In Bourne Legacy A 1.5% Gain Of Mitochondrial Function Yielded Super Soldiers

thescientist |  Since the 1970s, when researchers turned up similarities between DNA in eukaryotes’ mitochondria and bacterial genomes, scientists have suspected that the organelles descended from symbionts that took up residence within larger cells. A diverse class of bacteria called Alphaproteobacteria soon emerged as a likely candidate for the evolutionary origins of mitochondria. 

But a new analysis, published today (April 25) in Nature, suggests that mitochondria are at best distant cousins to known alphaproteobacteria lineages, and not descendents as previously thought.

“We are still left hungry for the ancestor of mitochondria,” says Puri Lopez-Garcia, a biologist at the University of Paris-South who was not involved in the study.

While it’s generally agreed that Alphaproteobacteria includes the closest bacterial relatives of mitochondria, that relationship doesn’t reveal much about how mitochondrial ancestors made a living or how they made the jump to acting as organelles. That’s because “Alphaproteobacteria is a particularly diverse group of organisms in terms of kinds of metabolism,” Lopez-Garcia explains. 

“You find more or less everything in there.” Some studies have found genetic similarities between mitochondria and an order of alphaproteobacterial symbionts known as Rickettsiales, but other, free-living candidates have also emerged.

The question of where on the alphaproteobacteria family tree the mitochondrial ancestor fell has pestered study coauthor Thijs Ettema throughout his scientific career. “Now, with all the available data from all these new lineages in all sorts of environments, we thought we should just do one bold approach and see where this ends up,” says Ettema, an evolutionary biologist at Uppsala University in Sweden.

Much of the genomic data he and colleagues used in their analysis came from the Tara Oceans dataset, which includes metagenomic sequences from microbes in ocean waters sampled from various depths. “For reasons that are not extremely clear . . . it seems that oceanic waters are extremely enriched for Alphaproteobacteria, and not just one species—it seems to be a whole array,” Ettema explains. The datasets were “good and deep enough to make an effort to reconstruct near-complete genomes.”