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.

Marine animals show an astonishing diversity in how their bodies are built and how they function, yet many of the underlying biological mechanisms are surprisingly similar across species. This project focuses on a little-known phylum of marine organisms called chaetognaths, or arrow worms, which provide unique insights into how complex traits such as nervous systems, body patterning, and attachment mechanisms evolved. One major part of this project investigates how nervous systems form during chaetognath development. By studying gene activity in early embryos and juveniles, we found that the same genes used to build nervous systems in humans and other animals are also active in the chaetognath Spadella. This suggests that key aspects of nervous system development are ancient and were already present in early animal ancestors. At the same time, arrow worms also show unique features, helping us understand how nervous systems diversified over evolutionary time. We also explored how the head-to-tail body axis is established and showed that ancestral genetic patterns are shared with other animals, while Spadella also displays lineage-specific innovations. This helps clarify how fundamental body plans evolved and diversified. Using modern techniques such as single-cell RNA sequencing, genomics, and proteomics, we were also able to identify and characterize many different cell types in these animals. This includes cells involved in attachment, the nervous, sensory or muscle systems. Notably, we were able to describe for the first time which cell types are present during an early embryonic stage, the gastrula, and how tissues such as muscles and the nervous system originate. We identified specific cell types which are part of the chaetognath excretory system which has not been identified before. Spadella lives in marine environments with strong currents, and staying attached is key for survival. We discovered that Spadella uses specialized cells and a glue composed of a complex mixture of biological molecules, including sugars and proteins, to temporarily stick to surfaces. These adhesive systems show similarities to those found in other marine organisms, even though they evolved independently, highlighting how evolution can arrive at similar solutions to common challenges. Overall, the outcome of this project demonstrates that arrow worms combine ancient, conserved biological mechanisms with unique adaptations. By studying them, we gain a better understanding of how animal body plans, nervous systems, and functional traits originated and evolved, insights that are relevant not only for evolutionary biology but also for fields such as biomaterials and biotechnology.