Mysterious Eukaryote Missing Mitochondria

Researchers uncover the first example of a eukaryotic organism that lacks the organelles.

By | May 12, 2016

Monocercomonoides sp. PA203VLADIMIR HAMPL, CHARLES UNIVERSITY, PRAGUE, CZECH REPUBLICScientists have long thought that mitochondria—organelles responsible for energy generation—are an essential and defining feature of a eukaryotic cell. Now, researchers from Charles University in Prague and their colleagues are challenging this notion with their discovery of a eukaryotic organism, Monocercomonoides species PA203, which lacks mitochondria. The team’s phylogenetic analysis, published today (May 12) in Current Biology, suggests that Monocercomonoides—which belong to the Oxymonadida group of protozoa and live in low-oxygen environmentsdid have mitochondria at one point, but eventually lost the organelles.

“This is quite a groundbreaking discovery,” said Thijs Ettema, who studies microbial genome evolution at Uppsala University in Sweden and was not involved in the work.

“This study shows that mitochondria are not so central for all lineages of living eukaryotes,” Toni Gabaldon of the Center for Genomic Regulation in Barcelona, Spain, who also was not involved in the work, wrote in an email to The Scientist. “Yet, this mitochondrial-devoid, single-cell eukaryote is as complex as other eukaryotic cells in almost any other aspect of cellular complexity.”

Charles University’s Vladimir Hampl studies the evolution of protists. Along with Anna Karnkowska and colleagues, Hampl decided to sequence the genome of Monocercomonoides, a little-studied protist that lives in the digestive tracts of vertebrates. The 75-megabase genome—the first of an oxymonad—did not contain any conserved genes found on mitochondrial genomes of other eukaryotes, the researchers found. It also did not contain any nuclear genes associated with mitochondrial functions.

“It was surprising and for a long time, we didn’t believe that the [mitochondria-associated genes were really not there]. We thought we were missing something,” Hampl told The Scientist. “But when the data kept accumulating, we switched to the hypothesis that this organism really didn’t have mitochondria.”

Because researchers have previously not found examples of eukaryotes without some form of mitochondria, the current theory of the origin of eukaryotes poses that the appearance of mitochondria was crucial to the identity of these organisms.

“We now view these mitochondria-like organelles as a continuum from full mitochondria to very small vesicles called mitosomes,” Hampl explained.

Some anaerobic protists, for example, have only pared down versions of mitochondria, such as hydrogenosomes and mitosomes, which lack a mitochondrial genome. But these mitochondrion-like organelles perform essential functions of the iron-sulfur cluster assembly pathway, which is known to be conserved in virtually all eukaryotic organisms studied to date.

Yet, in their analysis, the researchers found no evidence of the presence of any components of this mitochondrial pathway.

Like the scaling down of mitochondria into mitosomes in some organisms, the ancestors of modern Monocercomonoides once had mitochondria. “Because this organism is phylogenetically nested among relatives that had conventional mitochondria, this is most likely a secondary adaptation,” said Michael Gray, a biochemist who studies mitochondria at Dalhousie University in Nova Scotia and was not involved in the study. According to Gray, the finding of a mitochondria-deficient eukaryote does not mean that the organelles did not play a major role in the evolution of eukaryotic cells.

To be sure they were not missing mitochondrial proteins, Hampl’s team also searched for potential mitochondrial protein homologs of other anaerobic species, and for signature sequences of a range of known mitochondrial proteins. While similar searches with other species uncovered a few mitochondrial proteins, the team’s analysis of Monocercomonoides came up empty.

“The data is very complete,” said Ettema. “It is difficult to prove the absence of something but [these authors] do a convincing job.”

To form the essential iron-sulfur clusters, the team discovered that Monocercomonoides use a sulfur mobilization system found in the cytosol, and that an ancestor of the organism acquired this system by lateral gene transfer from bacteria. This cytosolic, compensating system allowed Monocercomonoides to lose the otherwise essential iron-sulfur cluster-forming pathway in the mitochondrion, the team proposed.

