First eukaryotes found without a normal cellular power supply

Mitch Leslie in Science:

You can’t survive without mitochondria, the organelles that power most human cells. Nor, researchers thought, can any other eukaryotes—the group of organisms we belong to along with other animals, plants, fungi, and various microscopic creatures. But a new study has identified the first eukaryote that has ditched its mitochondria, suggesting that our branch on the tree of life may be more versatile than researchers thought. “This is a discovery of fundamental importance,” says evolutionary biologist Eugene Koonin of the National Center for Biotechnology Information in Bethesda, Maryland, who wasn’t connected to the study. “We now know that eukaryotes can live happily without any remnant of the mitochondria.”

Mitochondria are the descendants of bacteria that settled down inside primordial eukaryotic cells, eventually becoming the power plants for their new hosts. Although mitochondria are a signature feature of eukaryotes, scientists have long wondered whether some of them might have gotten rid of the organelles. The diarrhea-causing microbe Giardia intestinalis for a time seemed mitochondria-free, but on closer investigation, it and other suspects proved to be false alarms, containing shrunken versions of the organelles. For the new study, a team led by evolutionary biologist Anna Karnkowska, a postdoc, and her adviser, Vladimir Hampl, of Charles University in Prague, checked another candidate, a species in the genus Monocercomonoides. The single-celled organism came from the guts of a chinchilla that belonged to one of the lab members. The team decided to test it because it belonged to a group of microbes that scientists posited had lost their mitochondria. When the researchers sequenced Monocercomonoides’s genome, they found no signs of mitochondrial genes (the organelles carry their own DNA). Digging deeper, they determined that it lacks all of the key proteins that enable mitochondria to function. “The definition of eukaryotic cells is that they have mitochondria,” says Karnkowska, who is now at the University of British Columbia, Vancouver, in Canada. “We overturn this definition.”

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