TRANSITIONS · Key transitions in the evolution of endosymbioses

Endosymbiosis merges organisms and leads to evolutionary innovation. Hence endosymbiotic interactions play key roles in ecosystems and have shaped eukaryotic evolution, e.g., the formation of mitochondria and chloroplast organelles. The evolution of endosymbioses entails two key transitions: the transition from free-living to endosymbiont and from endosymbiont to organelle. These critical phases are difficult to study because extant relationships are often highly derived, and thus substantial knowledge gaps exist.Recent advances in technology make it now possible to employ valuable, but previously underutilised, protist systems in endosymbiosis research. This proposal will exploit the transitional nature of two protists: Paulinella chromatophora and the Paramecium bursaria–Chlorella association. First, I will combine partner-preference tests and single-cell approaches to study the mechanistic processes within an individual as it becomes endosymbiotic. Second, I will characterise metabolic integration during early-stage organelle formation using cutting-edge chemical imaging alongside a long-term evolution experiment. Finally, a year-long field survey will discover how changing environmental conditions influence the success of these partnerships in situ. My unique skillset of evolution experiments, single-cell genomics, metabolomics, and culturing these protists makes this ambitious proposal feasible.This project leverages transitional protists and pairs them with a powerful array of techniques to tackle fundamental questions underlying the key transitions within endosymbiosis. This original combination is at the forefront of endosymbiosis and plastid research. When completed, this proposal will provide unprecedented detail of the molecular mechanisms underpinning these transitions. As such it will fundamentally shift our perspective towards viewing these transitions as a gradient of processes rather than binary states, and will open up avenues for future research.