ORIGIN OF MITOCHONDRION. “Polintoviruses appear to have evolved from bacteriophages of the family Tectiviridae, at the onset of eukaryogenesis. This ancestral tectivirus most likely entered the protoeukaryotic cell along with the α-proteobacterial endosymbiont that subsequently gave rise to the mitochondrion.”
http://onlinelibrary.wiley.com/doi/10.1111/nyas.12728/full