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May 13, 2016

Mitochondria? We Don’t Need No Stinking Mitochondria!

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    Diagram comparing typical eukaryotic cell to the newly discovered mitochondria-free organism. [Karnkowska et al., 2016, Current Biology 26, 1–11]

    The organelle that produces a significant portion of energy for eukaryotic cells would seemingly be indispensable, yet over the years, a number of organisms have been discovered that challenge that biological pretense. However, these so-called amitochondrial species may lack a defined organelle, but they still retain some residual functions of their mitochondria-containing brethren. Even the intestinal eukaryotic parasite Giardia intestinalis, which was for many years considered to be mitochondria-free, was proven recently to contain a considerably shriveled version of the organelle.  

    Now, an international group of scientists has released results from a new study that challenges the notion that mitochondria are essential for eukaryotes—discovering an organism that resides in the gut of chinchillas that contains absolutely no trace of mitochondria at all.

    "In low-oxygen environments, eukaryotes often possess a reduced form of the mitochondrion, but it was believed that some of the mitochondrial functions are so essential that these organelles are indispensable for their life," explained lead study author Anna Karnkowska, Ph.D., visiting scientist at the University of British Columbia in Vancouver. "We have characterized a eukaryotic microbe which indeed possesses no mitochondrion at all."

    In this new study, the researchers sequenced the genome of Monocercomonoides sp., and to their surprise found that the organism completely lacks all mitochondrial proteins. Interestingly, Monocercomonoides has been known for more than 80 years and is related to the human pathogens Giardia and Trichomonas—all of which belong to a group known as Metamonada, which lives exclusively in low-oxygen environments. 

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    This is a light micrograph of Monocercomonoides sp. (PA203). [Dr. Naoji Yubuki]

    Speaking with Science News, evolutionary biologist Eugene Koonin, Ph.D., a senior investigator at the National Center for Biotechnology Information, who wasn’t connected to the study, remarked “This is a discovery of fundamental importance. We now know that eukaryotes can live happily without any remnant of the mitochondria.”

    Monocercomonoides seems to have gotten by without mitochondria thanks to a cytosolic sulfur mobilization system (SUF) acquired from bacteria, which appears to substitute for essential mitochondrial functions. Through a unique combination of events, including the loss of many mitochondrial functions and the acquisition of this critical machinery from prokaryotes, "this organism has evolved beyond the known limits that biologists circumscribed," Dr. Karnkowska noted.

    Investigators have been looking for organisms lacking mitochondria for decades, and as the years passed, it seemed unlikely that a eukaryote truly lacking mitochondria would ever be found. Nevertheless, Dr. Karnkowska and her colleagues now say there may be others.

    The findings from this study were published recently in Current Biology in an article entitled “A Eukaryote without a Mitochondrial Organelle.”

    "This amazing organism is a striking example of a cell which refuses to adhere to the standard cell biology text book, and we believe there may be many more similar examples in the so far hidden diversity in the world of microbial eukaryotes—the protists," Dr. Karnkowska stated.

    The research team said that they'd now like to learn more about how these organisms function. Moreover, they would like to characterize Monocercomonoides and its relatives better to understand their discovery in a broader evolutionary context.

    "It is very likely that the mitochondrion is absent in the whole group called oxymonads," said senior study author Vladimir Hampl, Ph.D., a professor at Charles University in Prague. "We would like to know how long ago the mitochondria were lost."

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