Evolution of the chordate body plan: New insights from phylogenetic analyses of deuterostome phyla

The deuterostome phyla include Echinodermata, Hemichordata. and Chordata. C hordata is composed of three subphyla. Vertebrata, Cephalochordata (Branchi ostoma), and Urochordata (Tunicata). Careful analysis of a new 18S rDNA dat a set indicates that deuterostomes are composed of two major clades: chorda tes and echinoderms + hemichordates. This analysis strongly supports the mo nophyly of each of the four major deuterostome taxa: Vertebrata + Cephaloch ordata, 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 mo rphology of extant colonial pterobranchs resembles the ancestral deuterosto me. We present a molecular phylogenetic analysis of hemichordates that chal lenges this long-held view. We used 18S rRNA to infer evolutionary relation ships of the hemichordate classes Pterobranchia and Enteropneusta. Our data show that pterobranchs may be derived within enteropneust worms rather tha n being a sister clade to the enteropneusts. The nesting of the pterobranch s within the enteropneusts dramatically alters our view of the evolution of the chordate body plan and suggests that the ancestral deuterostome more c losely resembled a mobile worm-like enteropneust than a sessile colonial pt erobranch.