostome molecular phylogeny, nuclear and mitochondrial genomics, and evolutionary developmental biology, have unambiguously demonstrated that echinoderms and hemichordates form a clade, and that urochordates, cephalochordates and vertebrates form another distinct clade (figure 1b) [12,[38][39][40]. The former is called the Ambulacraria, with similarities in coelomic systems and larvae [41], and the latter Chordata.…”

Developmental biology of pterobranch hemichordates: History and perspectives

“…First, it is now apparent that hemichordates and echinoderms form a clade, the Ambulacraria, which is sister to the chordates (Bourlat et al, 2003;Halanych, 1995;Turbeville et al, 1994;Wada and Satoh, 1994;Winchell et al, 2002). This conclusion does not, however, reveal the nature of the ancestral hemichordate, or indeed ancestral ambulacrarian.…”