UNCOVERING MOLECULAR BLUEPRINTS OF THE UNIQUE TARDIGRADE BODY

Embryonic development guides the transformation from a single cell to a complex multicellular organism. How development has been tuned through evolutionary history to form diverse tissue and organism shapes is poorly understood. Stepping back in evolutionary time to observe what changes made to developmental processes led to evolution of new species is impossible, yet hypotheses about the evolution of development can be formed using comparisons made between existing organisms, with reference made to their likely evolutionary relationships. This comparative process helps inform the evolution of many developmental processes, including cell fate acquisition, body patterning, tissue morphogenesis, and ultimately adult body development. In this dissertation, I present my work uncovering important aspects of development in an emerging model system, the tardigrade Hypsibius exemplaris, with the goal that the information I reveal will provide useful points of comparison for building our understanding of the evolution of diverse species.Primordial germ cells (PGCs) are an essential cell type for many organisms, yet there is limited knowledge of where PGCs originate in most species. Identifying where PGCs originate in embryos is a key step towards understanding how they arise during development. In Chapter 2, I present my work uncovering the embryonic origin of PGCs in the tardigrade H. exemplaris. I also present an updated lineage of early H. exemplaris embryos, which is a resource for the tardigrade community. This work lays the foundation for future research to explore how PGCs are specified in this animal.Development draws from a small number of conserved signaling pathways to guide patterning of diverse organisms. Harnessing this conservation, I explored gene expression of the BMP and FGF pathways in the tardigrade H. exemplaris (Chapter 3). In combination with what is already known from well-studied model organisms, I generated hypotheses of functions of BMP and FGF in H. exemplaris development. Testing these hypotheses will help inform how conserved pathways are deployed in unique ways to guide development of different species.This work adds to our understanding of the molecular organization of developing tardigrades, which in comparison with organization of differently-shaped organisms, will help inform the evolution of diverse forms.