The origin of the body plan, major traits, and modes of development of chordates will be sought in the next grant period by our making a molecular developmental analysis of hemichordates, the phylum of bilateral animals that last shared an ancestor with chordates. Traits possessed by modern members of both phyla were probably possessed by the common ancestor and just brought forward in the chordate lineage, not evolved anew by chordates. Hemichordates have long beer thought to possess chordate-like features, but anatomical comparisons of the 2 groups, while suggestive, have not allowed definitive conclusions. We have found that hemichordates are remarkably Iike chordates in the anteroposterior dimension in terms of their map of 28 domains of ectodermal expression of orthologs of transcription factors which are important for neural patterning in chordates. Furthermore, hemichordates possess 4 signaling centers (secreted Wnts, Fgfs, Hh) along this dimension, similar to those in chordates, such as the anterior neural ridge and the isthmic organizer. The anteroposterior similarities prevail despite the fact that the hemichordate nervous system is diffuse throughout the ectoderm, not centralized as in chordates. In the dorsoventral dimension hemichordates possess 2 opposed signaling centers, 1 for Bmps and 1 for Chordin, as do chordates, but the orientation is more like that of protostomes such as Drosophila (Bmp dorsal;Chordin ventral). We propose to analyze hemichordate development further to determine the formation and function of the 4 anteroposterior signaling centers and 2 dorsoventral signaling centers in patterning the hemichordate embryo and to compare our findings with those on chordates to further elucidate ancestral developmental modes. Furthermore, the development of isolated ectoderm will be studied to see if it has a neural default circuitry or whether it is autonomously specified for neurogenesis, independent of Bmp. Then, the mesendoderm will be examined for the expression of orthologs of genes expressed in Spemann's organizer, which is a major developmental trait of chordates. siRNA interventions will be used to evaluate the role of the encoded gene products in development.
