SWALLA, B.J.; University of Washington, Seattle: Evolution of the Chordates: Worms or Squirts?

The Deuterostomia are a monophyletic group of animals that diverged from the Ecdysozoa and Lophotrochozoa some time before the Cambrian period, as all of the four major clades of deuterostomes are present in the Cambrian fauna. The four major clades of deuterostomes are echinoderms, hemichordates, urochordates (tunicates) and chordates (vertebrates and amphioxus). Echinoderms and hemichordates are sister groups as has been shown by rRNA phylogenies, mitochondrial data and larval morphology, but the monophyly of the chordates and urochordates is hard to recover by molecular analysis. Using developmental and morphological characters to compare the four major deuterostome groups it appears that echinoderm and urochordate adult body plans appear to have become highly derived at the time of their divergences from the other major clades. Previous theories of chordate origins propose that chordates evolved from a sessile urochordate tadpole larva. However, the deuterostome ancestor was likely to have been a motile, benthic, filter-feeding worm with gill slits, an organized cartilaginous skeleton, coelom, and perhaps even neural crest cells. Therefore, the chordate ancestor was likely to have been a motile worm, instead of a sessile urochordate. This scenario would suggest that tunicates evolved from a motile worm-like chordate ancestor by dissociation of the ancestral adult and larval developmental modules. The non-feeding larva contains the notochord and the gut and adult organs are all developed after metamorphosis. In this scenario, the deuterostome and chordate ancestors are similar, except the chordate ancestor would be expected to have somites. Ongoing experiments being done in my lab to test these hypotheses will be discussed.