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

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

^*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; and ^§Zoology Department, University of Washington, Seattle, WA 98195

Edited by Walter M. Fitch, University of California, Irvine, CA, and approved February 24, 2000 (received for review January 12, 2000)

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.

Footnotes

↵ ¶ To whom reprint requests should be addressed. E-mail: bjswalla{at}u.washington.edu.
This paper was submitted directly (Track II) to the PNAS office.
The sequences reported in this paper have been deposited in the GenBank database (accession nos. AF236798, AF236799, AF236800, AF236802, and AF236803).
Abbreviations:

MP,

maximum parsimony;

ME,

minimum evolution;

NJ,

neighbor joining;

ML,

maximum likelihood