Elucidating molluscan evolution by single cell sequencing

Important questions in evolutionary developmental biology are how body plans develop and how animal diversity has evolved. Inferring how the last common ancestor of all bilateral symmetrical animals looked alike is still controversial, also due to unresolved phylogenetic relationships, in particular among the Lophotrochozoa. This bilaterian superphylum comprises diverse groups such as mollusks (e.g. snails & cephalopods) and annelid worms but it lacks animals that have been extensively studied by means of molecular tools, so called model organisms. Sole exception is the annelid Platynereis dumerilii that possesses certain cell types that have been recently inferred to be of the same evolutionary origin as neurons found in the vertebrate brain. This project focuses on the identification of evolutionary conserved cell types in the Mollusca, the largest lophotrochozoan clade. Polyplacophoran mollusks exhibit ancestral lophotrochozoan traits such as a trochophore larva, molluscan traits like a shell and a foot, as well as polyplacophoran characteristics such as eight shell fields giving rise to eight shell plates. They also possess the most uncentralized, cord-like, nervous system among mollusks. This project addresses the following research questions: (I) What is the evolutionary origin of the molluscan shell fields? (II) Did the last common bilaterian ancestor possess a complex or simple nervous system? To address these questions the applicant will analyze the anatomy of cell types and gene combinations that are active, i.e. expressed, in cells of the polyplacophoran shell fields and nervous system. A cellular expression atlas of the trochophore larva of the polyplacophoran Acanthochitona crinita will provide details on expressed genes and neurotransmitters in the cells and about their anatomies. Subsequently, the polyplacophoran cell types will be compared with those found in the yet existing cellular expression atlas of the trochophore of the annelid P. dumerilii and with vertebrate cell types. A cellular expression atlas allows to sequence randomly picked cells from a dissociated larva and to map these back into their original location. This will eventually lead to a 3D model of an averaged larva that provides information on the single cell level about the anatomy and combinations of genes expressed. If single cells were evolutionarily conserved among mollusk, annelid, or vertebrate, then similar gene combinations would be expressed in these cells, in addition to similar cellular anatomical similarities. This project is innovative since it will allow for detailed comparisons between bilaterian cell types and organ systems. Tim Wollesen is applying for a Schrödinger fellowship and will be hosted by Detlev Arendt at the European Molecular Biology Laboratory in Heidelberg for two years. Afterwards he will return to the laboratory of Andreas Wanninger in Vienna for a year of project extension.