The phylogenetic analyses of the few genes that are still encoded in the genomes of modern mitochondria suggest an alphaproteobacterial origin for this endosymbiont, in an ancient episode of symbiogenesis early in the history of the eukaryotes.
[4] The phylogenetic tree of the Rickettsidae has been inferred by Ferla et al. from the comparison of 16S + 23S ribosomal RNA sequences.
[5] Geiger et alii (2023) propose placing the recently-discovered (2016) genus Iodidimonas, found in a sister clade to Rickettsidae, the Caulobacteridae,[5] as the closest free-living relative of mitochondria, as it possesses more metabolic products matching that of mitochondria today, such as cardiolipins and sphingolipids, and important genetic markers such as the COX operon and a counterpart to mitochondrial complex III, the bc1 complex.
Holosporales Iodidimonas Proto-mitochondria Geiger et al 2023 Pelagibacter Subgroups Ib, II, IIIa, IIIb, IV, and V Proto-mitochondria Emelyanov 2001 Ehrlichia Anaplasma Wolbachia Neorickettsia Midichloria Rickettsia Orientia Proto-mitochondria Martijn et al 2018 Gabaldón & Huynen (2003) reconstructed the proteome (the entire set of proteins expressed by a genome) 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 respiring lipids, glycerol and other compounds provided by the host.