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  • [♩INTRO]

  • There are a few things in life that we, as animals,

  • absolutely cannot live without.

  • One of those things is oxygen.

  • It's so essential that every multicellular species we've ever studied

  • has the ability to use oxygen to create energy.

  • That is, every species except one.

  • Because in 2020, biologists reported that they'd found an animal

  • that not only doesn't breathe oxygenas far as they could tell, it can't.

  • This species is called Henneguya salminicola,

  • because it infects salmon during a couple of stages of its development.

  • It's a type of animal called a myxozoan, making it a sort of tiny parasitic jellyfish

  • with a complex life cycle.

  • We know it spends part of its time in salmon, but not where it goes after that.

  • And it doesn't seem to have much in the way of mitochondria.

  • Say it with me, now: Mitochondria are the powerhouses of the cell.

  • They take sugars and oxygen and turn them into the molecules that carry energy.

  • All eukaryotic life -- everything with a nucleus,

  • both single-celled and multicellular -- has these in its evolutionary history.

  • Even life forms that have adapted to oxygen-poor environments

  • tend to hold onto some version of their mitochondria.

  • And all animals have some capacity to use their mitochondria to metabolize oxygen.

  • Even though they live inside other cells,

  • mitochondria have their very own genomes -- called mitochondrial or mtDNA.

  • In this study, published in the journal PNAS,

  • researchers were trying to compare the mitochondrial genomes

  • of H. salminicola and another, closely-related species.

  • Except they discovered thatthey couldn't.

  • Because H. salminicola didn't seem to have mtDNA

  • something we've never confirmed in a multicellular organism before.

  • DNA, both mitochondrial and regular, is what tells a cell how to make different proteins.

  • And without mtDNA, this parasite is missing the instructions

  • to make proteins it would need to turn oxygen into energy.

  • The team was understandably intrigued,

  • and they decided to look at its regular DNA, too.

  • Specifically, they were looking for genes that we know help mitochondria

  • turn oxygen into energy, some of which are in the regular genome

  • rather than the mitochondrial one.

  • And H. salminicola only had 7, while other myxozoans usually have

  • somewhere between 18 and 25.

  • These guys still have structures similar to mitochondria,

  • called mitochondria-related organelles.

  • And these structures probably do help with energy production.

  • Justnot from oxygen.

  • So why'd our salmon-infecting friend toss its ability to use oxygen out the window?

  • It lives in an oxygen-poor environment anyway, such as

  • inside a salmon.

  • So it must get its energy through less efficient, oxygen-independent means

  • specifically, by breaking down sugars without oxygen.

  • Life that does breathe oxygen, like us, can also break down sugars this way,

  • but you get much less energy out of it.

  • We know that when genes go unused for many generations, they can simply

  • be lost.

  • In the case of this parasite, losing its mtDNA could be an evolutionary adaptation.

  • It takes energy to maintain genes.

  • And especially in a creature with a tiny genome like H. salminicola,

  • losing DNA that it wasn't using anyway

  • could have saved energy it could otherwise use to survive.

  • Or something similar could have happened as a fluke.

  • Natural selection can drive genetic change, but so can pure random chance.

  • The team points out that besides revealing the first known animal

  • to not breathe oxygen, this discovery could also be useful on a practical level.

  • This myxozoan parasite can actually be a big problem for salmon farmers,

  • and knowing that is doesn't breathe oxygen

  • could help us make drugs that are better at targeting it.

  • So maybe it wasn't that great a trick, after all.

  • Thanks for watching this episode of SciShow, which is produced by Complexly.

  • If you like learning about super small stuff,

  • we think you'll really like our sister channel, Journey to the Microcosmos.

  • The show's whole MO is slow, calming descriptions of microscopic life,

  • paired with fantastic microscopy and soothing music.

  • It's what we all deserve right now.

  • Check it out at the link in the description!

  • [♩OUTRO]

[♩INTRO]

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B1 中級

酸素を吸えない唯一の動物 (The Only Animal That Can't Breathe Oxygen)

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    林宜悉 に公開 2021 年 01 月 14 日
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