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MBE Advance Access originally published online on November 9, 2005
Molecular Biology and Evolution 2006 23(3):541-549; doi:10.1093/molbev/msj055
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© The Author 2005. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

Research Article

Evolution and Development of the Chordates: Collagen and Pharyngeal Cartilage

Amanda L. Rychel*,{dagger}, Shannon E. Smith*, Heather T. Shimamoto* and Billie J. Swalla*,{dagger}

* Center for Developmental Biology and Biology Department, University of Washington, Seattle; and {dagger} Friday Harbor Laboratories, University of Washington, Friday Harbor

E-mail: bjswalla{at}u.washington.edu.

Chordates evolved a unique body plan within deuterostomes and are considered to share five morphological characters, a muscular postanal tail, a notochord, a dorsal neural tube, an endostyle, and pharyngeal gill slits. The phylum Chordata typically includes three subphyla, Cephalochordata, Vertebrata, and Tunicata, the last showing a chordate body plan only as a larva. Hemichordates, in contrast, have pharyngeal gill slits, an endostyle, and a postanal tail but appear to lack a notochord and dorsal neural tube. Because hemichordates are the sister group of echinoderms, the morphological features shared with the chordates must have been present in the deuterostome ancestor. No extant echinoderms share any of the chordate features, so presumably they have lost these structures evolutionarily. We review the development of chordate characters in hemichordates and present new data characterizing the pharyngeal gill slits and their cartilaginous gill bars. We show that hemichordate gill bars contain collagen and proteoglycans but are acellular. Hemichordates and cephalochordates, or lancelets, show strong similarities in their gill bars, suggesting that an acellular cartilage may have preceded cellular cartilage in deuterostomes. Our evidence suggests that the deuterostome ancestor was a benthic worm with gill slits and acellular gill cartilages.

Key Words: evolution and development • chordate evolution • tunicates • cephalochordates • hemichordates • cartilage • collagen


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