“This work shows the great evolutionary plasticity of the eukaryotic cell,” said Karnkowska, who participated in the study while she was a postdoc at Charles University. Karnkowska, who is now a visiting researcher at the University of British Columbia in Canada, added: “This is a striking example of how far the evolution of a eukaryotic cell can go that was beyond our expectations.”

“The results highlight how many surprises may await us in the poorly studied eukaryotic phyla that live in under-explored environments,” Gabaldon said.

Ettema agreed. “Now that we’ve found one, we need to look at the bigger picture and see if there are other examples of eukaryotes that have lost their mitochondria, to understand how adaptable eukaryotes are.”

A. Karnkowska et al., “A eukaryote without a mitochondrial organelle,” Current Biology, doi:10.1016/j.cub.2016.03.053, 2016.

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Avatar of: rborges

rborges

Posts: 1

May 13, 2016

Dear Sir,

 

Giardia lamblia is a well-known eukaryote cell without mitochondria. 

May 13, 2016

Please do some what of a literature review before you make statemnts like this.  In my parasitology class in 1972 we learned about Giardia not having a mitochondria.  Many protozoa and worms live in environments that are anaerobic and have no mitochondria and other use their's only to cycle amino acids in the Kreb cycle.

Avatar of: dogrt

dogrt

Posts: 1

May 13, 2016

The title of The Scientist article should have been "First eukaryote that lacks a mitochondrial organelle." There are hundreds of eukaryotic taxa (e.g., GiardiaHexamitaTrichomonasEntamoebaEncephalitozoonNosema)that do not have mitochondria. However, phylogenetic analyses have shown that the lack of mitochondria was always a derived state—a secondary loss of the organelle from a lineage that once possessed mitochondria. Monocercomonoides is no exception. 

Moreover, detailed cell biology and genomic investigations have shown that all so-called amitochondriate eukaryotes examined so far contain a double-membrane-bounded organelle of mitochondrial ancestry, a mitochondrial organelle, and thus are not amitochondriate after all. Is Monocercomonoides an exception in this respect? First, the cell biology part in the Karnkowska et al. paper is meager, so they should have withhold judgment on whether or not a mitochondrial remnant exists or not. As we all know, absence of evidence does not equal evidence of absence.  

Moreover, a friend of mine ran a search of the Monocercomonoides protein coding genes sequence and found that 75% of its genes are eubacterial. So far, there is no evidence for an additional eubacterial contributors in any eukaryotes (Ku et al. PNAS. 2015), so until proven otherwise, the mitochondrion origin of the eubacterially derived genes stays.

In summary, the conclusions of the Karnkowska et al. paper are premature, and The Scientist commentary is misleading.

Dan Graur

Avatar of: SMC

SMC

Posts: 1

May 13, 2016

It's kind of odd. This significant published study is authored by Anna Karnkowska (et al) yet she is decribed as having "participated" in the study. Meanwhile, Vladimir Hampl is quoted first as if he is the leader of the study. Whose idea was this originally? Who did the work? And who deserves the main credit?

If it's Karnkowska, then the reporter should quote her first to give her lead credit for the work. And Hampl maybe should be more laudatory of her creative effort in this achievement. But if it's Hampl who is the lead, then the article should be authored by Hampl et al.

Hopefully, this is not a case of an older male academic horning on the significant achievement by a female underling. Maybe it's a team effort and Hampl wants to help Karnkowska by allowing her to get the credit for PhD efforts, etc. But there's no need for The Scientist to help in this effort. 

Who did the work and who gets the credit? Not a small matter. These early articles often set the tone for future descriptions and credit for this scientific discovery.

Avatar of: Bigsteve111

Bigsteve111

Posts: 1

May 14, 2016

My master's work involved the study of Verongia fistularis, a marine sponge. What is remarkable is that about 1/3 of its extracellular volume is occupied by bacteria.

Part of the experiment involved exposing the sponge in situ (at McGill University's Bellairs Research Institute in Barbados) to a broad spectrum antibiotic called gentamicin.

My "aha" moment came when I realized I had never seen any mitochondria in the electron micrographs but samples taken 8 hours later showed them inside various cells including the amoebocytes.

Alas, I never published but for anyone interested, the thesis should be in the archives at McGill.

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