Development of transgenic and genome editing tools to study the role of Notch signalling in Hydractinia echinata

Abstract

Hydractinia is a colony forming marine invertebrate and a member of the phylum Cnidaria. Members of this genus have been models for developmental biologists for over a century and feature many characters of evolutionary significance, most notably a stem cell system called i-cells, the evolution of which is not well understood. Hydractinia is still a relatively new genetic model organism and so nearly any work involves the development of new tools. Here I present novel transgenesis tools in Hydractinia and discuss the implications of the successes and failures of this work on future work in Hydractinia. In particular I present, for the first time, genome editing via CRISPR-Cas9 in Hydractinia. Using this tool as well as classical pharmacological inhibition experiments I analysed the role of Notch signalling in Hydractinia. I show that Notch signalling plays a role in tentacle patterning, a function conserved across studied cnidarians. Additionally, I show a role for Notch signalling in nematocyte differentiation in Hydractinia, again a function which is conserved among cnidarians. Most interestingly, I show that Notch signalling is not required for early nervous system development or neural commitment in Hydractinia, a feature which apparently emerged within hydrozoans, as anthozoans and most studied metazoans appear to use Notch signalling in a similar manner during neurogenesis. I further discuss this loss in terms of the evolution of i-cells in hydrozoans and hypothesise that loss of Notch signalling during neurogenesis would have been a major event which facilitated the evolution of i-cells by removing the requirement for neural progenitors to emerge and exist in an epithelial context.