Endosymbiosis is a phenomenon where one cell lives within another. This interaction can be beneficial for both partners. It can also be negative for one or -at times- both partners. As such, these interactions between cells can be unstable resulting in the interaction breaking down. Sometimes these interactions evolve so the partners become interdependent and inseparable. This can lead to the integration of one cell within the other. In these cases, the two cells have become one and they now have the same evolutionary destiny.
This process is incredibly important for evolution as it changes the characteristics of the interacting cells and creates novel evolutionary trajectories, allowing radical changes in cell function and organismal ecology to occur. This process has been fundamental for the evolution of cellular complexity and, indeed, played a critical role in the evolution of eukaryotes, a process that eventually led to the evolution of plants, fungi, and animals (including humans). There was probably no more important evolutionary transition in the history of life.
We understand very little about how endosymbiotic interactions can arise, and which cellular mechanisms underpin an interaction to allow such systems to become stable. The aim of this project was to develop an experimental system for identifying the cell biology that controls an endosymbiotic interaction, allowing two different species to manifest a long-term and intimate interaction. To do this we focussed on a microbe, a single-cell protist, called Paramecium bursaria, found in most ponds around the world, which can incubate hundreds of photosynthetic green algal microbes within its cell. This is an excellent system for understanding endosymbiotic interactions because the interaction has yet to become an obligate interaction, which means we can experimentally manipulate both partners separately and together. These organisms form an endosymbiotic partnership, exchanging metabolites and providing other functions important for their collective ecology.
The objective of this project was to develop approaches to understand how stable endosymbiotic interactions come to be and use these to identify the cellular systems that control endosymbiosis. Using our new approaches, we identified: the cellular systems that enabled host to recognise their endosymbiotic partners, the proteins that control metabolic trade between host and endosymbiont, the proteins that constitute and control the compartment that hosts the endosymbiotic partner cells, and how both partners can act to punish each other. Importantly, because our approach identified the genes that control this process, we then used phylogenomic analysis to investigate how these genes evolved so we understand the evolutionary changes that underpinned the evolution of an endosymbiotic systems.
This work allows us to compare and contrast different endosymbiotic systems and understand how such interactions can evolve. The work also provides us with the experimental capabilities to modify and perturb these interactions, providing understanding of the cellular and ecological factors that drive these important symbioses.
