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Bdelloid rotifers are microscopic animals that look like cigars with a brush on the end.
They are not very impressive looking.
But they are some of the toughest creatures in the animal world.
Their superpower?
If their DNA is shredded to pieces, whether because of a lack of water or a blast of radiation,
they can put it back together.
They might even use the situation to their advantage and acquire new genes in the process.
It’s all in the name of survival in aquatic habitats that can dry out
and leave the rotifers without a place to live.
You might be thinking, wait, this sounds familiar,
because tardigrades famously survive under similar conditions.
We talked about them way back in SciShow’s fifth video ever.
But stay with me, because rotifers are -- arguably -- even more hardcore.
And they do it in their own way.
There are over 400 species of bdelloid, and many of them can tolerate desiccation,
or drying out.
They live in droplets on mosses or in temporary ponds, which can dry out and force the rotifers
to enter a sort of suspended animation known as anhydrobiosis.
Their bodies contract into a dried-out form that’s less than 10% water, which is too
dry for biochemical reactions to happen.
It’s hitting a complete pause button on being alive.
There are serious challenges to surviving this dried-out state.
One of them is the fact that it’s extremely bad for DNA.
At this level of desiccation, DNA molecules just snap.
Both strands of the backbone holding them together give way in what’s termed a double-strand break.
And DNA doesn’t work if there’s a break in the middle of a sequence the cell needs to use.
Even worse, most organisms aren’t very efficient at repairing double-strand breaks.
The usual repair processes might stick the wrong ends together or introduce new base pairs,
either of which would cause potentially dangerous mutations.
Bdelloids are known for their resistance to radiation, which also causes double-strand breaks.
A 2008 study by researchers in Massachusetts
found that bdelloids could withstand a dose of radiation that caused about 500 breaks per copy of their genome
and only suffer a drop in their ability to reproduce of about 20%.
But their resistance to radiation is probably a coincidence,
an unintended, but super awesome side effect of the fact that both radiation and drying out cause the same DNA-snapping problem.
The weirdest thing of all is that bdelloids don’t seem to fix their DNA until they rehydrate.
It’s not clear how they do this, but it seems like maybe the proteins that fix their DNA
pull through the desiccation process even when the DNA is damaged.
That was what researchers found in a 2012 study in the journal PNAS,
bdelloids experienced DNA damage from radiation at about the same level as other, similar animals, like nematodes.
But the damage to their proteins was kept relatively in check.
So maybe when the rotifers are exposed to water again, the proteins fix the DNA,
and the rotifers swim away.
But stitching up their shredded DNA isn’t the only weird thing rotifers can do.
They can also borrow genes from other organisms, including bacteria, plants, and fungi.
Acquiring genes in the absence of sexual reproduction, which, by the way, bdelloids don’t seem to go for,
is called horizontal gene transfer.
Bacteria do it all the time, but it’s incredibly uncommon in animals.
And yet, as many as 8-9% of bdelloid genes may come from other organisms.
At least one study has found that desiccating species of bdelloids acquire more foreign DNA
than their cousins who don’t dry out.
Which leads to the hypothesis that they actually do it WHILE stitching their DNA back together.
Got some free DNA ends here?
Got a spare gene?
Stick it in there.
Maybe it’ll even work.
We don’t know for sure that this is actually what’s going on,
especially because even bdelloids that don’t dry out still have some foreign genes.
But this chaotic borrowing leads to perhaps the strangest example of how rotifers are
so good at not dying.
A 2015 study in the journal PLoS One showed how horizontal gene transfer could help bdelloids survive desiccation,
as well as shed new light on an old mystery.
Many desiccation-resistant organisms, including tardigrades,
fill their bodies with something to take the place of water when they dry out.
For a lot of these organisms, that something is a sugar called trehalose.
Trehalose hasn’t been found in bdelloids, but when the researchers took a closer look at one species’ genome,
they found that it had some genes for both making and breaking down trehalose.
One seems to have been acquired from plants, and one from bacteria.
Both appeared to be expressed in the bdelloid’s genome,
so it was using the genes to make something,
although we don’t know whether the products were biochemically active.
The breakdown gene was expressed more than the building one,
leading the researchers to speculate that trehalose could be produced in bdelloids,
then broken down too quickly to be detected.
Whether this is an actual mechanism for bdelloids to survive desiccation is far from clear.
Still, it’s pretty incredible for an organism to potentially piece together an entire biochemical pathway
from bits it finds laying around.
Which is what makes rotifers some of the most incredible survivors of the animal world.
Like some bizarre zombie MacGyver,
they’re able to piece themselves together all the way back from the practically dead.
We’ve still got a lot to learn about how they do it,
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