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[♪ INTRO]
Most animals have two sets of chromosomes. That’s nice for them.
It’s neat and tidy. Makes sense.
And then there’s plants.
It turns out it’s super common for plants to play around with polyploidy,
the technical name for having more than two full sets of chromosomes.
Even in the food we eat. Maybe especially in the food we eat.
So in this episode, we’re going to talk a little bit about the what, why, and how.
You’d never guess just from looking at them,
but potatoes can have four sets of chromosomes.
That’s known as 4n - n for “number of copies”.
Kiwis can be 6n, and strawberries…
Strawberries are 8n.
Let’s quickly recap what exactly that means.
In plants and animals, our DNA isn’t one long string,
but instead it is packed into discrete units called chromosomes.
And we usually have two copies of each chromosome,
because we inherit one copy from each parent.
Polyploidy means going above and beyond that very logical system.
This can sometimes happen in animals, but it’s generally
more of a genetic accident than a life strategy.
A lot of the time, when animals are polyploid, they’re triploid,
meaning 3n: three sets of chromosomes.
But that’s a problem, because it generally makes that animal sterile.
Three is an odd number, and cells can’t split three sets up evenly when it comes
time to make sperm and egg cells, so those cells end up failing at their
basic job of passing on the genome.
This is actually how seedless watermelons are made.
We intentionally create triploid watermelons.
But for a lot of plants, polyploidy isn’t just a one-generation accident. It’s built in.
With even numbers of copies, like 4n or 6n, they can not just survive,
but reproduce and create whole polyploid lineages and species.
In fact, all plant species may have some episode of
genome duplication in their past.
Sometimes the plants keep their new copies, like strawberries.
Other times, they edit things back down to 2n,
keeping some versions of their genes and ditching others.
But all plants seem to have done this at some point.
As for how plants can get away with this when animals generally can’t?
It’s not entirely clear.
It’s been suggested that it might have to do with
how plants and animals control their chromosomes.
Many animal species tightly control the “dose” of information
coming from each chromosome.
Like, mammals who have two X chromosomes only need one.
So they turn the other one off.
Whereas plants might not have to worry about it.
Or maybe it’s because plants have more flexible body plans than animals.
We have to have our arms and legs just so, but plants can just sort of have a leaf,
and then another leaf, and then another one somewhere else.
But it’s not just something plants can get away with.
Polyploidy seems to confer some genuine advantages.
Having multiple copies of a gene might make
harmful variants of that gene less problematic, for instance.
But the really cool part is that extra copies of genes can
be kind of a blank slate for evolution to play around with over time.
With other copies of those genes still doing their thing, the extra copies
of genes are then free to change things up and acquire new functions.
It’s like turning a spare bedroom into an office.
Over time, this could even lead to new species.
And it might actually be related to plants recovering from mass extinctions.
The timing of these duplication events in plant genomes seems
to correspond closely with extreme changes, disasters, and mass extinctions.
Polyploidy may have provided an advantage in the form of increased adaptability.
Or it may have hitched a ride with a different trait, like asexual reproduction.
It’s been suggested that asexual reproduction makes polyploidy more likely.
And in the aftermath of a mass extinction, not needing to find a mate
before having babies might have been a winning strategy in harsh times.
Strawberries, for instance, can reproduce via seeds, but also via runners,
which result in clones of the parent plant.
That said, there are potential disadvantages to polyploidy.
More copies of chromosomes means more DNA,
and that makes the nucleus of the cell, which houses the DNA, bigger.
And that can cause problems.
Like, it’s just generally harder to find the right gene in all of that mess.
There might also be problems with gene regulation,
since suddenly doubling things can throw the balance of the cell out of whack.
So what does this mean for your strawberry torte?
Well, it turns out strawberries are as difficult as they are delicious.
Even molecular biologists admit they are notoriously complex to study.
Part of the reason is that eight copies of chromosomes is just, like,
a lot to deal with as you’re doing science.
Literally; it’s hard to tell one copy of a chromosome from its seven other buddies.
Trying to piece together their genome can be a bit like sticking eight copies of
Hamlet into a blender and then trying to figure out
where all the “thees” and “thous” go.
This makes answering the question of why they have eight copies,
what specific event or factor led to this genetic traffic jam, kind of hard.
Nevertheless, there has been a good amount of progress lately.
And it looks like our modern strawberries are a hybrid of naturally octoploid
species, further tweaked by domestication in the roughly
300 years or so of mass strawberry consumption.
So while domestication can be linked to polyploidy, it turns out strawberries
may have been as weird as they are long before we got our hands on them.
Anyways, while we are still looking for the secret to the strawberry riddle,
this is kind of a cool subject to dive into generally, as studying
the genetics of plants can help us fight diseases or create new varieties.
It’s also a cool reminder about the diversity of life.
It’s pretty easy to get caught up with how weird and wonderful and wacky
animals are and to think the other kingdoms of life are just kind of the background.
But fungi, microbes, and, yes, even the humble strawberry are just as strange
and wonderful as any other organism on Earth.
Thanks for watching this episode, and thank you to
HelloFresh for sponsoring this episode!
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