New evidence on the origins of mitochondria
Mitochondria, the organelles often referee to as 'power-houses' of the cell are vital structures for almost all living eukaryotes. It is widely accepted that they originated through an endosysmbiosis event, where an oxygen using bacterium was taken up by a larger, anaerobic archaeal cell. Originally this was viewed as being of mutual benefit; the bacterium gained resources and protection from its host while the archaean was able to survive in increasingly oxygen-rich environments. Since the advent of gene sequencing our views of the identity of the bacterium and archaean have been refined. The mitochondria seem to be related to the Alphaproteobacteria while the host cell is now firmly placed in the bizarre Asgard group of Archaea. For the mitochondria, the closest relationship within the Alphaproteobacteria has been identified as the Rikketsiales. This is interesting because many of these are symbionts or parasites, raising the possibility that the original even was not endosysmbiosis, but parasitism.
The most thorough comparison of mitochondria and Alphaproteobacteria has just been published, concluding that the relationship is not as close as had been thought. In fact, mitochondria seem to lie outside the Alphaproteobacteria, suggesting an origin from some other bacterium related to that group. At present it is impossible to say what this means, other than there is no longer strong evidence of a parasitic origin for mitochondria.
The big problem with all of these studies is how little we know about bacteria; this new study included 40 Alphaproteobacteria genomes, many sampled from environmental DNA from sea-water, rather than from individual bacteria. Whilst this is a great increase in sampling, it remains a miniscule fraction of the group. It is highly likely that future studies will change the story yet again.
Source: Joran Martijn, Julian Vosseberg, Lionel Guy, Pierre Offre & Thijs J. G. Ettema. 2018. Deep mitochondrial origin outside the sampled alphaproteobacteria Nature (2018) doi:10.1038/s41586-018-0059-5