English: Proposed model for the evolution of phototrophy in Paulinella chromatophora. Heterotrophic and phototrophic Paulinella species share a common bacterivorus predecedor (left). 1a A mixotrophic cell evolved by initially delaying digestion of a particular type of α-cyanobacteria and exploiting their photosynthetic ability. During this stage EGTs and HGTs were possible. The host started to insert transporters into the symbiont-surrounding membranes and target protein into the bacterial symbionts. 1b Heterotrophic Paulinella species did not acquire the ability to house cyanobacterial endosymbionts. 2 As the host cell continues to target proteins into the endosymbiont and regulate metabolic fluxes between the cytoplasm and the endosymbiont, it gains control over the endosymbiont’s growth and division. This underpins a stable vertical inheritance. Efficient metabolite exchange makes phagotrophy dispensable (although organic solutes might still be taken up from the environment) and functional constraints relax for hundreds of chromatophore genes leading to massive chromatophore genome reduction. Lysis of chromatophores becomes less frequent and the rate of EGT slows down. 3a,b Divergent evolution results in different phototrophic Paulinella species (represented by strain CCAC 0185 and FK01). Speciation is accompanied by continued chromatophore genome reduction that leads to differential gene losses (and probably differential EGTs) in CCAC 0185 vs. FK01. Colored sections in the nucleus represent contribution of HGT (brown) and EGT (green) to the nuclear genome, the thickness of the arrows represents prevalence of the particular type of gene transfer during different evolutionary stages. At which point the number of chromatophore-surrounding membranes was reduced from 3 to 2 is uncertain.