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, 97 (9), 4469-74

Evolution of the Chordate Body Plan: New Insights From Phylogenetic Analyses of Deuterostome Phyla

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Evolution of the Chordate Body Plan: New Insights From Phylogenetic Analyses of Deuterostome Phyla

C B Cameron et al. Proc Natl Acad Sci U S A.

Abstract

The deuterostome phyla include Echinodermata, Hemichordata, and Chordata. Chordata is composed of three subphyla, Vertebrata, Cephalochordata (Branchiostoma), and Urochordata (Tunicata). Careful analysis of a new 18S rDNA data set indicates that deuterostomes are composed of two major clades: chordates and echinoderms + hemichordates. This analysis strongly supports the monophyly of each of the four major deuterostome taxa: Vertebrata + Cephalochordata, Urochordata, Hemichordata, and Echinodermata. Hemichordates include two distinct classes, the enteropneust worms and the colonial pterobranchs. Most previous hypotheses of deuterostome origins have assumed that the morphology of extant colonial pterobranchs resembles the ancestral deuterostome. We present a molecular phylogenetic analysis of hemichordates that challenges this long-held view. We used 18S rRNA to infer evolutionary relationships of the hemichordate classes Pterobranchia and Enteropneusta. Our data show that pterobranchs may be derived within enteropneust worms rather than being a sister clade to the enteropneusts. The nesting of the pterobranchs within the enteropneusts dramatically alters our view of the evolution of the chordate body plan and suggests that the ancestral deuterostome more closely resembled a mobile worm-like enteropneust than a sessile colonial pterobranch.

Figures

Figure 1
Figure 1
Photographs of the adults of the hemichordate species represented in this study. (a) Ptychodera bahamensis; (b) Harrimania species; (c) Cephalodiscus gracilus individuals; (d) Cephalodiscus gracilus colony. Our results suggest that members of the family Ptychoderidae (a) form one clade of Enteropneusta, whereas the family Harrimanidae (b) plus Pterobranchia (c and d) form another.
Figure 2
Figure 2
Phylogenetic tree of the deuterostomes when sequences with similar evolutionary rates (16 taxa ≤0.12 substitutions per site) were analyzed with gambit. Key characters are mapped to the deeper nodes. The deuterostomes form two great clades, one containing the hemichordates and echinoderms and the other made up of urochordates and chordates (cephalochordates and vertebrates). Major differences in adult body plan between Cephalochordata + Vertebrata (myotomes) and Urochordata (tunic) are marked. These results, combined with morphological data, suggest that Chordata should be restricted to Cephalochordata + Vertebrata and that Urochordata is an independent phylum and the sister group to Chordata. Note that the tripartate coelom of hemichordates is considered homologous to the three pairs of echinoderm coeloms.
Figure 3
Figure 3
Analyses of hemichordate phylogeny. Branches are drawn to scale (Kimura two-parameter distances) to emphasize the potential for artifacts because of unequal rate effects. The same topology was obtained from NJ with Kimura two-parameter distances (bootstrap values above each branch), gambit paralinear distances with correction for site-to-site variation (bootstrap values below each branch), and gambit MP (bootstrap values to the right of each branch). See text for details. Hemichordate classes (bold) and families are indicated (Right). S. barkleyii and H. planktophilus are undescribed species (33).

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