What separates animals, plants, fungi and even amoebas from bacteria? Well, there's one feature that no longer universally makes us distinct, thanks to a new study that redefines what it means to belong to our limb on the tree of life.
That limb includes all eukaryotes, multi-celled organisms and more complex single-celled organisms with cells that contain specialized mini-organs called organelles.
Organelles include the nucleus, which holds most of the cell's DNA and mitochondria, a "powerhouse" that uses oxygen to produce energy – things that bacteria and archaea from the two other "domains," or limbs, on the tree of life don't have.
But now, for the first time, scientists have discovered a eukaryote with no mitochondria – forcing them to reconsider what it means to be a eukaryote.
"In my view this is an important finding because it changes our concept of what minimally defines a eukaryotic cell," said Andrew Roger, a Dalhousie University researcher who co-authored the study published in the journal Current Biology.
"Until now, all eukaryotes known had some form of mitochondria in their cells and so mitochondria were considered to be fundamental components of eukaryote cell biology."
The mitochondria-less organism, called Monocercomonoides, lives inside the gut of a chinchilla, a rodent from the Andes mountains of South America known for its luxurious fur.
Related to human parasites
It's somewhat related to the bug that causes the diarrheal infection giardiasis or "beaver fever," and the one that causes the vaginal infection trichomoniasis in humans, although there's no evidence that Monocercomonoides harms the chinchilla in any way.
The guts of other animals don't contain a lot of oxygen, so organisms that live inside them don't use oxygen the same way and don't need mitochondria to produce energy using oxygen.
Such organisms don't have visible mitochondria when viewed under the microscope, and some were once thought not to have mitochondria. But up until now, traces of mitochondria and their genes and proteins have been found inside most such organisms that have been studied in detail.
Anna Karnkowska, the researcher who led the new study, said that's because we use mitochondria for much more than generating energy. For example, they help assemble sulphur and iron into important proteins that are essential for many biological processes.
Still, when she and her colleagues first started studying Monocercomonoides, she suspected it might be an exception that didn't have mitochondria.
"We believed this from the very first day," said Karnkowska, who did the work while she was a postdoctoral researcher with Vladimir Hampl at Charles University in Prague. "But it's intellectually challenging to show something is not there. It's always easier to say, 'It's here.'"
For three years, she studied Monocercomonoides as part of a project initiated and coordinated by Hampl, hunting for traces of mitochondria. In the process, she discovered that it had an alternative system for doing the mitochondria's one remaining essential job – a gene stolen from bacteria was helping it assemble iron-sulphur proteins. That may have happened through a rare, random process such as an ancestor eating a bacterium, and incorporating some of its DNA instead of digesting it all.
That gene was crucial because it made the mitochondria's job redundant, allowing Monocercomonoides to do without mitochondria.
"The organelle could disappear," said said Karnkowska, now a visiting scientist at the University of British Columbia. "That very nicely explained the whole thing."
She emphasized, however, that the new organism is exceptional, and all other known eukayrotes still have (and need) mitochondria: "Our cells would be not fine without mitochondria."
While this may alter scientists' definition of mitochondria, Karnkowska said definitions in biology are often not precise because there are exceptions to the rule.
"I think we just have to, in this definition, be a little more open for the great diversity of eukaryotic cells."
Her Dalhousie co-author, Roger, said the study shows that eukaryotic cells can do without "essential" components if they manage to acquire a gene from other organisms, such as bacteria, that perform the same function – something that happens much more often than scientists previously thought.
"This is a clear example of how important it can be in eukaryotic evolution," he added.
Gertraud Burger, a University of Montreal researcher who studies the evolution of organelles in eurkaryotes and was not involved in the study, said the research was well done.
"There's no doubt about what they claim," she said. Based on previous research, she had expected an organism like this would eventually be found. "This is the last link that was missing so far and it's nice to have it."
Claudio Slamovits, a Dalhousie researcher who also studies the evolution of eukaryotes, but wasn't involved with this study, said the findings support the idea that all eurkaryotes originally had mitochondria, but can exist without them under specific circumstances.
He added this type of research was "extremely difficult," and likened it to looking for a needle in a haystack. Given the researchers' exhaustive efforts, he said, "one can be pretty sure the needle is not there."