Evolution of deuterostome axial patterning and nervous systems: Insights from hemichordates

Vertebrate origins have long fascinated biologists, but the large number of vertebrate novelties and dearth of uncontroversial homologies with other animals have made it challenging to test hypotheses. These issues have been especially acute for reconstructing early vertebrate brain evolution. Debates on brain origins have recently been invigorated by data suggesting possible gene regulatory innovations associated with novel brain structures, along with molecular studies concluding extensive homologies between vertebrate and protostome nervous systems.;Here, I first utilize the hemichordate Saccoglossus kowalevskii as a system to characterize the origins of three vertebrate brain signaling centers, the anterior neural ridge, zona limitans intrathalamica and isthmic organizer, that regulate brain anteroposterior patterning. Homologous centers are absent or divergent in amphioxus and ascidians, which led to the hypothesis that they are vertebrate novelties possibly associated with morphological innovations of the vertebrate brain. However, I present descriptive and functional evidence that homologous signaling center- like genetic programs predate vertebrates and regulate ectodermal anteroposterior patterning in S. kowalevskii embryos. My findings suggest these mechanisms were ancestral to deuterostomes, but degenerated in invertebrate chordates, and are not strictly coupled to specific CNS structures.;Hemichordate neuroanatomy has featured prominently in theories on bilaterian nervous system evolution, but nervous system organization in adult hemichordates remains uncertain. Here, I use molecular techniques to identify distributions of neurons and neuronal cell types in late juvenile and adult S. kowalevskii. The nervous system is highly regionalized and includes a circumferential neural center in the proboscis base, an internalized, dorsal nerve cord in the collar, dorsal and ventral trunk nerve cords, and nerve rings in the anterior and posterior collar. I do not find strong support for CNS homologies between S. kowalevskii and other animals.;Patterning mechanisms provide another way to compare nervous systems across diverse animals, and perhaps to assess anatomical homologies. I found that homologs of many genes that regulate vertebrate anteroposterior CNS patterning are expressed in similar arrangements in the S. kowalevskii late juvenile and adult nervous system, including domains in circumferentially organized proboscis and collar CNS regions without proposed chordate homologs. Adult hemichordates also provide a system to test the theory that there was a dorsoventral body inversion in chordates linked with inversion of a BMP-directed molecular axis. Dorsoventral patterning could also help resolve potential affinities between the hemichordate dorsal and ventral trunk nerve cords and the single nerve cords of other bilaterian models systems.;I found homologs of genes involved in dorsoventral patterning in the vertebrate CNS are often expressed in either the dorsal or ventral nerve cord in the S. kowalevskii trunk, and give little support for homologies with protostome or chordate nerve cords. These data do not strongly support a dorsoventral axis inversion in stem chordates and suggest that some aspects of the anatomical and molecular dorsoventral axes may not have been tightly linked during deuterostome evolution. If the hemichordate collar cord and chordate CNS are homologous it would indicate that two main features of the `inverted' chordate body plan, a dorsal nerve cord and ventral mouth, were already present in stem deuterostomes despite an opposite BMP- Chordin axis in embryos. More broadly, my data indicate that conserved genetic mechanisms can be loosely coupled with anatomies, which illustrates challenges with using molecular data to reconstruct ancestral morphologies or draw conclusions on anatomical homologies. These data also highlight the critical importance of morphologically divergent groups for unraveling the relationships between developmental genetic and anatomical characters across phyla.