Researchers at Tel Aviv University (TAU) have discovered a non-oxygen
breathing animal. The unexpected finding changes one of science’s assumptions
about the animal world.
A study on the finding was published on February 25 in the Proceedings
of the National Academy of Sciences by TAU researchers led by
Prof. Dorothee Huchon of the School of Zoology at TAU’s Faculty of Life
Sciences and Steinhardt Museum of Natural History.
The tiny, less than 10-celled parasite Henneguya salminicola lives
in salmon muscle. As it evolved, the animal, which is a myxozoan relative of
jellyfish and corals, gave up breathing and consuming oxygen to produce energy.
“Aerobic
respiration was thought to be ubiquitous in animals, but now we confirmed that
this is not the case,” Prof. Huchon explains. “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.”
Some other
organisms like fungi, amoebas or ciliate lineages in anaerobic environments
have lost the ability to breathe over time. The new study demonstrates that the
same can happen to an animal — possibly because the parasite happens to live in
an anaerobic environment.
Its genome was
sequenced, along with those of other myxozoan fish parasites, as part of
research supported by the U.S.-Israel Binational Science Foundation and
conducted with Prof. Paulyn Cartwright of the University of Kansas, and Prof.
Jerri Bartholomew and Dr. Stephen Atkinson of Oregon State University.
The parasite’s anaerobic nature was an accidental discovery. While
assembling the Henneguya genome, Prof. Huchon found that it did
not include a mitochondrial genome. The mitochondria is the powerhouse of the
cell where oxygen is captured to make energy, so its absence indicated that the
animal was not breathing oxygen.
Until the new
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,” Prof. Huchon says. “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.”
According to
Prof. Huchon, the discovery bears enormous significance for evolutionary
research.
“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,” she concludes. “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.”
Journal article: https://www.pnas.org/content/early/2020/02/18/1909907117
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