Researchers thought that it was impossible for cells to live without mitochondria, the organelles that power most human cells and other eukaryotes, a category that contains numerous other animals, plants and fungi. Now, a new study reveals a eukaryote that does not possess mitochondria, suggesting that we might not know as much as we think about our branch on the tree of life and its relationship to this cellular power supply.
"This is a discovery of fundamental importance," said Eugene Koonin of the National Center for Biotechnology Information in Bethesda, Md., who wasn't involved in the study. "We now know that eukaryotes can live happily without any remnant of the mitochondria."
Mitochondria are bacterial remnants that eventually settled inside primordial eukaryotic cells and over time became the main power plants for their new hosts. During the new study, the team examined a species in the genus Monocercomonoides, a single-celled organism that originated from the guts of a chinchilla. They chose this species because it belongs to a group of microbes that scientists believed might have lost their mitochondria.
After sequencing the Monocercomonoides's genome, the team found no signs of mitochondrial genes, and upon further examination, they discovered that it also lacks all of the proteins necessary for mitochondria to function.
"The definition of eukaryotic cells is that they have mitochondria," said Anna Karnkowska, who is now at the University of British Columbia in Canada and first author of the study. "We overturn this definition."
Why does Monocercomonoides lack such a crucial element to eukaryotic function? The team believes that this could stem from where it lives - inside the intestines of a chinchilla, where nutrients are in ample supply. For oxygen, the organism likely uses enzymes in its cytoplasm to break down food and for iron and sulfur synthesis, it appears to borrow bacterial genes that do just this.
"It's a very solid paper experimentally," said B. Franz Lang of the University of Montreal in Canada, who was not involved in the study. "If you'd like me to bet, I'd give them 90% probability that they are correct."
Further research will likely focus on detailed microscopic analysis in order to support the findings. Nevertheless, the current results force us to expand our current views on cellular functioning and the role of mitochondria in eukaryotes.
"It shows that eukaryotic life is more flexible than what the textbooks say," Lang said.
The findings were published in the May 12 issue of the journal Current Biology.