Functional Consequences of Being Multicellular: Predation by Protozoans on Unicellular vs. Multicellular Choanoflagellates

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Animal bodies are made up of many cells, but the ancestors of animals were single-celled organisms ("protozoans"). A pivotal step in the evolution of life was the transition from being unicellular to becoming multicellular. The closest protozoan relatives of animals are aquatic "choanoflagellates", which are unicellular but can also form multicellular colonies. For multicellularity to evolve, colonies of animal ancestors must have performed functions affecting their survival better than did solitary cells. This study examines whether unicellular or multicellular choanoflagellates are better able to avoid being eaten by protozoan predators, and the mechanisms involved. This project at the interface between biology and physics (hydrodynamics of how aquatic predators and prey interact) will give teams of undergraduate students from different majors hands-on research experience and will involve them in interdisciplinary research, which is important because many future discoveries in science and technology will be made at interfaces between different fields. Public outreach includes an interactive online site where citizen scientists can calculate the predation rates on unicellular and multicellular choanoflagellates by different types of predators. Work with women and dyslexics by the PIs will encourage success in science and technology fields. A novel microfluidic system will be developed to determine how performance and mechanisms of predator-prey interactions (usually studied in still water) are affected by realistic patterns of fluctuating shear that microscopic swimmers experience in natural, turbulent water flow. This system will be useful for studying other aquatic microoganisms interacting with each other in nature. Protozoans, both unicellular and colonial, play important roles in aquatic food webs. Chonaoflagellates, the closest protozoan relatives of animals, will be used to study mechanisms underlying resistance to predation by uni- vs. multicellular protozoans. Data about choanoflagellates will also enable informed inferences about the possible role of predation as a selective factor in the evolution of multicellularity. The origin of multicellular animals from unicellular protozoans represents a pivotal transition in life's history. This study focuses on resistance to being eaten by protozoan predators (before animals evolved, predators on chonaoflagellates were other protozoans). Research goals are to: 1) quantify performance differences between unicellular and colonial choanoflagellates in avoiding predation by protozoan predators using different modes of prey capture; 2) elucidate fluid dynamics of interactions between predators and choanoflagellate prey to identify mechanisms underlying performance differences; 3) determine how fluctuating shear encountered by protozoans carried in turbulent water flow in nature affects interactions of unicellular vs. colonial prey with predators. High-speed videomicrography of predators, prey, and flow-marking microbeads will record interactions of choanoflagellates with different protozoan predators. Digitized protozoan trajectories enable calculation of encounter and predation success rates, and behavioral interactions. Particle tracking velocimetry will quantify instantaneous flow fields produced by predators and prey, and hydrodynamic signals during their interactions. A novel contribution is development of microfluidic techniques to determine how mechanisms of predator-prey interactions (usually studied in still water) are affected by realistic patterns of fluctuating shear that microscopic swimmers experience in turbulent ambient water flow.