The emergence of organelles

Over a billion years ago, one group of single-celled organisms engulfed a second group and gained a partner rather than a meal. Thus, the endosymbionts called mitochondria and chloroplasts were formed.  


Originally, these organelles were individual cells with their own complete genomes.  Today, mitochondria and chloroplasts still contain their own genes (a huge benefit for genealogical and evolutionary studies), but not a complete genome.  Many of the genes required to build the organelles are found not within the organelles themselves, but in the nuclei of the host cells.  At some point, genes were transferred from the proto-organelles to the host genome.  After that time, proteins critical for making organelles were assembled in the cytoplasm and delivered to those organelles, making the organelles completely dependent upon their hosts. 

It can be difficult to reconstruct processes that occurred over a billion years ago. Luckily, an organism with a more recent foray into endosymbiosis has been discovered. The amoeba Paulinella chromatophora has chromatophores that resemble chloroplasts (they too originated as cyanobacteria, are separated from the host cell by a double membrane and have transferred most of their original genes to the host nucleus) but were incorporated into the amoebae only about 60 million years ago. By studying this amoeba, Eva Nowack and Arthur Grossman of Stanford University have been able to see how proteins encoded in the host genome are transported to the chromatophores.

Interestingly, most of the genes required for photosynthesis, the very raison d'être for chloroplasts or chromatophores, are encoded within the nuclear genome. This gives the host complete control of the organelles’ activities. Because the process of transferring organelle genes to the host nucleus had already occurred in P. chromatophora, it may be an early process in the conversion from two individual cells to a single cell.