Mitochondrial parasites

The story always is told that the symbiosis between early, free-ranging mitochondria and proto-eukaryotes was a virtual enslavement, with the eukaryotes consuming the mitochondria and making use of their energy-generation facility to the benefit of the eukaryotic cells.

Of course, the complete lack of free-ranging mitochondria and non-mitochondriated free-living eukaryotes today help to cast doubt on that account. Clearly the fate of both these ancestral organisms was to some extent sealed when they joined forces with each other.

Yet there is still the perception that the mitochondria somehow didn't gain as much as the eukaryotes. After all, almost all the genes of the mitochondria ended up being assimilated into the nuclear DNA. There is little left but a shadow of the original mtDNA. If the two could divorce, you have the sense that the nucleated cell would be much better off than its tiny organelle.

I wonder if it isn't equally plausible that the free-ranging mitochondria were parasites infecting eubacterial cells. The presumptive qualities of these proto-mitochondria would be really helpful for a parasite: they can pump out lots of ATP, which would be helpful in rapid infection and reproduction. Much of their DNA migrated readily into the nuclear DNA of eukaryotic cells, which would tend to support the idea that it was adapted to move outside the mitochondrion and manipulate host cell functions. And several kinds of non-eukaryotes appear to have remnant mitochondria-like organelles also (Emelyanov 2003, and citations therein).

This could also help to explain some other aspects of eukaryotic cells. For example, a nucleus and chromosome structure may both have originally protected host DNA from parasite manipulation. Differences in the tRNA population for mtDNA and nuclear genes may also stem from selection on parasitism.

I should mention that Emelyanov's 2003 paper also presents an interesting hypothesis for the origins of eukaryotes, and has much data from gene phylogenies to bring to bear. The idea is that the initial eukaryotes were chimeric with archaean and eubacterial elements, in an age when lateral gene transfer and merging of cells was more the rule. Could be.

References:

Emelyanov V. 2003. Mitochondrial connection to the origin of the eukaryotic cell. Eur J Biochem 270:1599. DOI link