alignments

^{*Department of Biological Sciences, University of Alberta, Edmonton, AB T6G 2E9, CANADA
†Station Biologique, BP°74, 29682 Roscoff Cedex FRANCE
‡Department of Biological Sciences, University of South Florida, Tampa, FL 33620-5150
§Zoology Department, University of Washington, Seattle, WA 98195}

Abstract. - The deuterostome phyla include Echinodermata, Hemichordata and Chordata.  Chordata is comprised of 3 sub-phyla, Vertebrata, Cephalochordata (Branchiostoma) and Urochordata (Tunicata).  Careful analyses 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.

Figure 1.  Photographs of the adults of the hemichordate species represented in this study. a) Ptychodera bahamensis b) Harrimania planktophilus c) Cephalodiscus gracilus individuals d) Cephalodiscus gracilus colony. Our results suggest that members of the family Ptychoderidae (a), form one clade of Enteropneusta, while the family Harrimanidae (b) plus Pterobranchia (c and d) form another.

Figure 2. Phylogenetic Tree of  the deuterostomes when sequences with similar evolutionary rates (16 taxa, ? 0.12 substitutions/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. Analyses of hemichordate phylogeny: Branches are drawn to scale (Kimura 2 parameter distances) to emphasize the potential for artifacts due to unequal rate effects.  The same topology was obtained from NJ with Kimura 2 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 to the right.