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Canadian study reveals hidden link between cells and bacteria

Mitochondria Guinea Pig Pancreas. Because mitochondria have their own separate DNA, researchers have long suspected that they started out as separate, free-living bacteria that were somehow co-opted into larger cells. But the circumstances of this merger remain mysterious.

Yale University School of Medicine

Scientists have added a new wrinkle – literally – to the story of how life as we know it evolved.

The find, published Thursday in the journal Current Biology, involves the wrinkle-like folds that are found inside mitochondria, tiny vessels that are the power plants of cells. Their folds – known as cristae – are useful because they give mitochondria more surface area to promote reactions that turn sugar and oxygen into ATP, the energy-storing chemical that drives much of what cells do.

Because mitochondria have their own separate DNA, researchers have long suspected that they started out as separate, free-living bacteria that were somehow co-opted into larger cells. But the circumstances of this merger remain mysterious.

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"It happened so long ago – about two billion years or so – that a lot of the evidence is basically erased," said Claudio Slamovits, a molecular biologist at the Dalhousie University in Halifax whose work is also supported by the Canadian Institute for Advanced Research.

To shed light on the question, Dr. Slamovits and Sergio Munoz-Gomez, a graduate student in his lab, decided to investigate what cristae can reveal about the origin of mitochondria.

They zeroed in on the genes that are responsible for a making the proteins that help form the convoluted folds. By analyzing the genomes of many different kinds of organisms – from plants, animals and fungi to single-celled creatures like amoebas – they discovered a fold-making gene that was common to all.

Next, they found the same gene in just one class of bacteria, called alphaproteobacteria, whose members are suspected of including the closest living cousins of the bacteria that were absorbed long ago and turned into mitochondria.

Taken together the evidence suggests that the ancestors of mitochondria already had cristae before they became a cell component. This suggests they were more complex and versatile creatures than some studies have argued in the past. The result casts doubt on a competing idea that mitochondria began as simple but highly specialized parasites.

"What they find is interesting and substantiates what some of us have been saying for a long time about the ancestor of mitochondria," said William Martin, head of the Institute of Molecular Evolution at Heinrich Heine University in Dusseldorf, Germany, who is not involved with the study.

The research was done by Dr. Slamovits's group in conjunction with another team that included Jeremy Wideman at the University of Alberta. The two teams joined forces once they discovered they were working on parallel tracks.

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The Canadian study comes just two weeks after European researchers reported the discovery of genes from a group of microbes that could be a bridge between eukaryotes – the type of cells that have mitochondria – and a more primitive lineage of single celled creatures known as archaea. Animals, plants and fungi are all made of eukaryotic cells. Scientists have speculated that eukaryotes may have begun the moment archaea combined with bacteria to form the first cell with mitochondria.

Both studies reveal the growing power of genomics to reveal clues about how our cells evolved, in much the way that Charles Darwin's detailed descriptions of different kinds of animals nearly two centuries ago led him to formulate his ideas about how species evolve.

"The ability to sequence mass quantities of DNA from all over the world is making it so that we're finding things we never knew existed," said Dr. Wideman, now based at the University of Exeter in Britain.

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