The proto-mitochondrion is the ancestral bacterial endosymbiont from which all mitochondria are thought to be derived.
The phylogenetic analyses of the few genes that are still encoded in the genomes of modern mitochondria suggest an alpha-proteobacterial origin for this endosymbiont. Although the order Rickettsiales has been proposed as the alpha-proteobacterial sister-group of mitochondria, there is no definitive evidence as to from which alpha-proteobacterial group the proto-mitochondrion emerged.
|Phylogeny of Rickettsiales
|Robust phylogeny of Rickettsiales from Williams et al. (2007)
Toni Gabaldón and Martijn Huynen (2003) reconstructed the proteome and corresponding metabolism of the proto-mitochondrion by comparing extant alpha-proteobacterial and eukaryotic genomes. They concluded that this organism was an aerobic alpha-proteobacterium catabolyzing lipids, glycerol and other compounds provided by the host. At least 630 gene families derived from this organism can still be found in the 9 eukaryotic genomes analyzed in the study.