Endosymbiotic Theory - From Endosymbionts To Organelles

From Endosymbionts To Organelles

According to Keeling and Archibald, 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, these authors argue 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. Nowack and his colleagues 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.