Biologists Discover Animal that Lacks Mitochondrial Genome, Doesn’t Need Oxygen to Live

An international team of biologists has discovered that a tiny parasite of salmon called Henneguya salminicola has no mitochondrial genome and thus has lost the ability to perform aerobic respiration.

Henneguya salminicola, also known as Henneguya zschokkei, has large nuclei but surprisingly no mitochondrial nucleosomes. Image credit: Yahalomi et al, doi: 10.1073/pnas.1909907117.

Henneguya salminicola, also known as Henneguya zschokkei, has large nuclei but surprisingly no mitochondrial nucleosomes. Image credit: Yahalomi et al, doi: 10.1073/pnas.1909907117.

Although aerobic respiration is a hallmark of eukaryotes, a few unicellular lineages, growing in hypoxic environments, have secondarily lost this ability.

In the absence of oxygen, the mitochondria of these organisms have lost all or parts of their genomes and evolved into mitochondria-related organelles.

“Aerobic respiration was thought to be ubiquitous in animals, but now we confirmed that this is not the case,” said team leader Professor Dorothee Huchon, a researcher in the School of Zoology at Tel Aviv University and the Steinhardt Museum of Natural History.

“Our discovery shows that evolution can go in strange directions. Aerobic respiration is a major source of energy, and yet we found an animal that gave up this critical pathway.”

In the study, Professor Huchon and colleagues from the University of Kansas, Oregon State University and Tel Aviv University sequenced and analyzed the genome of Henneguya salminicola, a tiny, less than ten-celled parasite of salmonid fish.

The analyses suggest that the microscopic animal lost not only its mitochondrial genome but also nearly all nuclear genes involved in transcription and replication of the mitochondrial genome.

TEM image of Henneguya salminicola mitochondrion-related organelle (center) with few cristae. Image credit: Yahalomi et al, doi: 10.1073/pnas.1909907117.

TEM image of Henneguya salminicola mitochondrion-related organelle (center) with few cristae. Image credit: Yahalomi et al, doi: 10.1073/pnas.1909907117.

Until this discovery, there was debate regarding the possibility that organisms belonging to the animal kingdom could survive in anaerobic environments.

The assumption that all animals are breathing oxygen was based, among other things, on the fact that animals are multicellular, highly developed organisms, which first appeared on Earth when oxygen levels rose.

“It’s not yet clear to us how the parasite generates energy,” Professor Huchon said.

“It may be drawing it from the surrounding fish cells, or it may have a different type of respiration such as oxygen-free breathing, which typically characterizes anaerobic non-animal organisms.”

“The discovery bears enormous significance for evolutionary research,” she said.

“It is generally thought that during evolution, organisms become more and more complex, and that simple single-celled or few-celled organisms are the ancestors of complex organisms.”

“But here, right before us, is an animal whose evolutionary process is the opposite. Living in an oxygen-free environment, it has shed unnecessary genes responsible for aerobic respiration and become an even simpler organism.”

The discovery is reported in a paper in the Proceedings of the National Academy of Sciences.

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Dayana Yahalomi et al. A cnidarian parasite of salmon (Myxozoa: Henneguya) lacks a mitochondrial genome. PNAS, published online February 24, 2020; doi: 10.1073/pnas.1909907117

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