Christopher J. Lowe
Assistant Professor, Hopkins Marine Station, Stanford University
A major finding from developmental biology has been that a highly conserved regulatory network of developmental genes is involved in early patterning and regionalization of the early neural plate to form the brain and central nervous system of all vertebrates. While parts of this network are conserved throughout almost all animals, much of it has been considered to be unique to vertebrates, and this conservation perhaps even facilitated the evolution of this complex structure. The conclusions from these studies were reached largely because more basal chordates, such as amphioxus and ascidians, have failed to exhibit such complex transcriptional regulation during the development of their anterior nervous systems, suggesting this was a later chordate innovation. My work has challenged this hypothesis: we found exquisite conservation in the relative expression domains of the transcription factors that define the three regions of the vertebrate brain during the development of the hemichordate Saccoglossus kowalevskii. This implies an ancient, rather than recent origin of this regulatory architecture. We have also recently extended this analysis to similarities in not only transcriptional regulation, but also signaling domains, critical in defining specific regions of the vertebrate brain. In vertebrates, division of the brain into functional domains is achieved by the action of localized regions of the brain that secrete diffusible proteins that act as morphogens. This method of nervous system patterning was again thought to have evolved in parallel with the evolution of a complex vertebrate brain. Our work suggests that this developmental strategy evolved much earlier in deuterostome history, and that much of the genetic complexity of vertebrate brain patterning was already in place before chordates branched off from the main deuterostome lineage.
Evolution of endomesoderm
We have begun a new project investigating the molecular similarities between hemichordates and vertebrates on how the endoderm and mesoderm are induced and patterned. Our preliminary data have identified a secreted protein that seems to play an important role in inducing the formation of mesoderm. This protein, FGF8, also plays a critical role in vertebrate mesoderm induction. We will fully characterize the role of this protein during mesoderm development in hemichordates, which will give insights into early evolution of deuterostome mesoderm induction.
After induction in vertebrates, the anterior, or head mesoderm, is distinguished from the trunk mesoderm by a distinctive developmental program. The anterior program has always been considered to be an innovation of the vertebrates. However, we have begun to find evidence of similarities in patterning between the anterior mesoderm of hemichordates and vertebrates. We will further investigate these similarities, both functionally and descriptively, and establish the degree of molecular patterning similarities between the two groups.
The other major deuterostome phylum is the echinoderms. This group has a very distinctive adult body plan. Unlike bilateral hemichorates and chordates, the adult is organized around a pentaradial body plan. Our understanding of how this body plan evolved from its bilaterian ancestors is not known. I have begun a large-scale genomic project in several species of echinoderms to investigate the molecular basis of body plan formation in this group. I hope this approach will help determine how the anteroposterior and dorsoventral axes have been modified during the evolution of this group.