Origins of the chordate bodyplan and mode of development will be sought in the hemichordates, the little studied sister taxon (with echinoderms) of chordates. Hemichordates possess: 1) a short stomacord which forms from the gut roof and contains vacuolated cells like a chordate notochord or prechordal plate, 2) a dorsal nerve cord formed by neurulation like a chordate dorsal hollow neural tube, 3) a long array of chordate-like gill slits, and 4) a transient post-anal tail in the juvenile of some species, like a chordate tail. They may also have left-right asymmetries like those of chordates. Except for the gill slits, none of these traits has been convincingly identified as chordate-like by anatomical scrutiny, and hemichordates also share traits with annelids and arthropods, such as a ventral nerve cord besides their dorsal cord. We are obtaining cDNA sequences from Saccoglossus kowalevskii, an enteropneust hemichordate, for in situ hybridization to assess hemichordate- chordate similarities more incisively. We find a set of chordate-homologous genes expressed in the dorsal midline of the hemichordate embryo (bmp2/4, twg) where the dorsal nerve cord forms and in the ventral midline (netrin, admp) where the ventral nerve cord forms. The animal's entire body wall contains an intraepidermal nerve net from which axons collect at the two midlines. It is like a chordate neural tube with bmp in the roofplate and netrin in the floorplate. We will continue to compare nervous systems and other traits. The three-part organization of hemichordates corresponds to the chordate anteroposterior axis in terms of expression domains of various homeobox genes (rtx, six3, pitx, otx, Hox3,4,9). The prosome is like the anterior head/ventral forebrain, the mesosome like the dorsal forebrain, midbrain and first branchial arch, and the metasome like the rest of the chordate body. These similarities are presumably conserved from a three-part common ancestor. Hemi-chordate development will be studied for the presence of one or more inductive parts of Spemann's organizer, which in chordates is involved in the development of all four specific traits. The stomochord region expresses otx and dkk genes, we find, as does the chordate head organizer. Knowledge of hemichordates will help to explain why chordates have certain aspects of their body plan and development, and by difference, what additions and modifications have been made only within the chordate line.
