DEPENDING on your viewpoint it’s either the ultimate in cooperation or it’s slavery. The mitochondria that provide our cells with energy and the chloroplasts in plant cells that harness light energy were once free-living bacteria. After becoming symbiotic, hundreds of millions of years ago, they lost their ability to survive independently.
Now two species of bacteria that live in insects have been found in the process of transition from free-living organism to organelle. Their genomes are the smallest ever sequenced.
One of the bacteria, Carsonella ruddii, (see above) has the fewest genes of any cell known in the world…
