Revealing ancestral bilaterian cell types in chaetognaths

The sheer diversity of animals with manifold body plans has fascinated generations of scientists. This diversity is also reflected in cell types that constitute organ systems. Arrow worms form an animal clade which has been neglected in developmental and evolutionary studies. These torpedo-shaped marine organisms include some species growing up to 120 millimeters and make up a huge portion of the marine plankton, feeding other organisms such as fishes and whales. Arrow worms are translucent predators that seize prey such as crustaceans with their characteristic jaw apparatus (scientific name: Chaetognatha = bristle-jaws). Until today, their evolutionary relationship to other organisms has been debated. Accordingly, they have been affiliated with very different animal clades due to similarities with regard to their anatomy and genetic makeup. Within the frame of this FWF-project and in collaboration with scientists of the Université Libre de Bruxelles in Belgium the project leader Tim Wollesen and his team will study cell type diversity during the development of the chaetognath Spadella cephaloptera. Modern single-cell sequencing experiments allow for a characterization and localizing of virtually every single cell type in the developing organisms based on shared ribonucleic acids in the individual cells. This powerful experimental approach has been performed on comparably few organisms and allows for the identification of cell types and organs and a subsequent unbiased comparison with cell types of other organisms. Wollesen and his team aim to elucidate which nerve cells of arrow worms are related to nerve cells of other animals. In addition, it is studied if cell types forming the jaw apparatus are closely related to those of other animals that also exhibit similar hard parts such as the rasping tongue of snails. This project will lead to important insights into the evolution and development of arrow worms and animals in general.