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Susie Bassham    Graduate Student Institute of Neuroscience 1254 University of Oregon Eugene, OR 97403-1254 USA Phone: 541-346-4495 / Fax 541-346-4548 sbassham@oregon.uoregon.edu http://evodevo.uoregon.edu/people/Bassham.html

Project Summary

A diverse assortment of animals belong to phylum Chordata. These are the vertebrates, the cephalochordates (amphioxus or lancelets) and the urochordates (also called tunicates, such as sessile seasquirts, and planktonic salps and larvaceans). Non-vertebrate chordates are generally small filter-feeders that lack the big brains and enhanced sensory structures enjoyed by vertebrates. Nevertheless, at some point during ontogeny, each chordate expresses a set of characters that unifies the phylum: a notochord, a perforated pharynx (gill slits), a dorsal nerve cord, and a post-anal tail.

Given that vertebrates are distinguished from their fellow chordates by some striking morphological differences, we'd like to know how vertebrates evolved and what was the nature of their chordate ancestor. In particular, we would like to understand 1) what kinds of structures might have been the precursors of the vertebrate morphological innovations which derive largely from placodes and neural crest tissues during development, and 2) what was the ancestral state of the vertebrate genome? Studying the development and genome structure of extant non-vertebrate chordates can help us make inferences about the chordate common ancestor. 

The urochordates are important for our understanding of this ancestor as they represent the only chordate outgroup to the cephalochordate-vertebrate clade. Since Garstang proposed his model for the origin of the vertebrates in 1928 (i.e. by paedomorphosis, or precocious sexual maturation in the larva of a sessile, ascidian-like ancestor), research attention has focused mainly on the ascidians as a model for Urochordata and for the chordate ancestral condition. Recent molecular phylogenies, however, place another urochordate class, the larvaceans, at a basal position in the urochordates. Larvaceans are tadpole-shaped as adults, while ascidians undergo a radical metamorphosis to a sessile adult that little resembles the chordate body plan of its larva. Consequently, some researchers (pre- and post-Gastang) have proposed that ascidians are derived, while larvaceans, cephalochordates and vertebrates retained their free-swimming body plan from a motile common ancestor.

I am approaching these large evolutionary questions by studying the molecular embryology of the larvacean Oikopleura dioica. In the relatively simple embryos of O. dioica, I am analyzing the expression of genes important for development of the brain and placodal structures in vertebrates. I have also analyzed the expression of a brachyury (T) orthologue, a gene important for notochord development, and found that the O. dioica gene shares a hindgut expression domain with embryos such as echinoderms, hemichordates, and arthropods, which are outgroups to the Chordata. This surprising pattern, which is different from the expression reported in ascidians, may reflect a deeper, bilaterian ancestry of T in patterning the hindgut.