Evolutionary origin of mitochondria predates the origin of Alphaproteobacteria

Mitochondria are known as the “energy factories” of most (animal) cells. Earliest bacteria that had originated on the planet do not have mitochondria, and have only one chromosome (haploid, seen in all prokaryotes) rather than chromosome-pairs (diploid, seen in all eukaryotes). Most eukaryotic cells have a nucleus, and then the cytoplasm outside the nucleus –– which contains many types of organelles –– including hundreds of mitochondria. These mitochondria are involved in various processes, of which ATP generation by means of oxidative phosphorylation (used by the cell to generate energy for itself and other cells) is a hallmark feature. Structures in plants are similar to mitochondria and called chloroplasts.

“The Endosymbiotic Theory” describes how the fusion of a large host eukaryotic cell, with one or more ingested bacteria, could easily have become dependent on one another for survival (hence, this is a topic of gene-environment interactions), resulting in a permanent relationship. After more than 2 billion years of evolution, mitochondria and chloroplasts have become more specialized, and today they cannot live outside the cell. In humans, while there are >20,000 genes in the nuclear genome, there are at least 37 important genes in the mitochondrial genome. During fertilization, a sperm (derived usually from the man) combines with an egg (derived usually from the woman), and only the egg has mitochondria in its cytoplasm. When there is a serious defect in a gene of the woman’s mitochondrial genome, today there is clinically successful in vitro fertilization scheme –– in which the male’s sperm, and nucleus of the female’s egg, is combined with mitochondria from a healthy woman –– following which a healthy baby is formed, derived from three persons.

To trace the evolutionary history of mitochondria and their role in the genesis of eukaryotes, detailed knowledge about the identity and nature of the mitochondrial ancestor is important. Alphaproteobacteria is a distinct Class of bacteria (which evolved later than early bacteria) in the Proteobacteria phylum; its members are highly diverse, some Taxa contain mitochondria, and certain Alphaproteobacteria can cause specific human (and agricultural) diseases, but nevertheless they share a common evolutionary ancestor. Despite the fact that the origin of mitochondria in Alphaproteobacteria is generally undisputed, efforts to resolve the phylogenetic position of mitochondria in the Alphaproteobacterial species tree have failed to reach an agreement.

Whereas most studies support the idea that mitochondria evolved from an ancestor related to Rickettsiales (an Order within Alphaproteobacteria that includes several host-associated pathogenic and endosymbiotic lineages), other studies suggest that mitochondria evolved from a free-living group. Authors [see attached publication] re-evaluated the phylogenetic placement of mitochondria. They used genome-resolved binning of oceanic meta-genome datasets and increased the genomic sampling of Alphaproteobacteria with twelve divergent clades, plus one clade representing a sister group to all Alphaproteobacteria. Subsequent phylogenomic analyses –– that specifically address long-branch attraction and compositional bias artifacts –– suggest that mitochondria did not evolve from Rickettsiales or any other currently recognized Alphaproteobacterial lineage. Rather, the analyses of these authors indicate that mitochondria evolved from a proteobacterial lineage that branched off before the divergence of all sampled Alphaproteobacteria. In light of this new finding, previous hypotheses about the nature of the mitochondrial ancestor will have to be re-evaluated. 🙂

Nature 3 May 2o18 557: 101–105

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