The endosymbiotic theory states that several key organelles of eukaryotes originated as symbioses between separate single-celled organisms. According to this theory, mitochondria and plastids (e.g.chloroplasts), and possibly other organelles, represent formerly free-living bacteria that were taken inside another cell as an endosymbiont. Molecular and biochemical evidence suggest that the mitochondrion developed from proteobacteria and the chloroplast from cyanobacteria.
The usual way to distinguish organelles from endosymbionts is by their reduced genome sizes. As an endosymbiont evolves into an organelle, most of their genes are transferred to the host cell genome. The host cell and organelle need to develop a transport mechanism that enables transfer back of the protein products needed by the organelle but now manufactured by the cell. However, using the example of the freshwater amoeboid Paulinella chromatophora, which contains chromatophores found to be evolved from cyanobacteria, it is argued that this is not the only possible criterion, another one being that the host cell has assumed control of the regulation of the former endosymbiont’s division, bringing it in synchrony with the cell’s own division. Biologists performed gene sequencing on the chromatophore (1.02Mb) and found that only 867 proteins were encoded by these photosynthetic cells. Comparisons with their closest free living cyanobacteria of the genus Synechococcus (having a genome size of 3Mb with 3300 genes) revealed that chromatophores underwent a drastic genome shrinkage. Chromatophores contained genes that were accountable for photosynthesis but were deficient in genes that could carry out other biosynthetic functions signifying that these endosymbiotic cells were highly dependent on their hosts for their survival and growth mechanisms. Thus, these chromatophores were found to be non-functional for organelle-specific purposes when compared to mitochondria and plastids. This distinction could have promoted the early evolution of photosynthetic organelles. Edited from endosymbiotic theory