Anteroposterior patterning in the enteropneust Schizocardium californicum [electronic resource] : molecular insights into the evolution of distinct larval and adult body plans in hemichordates
Keywords: Biology.

Many marine invertebrates have complex life cycles in which embryogenesis gives rise to a planktotrophic larva with its own body plan, distinct from the adult. In the phylum Hemichordata, both direct and indirect developing species occur. While data from the direct developing enteropneust Saccoglossus kowalevskii have provided insights into the evolution of the hemichordate adult body plan, comparatively little is known about the development and evolution of the tornaria larva of indirect developing species. In this dissertation, I establish the indirect developing hemichordate Schizocardium californicum as a new system for molecular developmental studies, and explore the evolution of the hemichordate larval body plan by investigating the molecular mechanisms that regulate development of its tornaria larva. In Chapter 1, I describe the information necessary to use Schizocardium californicum as a model system for developmental studies. I describe methods for adult collecting, induction of spawning and larval rearing in Schizocardium californicum, and provide a detailed description of embryonic development, larval development and metamorphosis, using light microscopy, immunolabeling and confocal microscopy. In Chapter 2, I ask whether the differences between larval and adult body plans in hemichordates are associated with fundamental changes in axial patterning. I characterize the expression patterns of a large number of transcription factors with conserved roles in deuterostome anteroposterior patterning throughout development of Schizocardium californicum. Using expression of these transcription factors to identify homologous body regions between larva and adult, I show that most of the ectoderm of the tornaria larva has an anterior identity, and lacks a body region equivalent to the adult trunk. Addition of a Hox-patterned trunk to this anterior larval territory prior to metamorphosis ensures the transition between larval and adult body plans. This work identifies a large-scale molecular correlate of a difference between larval and adult body plans, and demonstrates that modulating the timing of Hox expression may have played an important role in the evolutionary transition between direct and indirect development in hemichordates. In Chapter 3, I explore a potential cause of the delay in trunk development in Schizocardium californicum by characterizing the expression patterns of components of the Wnt signaling pathway. These results show that unlike Saccoglossus kowalevskii, early activation of Wnt signaling is insufficient to activate expression of trunk patterning genes. Together, these studies establish Schizocardium californicum as a new system for molecular developmental studies, with the potential to rapidly expand our knowledge of larval evolution in hemichordates, and identify large-scale differences in the molecular architecture of hemichordate larval and adult body plans. These results provide insights into the evolution of the body plan of the tornaria larva, and improve our ability to compare developmental data from hemichordates with other phyla.