The establishment of endosymbiosis: An experimental and computational investigation using the artificial symbiont Synechocystis

Ancient evolutionary events are difficult to study because their current products are derived forms altered by millions of years of adaptation, which obscures how these key transitions occurred. The primary endosymbiotic event formed the first photosynthetic eukaryote; this radiated to become plants and algae, and has therefore had vast consequences for life on earth. Modelling, if grounded correctly with biology, enables events such as this that cannot directly be studied, to be inferred by building analogous simulations. This project applies metabolic modelling, using flux balance analysis (FBA), to this evolutionary transition to study the metabolic adaptations necessary for the symbiont to transition from free-living to endosymbiotic.

The model was validated for the bacterial symbiont in isolation by testing the biomass composition, the nutrient limitation predictions, and the predictions for growth rates and metabolic fluxes in different environmental conditions. Subsequently the model was applied to a symbiotic state. Within the symbiotic state, a carbon compensation cost was used to manipulate the behaviour of the symbiont and therefore made it possible to compare the metabolism between a ‘selfish’ and a cooperative symbiont. These behaviour types are analogous to the expected difference between a newly established endosymbiosis that is governed by conflict and a well-developed one that has reached a mutual-benefit state. This project was able to develop predictions for these key states within the endosymbiotic context. Overall, the results show the applicability of FBA modelling to those ancient evolutionary transitions that were driven by metabolic exchanges, like endosymbioses.