S9-1.3 Saturday, Jan. 7 Origin, Evolution & Development of the Chordates: Notochord gain or retention? SWALLA, B.J.*; Univ. of Washington firstname.lastname@example.org
Hemichordates and echinoderms are crucial for understanding the evolution of the vertebrate central nervous system (CNS), deuterostome evolution and chordate origins. Hemichordates share many chordate features, including a post-anal tail, gill slits, and a CNS. Morphological and developmental evidence suggests that hemichordates have a CNS that rolls up from the ectoderm via neurulation, but do not have a notochord. Homologs to key neural proteins and developmental genes have been found in the Saccoglossus kowalevskii genome; this available genomic information was used to clone specific genes, and make RNA probes for in-situ hybridization on the Pacific hemichordate species S. bromophenolosus and an indirect developing Pacific species, Ptychodera flava. Results showed unusual complexity in the hemichordate nervous system with a set of cells expressing neuronal markers surrounding the stomochord and heart/kidney complex, but no recognizable notochord. The results suggest that the deuterosome ancestor was a complex benthic worm, with gill slits, a cartilaginous skeleton, and a CNS. Notochord loss in echinoderms and hemichordates is as parsimonious as notochord gain in the chordates. We are now investigating this question with genomics and development gene expression studies. We thank the NSF #DEB-0816892 for funding.