Proto-mitochondrion

Phylogeny

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.

Although the order Rickettsiales has been proposed as the alphaproteobacterial sister-group of mitochondria, no definitive evidence pinpoints the alphaproteobacterial group from which the proto-mitochondrion emerged, and some contradictory evidence, especially in the early, sparse genome samplings. Martijn et al found mitochondria are a possible sister-group to all other alphaproteobacteria. The phylogenetic tree of the Rickettsidae has been inferred by Ferla et al. from the comparison of 16S + 23S ribosomal RNA sequences. Geiger et alii (2023) propose placing the recently discovered (2016) genus Iodidimonas – found in a sister clade to Rickettsidae: the Caulobacteridae – 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 the bc₁ complex (Complex IV and Complex III in mitochondria, respectively).