It’s cold and dark. Ash and dust cloud the air, blocking out the sun.
And you’re stuck with nowhere to go. You can’t run or hide or burrow deep underground.
But you also can’t just do nothing. You’re
a living organism and survival is the name of the game.
So what happens to you?
This was the challenge plants faced in the wake of the asteroid impact that ended the
reign of dinosaurs, some 66 million years ago at the end of the Cretaceous Period.
And many of them didn’t make it. Roughly 60% of all plant species went extinct.
But the ancestors of plants we know today did survive… So, how’d they do it?
Well, for some of them, at least part of the answer may be hidden in their genomes.
It’s the legacy of a particular kind of mutation event, one that might only pay off
in times of extreme environmental stress.
And it affected many different plant lineages almost simultaneously – from the forebears
of potatoes, to lotuses, to legumes.
It looks like these ancient survivors doubled their DNA.
Today, angiosperms, or flowering plants, are the largest and most diverse group of plants
on the planet. They represent nearly 90% of all living plant species.
They first appeared in the fossil record maybe as early as the mid-Jurassic Period and rapidly
radiated into all kinds of environments by the Early Cretaceous.
And they’re the foundation of most terrestrial ecosystems today: from rainforests to meadows
to our farms, which grow mostly angiosperm crops.
Our world just wouldn't be the same without them.
And many aspects of their remarkable evolutionary
success has been pretty mysterious for a long time.
But over the last 20 or so years that we’ve been digging into their genomes, scientists
have found a strange pattern that might be an important piece of the puzzle.
Angiosperms have experienced many instances
of Whole Genome Duplication in their evolutionary history.
Which is exactly what it sounds like - ancient
events where their ancestors duplicated all of their DNA.
Now these events are pretty dramatic and sudden. And they happen totally by accident.
See, errors in the process of meiosis, which generates reproductive cells, end up leaving
them with twice as much DNA as they should have.
Then, when they fertilize other reproductive cells with doubled DNA, the offspring is what's
called a polyploid - an individual with extra copies of its genome.
And wherever we look in the angiosperm family tree, we see that living species come from
polyploid ancestors that experienced multiple whole genome duplications in deep time.
And this is way more common in angiosperms than any other group. Some lineages have even
had up to six duplication events.
Now, this is a bit of a conundrum, because many biologists thought that polyploidy should
generally be an evolutionary dead-end.
Organisms shouldn’t just be able to copy
their genomes all willy-nilly-like. And it often leaves
plants with issues that make them a worse fit for their niche.
So, in theory at least, polyploids should
be weeded out by natural selection over time…except
these weren’t. Instead, they seem to have been rampant across flowering plant evolution.
And in 2009, researchers comparing plant genomes noticed that the timing of these whole genome
duplication events didn't seem to be purely random.
They found a wave of them across angiosperms all dating to around 66 million years ago,
right at the end of the Cretaceous period.
Was this just a coincidence? Or could it be that individuals with double their usual DNA
had a survival advantage during this time?
While polyploidy usually comes with a bunch of problems, in the right environmental context,
there are also reasons to think it might be an evolutionary shortcut to success.
That’s because doubling the genome also
creates a lot of adaptability and evolutionary opportunity.
Natural selection suddenly has all that extra genetic raw material to innovate with.
It can repurpose redundant, extra copies of genes and genetic pathways for entirely new
roles, or use them to rewire existing pathways, changing how they work.
Evolution is, after all, a legacy system. It works with what it’s got, tweaking existing
features rather than creating brand new features from scratch.
So suddenly having twice the genome to play
with can allow rapid adaptation in ways that weren't possible before.
Polyploids may be less fit compared to non-polyploids
when conditions are stable and the name of
the game is being fine-tuned to fit a specific niche.
But in times of environmental upheaval, like, say, the aftermath of a global mass extinction,
they can tolerate a broader range of conditions and so, suddenly, they have an advantage.
And we have modern evidence that supports this idea, too.
Polyploidy seems to be especially common today in invasive plant species, and those that
live in stressful and unstable environments.
It seems to help them find their ecological footing.
Could the same process have been behind that ancient wave of whole genome duplications
we see in angiosperms right around the end-Cretaceous
mass extinction?
And if so, what kind of advantages could it have offered to help them withstand the cataclysm
and adapt to the post-apocalyptic conditions?
Well, in 2019 researchers in China published
a new study claiming to have found some answers to those questions.
They compared the genomes of 23 living species from across the angiosperm family tree, including
corn, rice, and bananas.
Using that data, they searched for duplicated genes in each lineage that could be traced
back to around 66 million years ago.
See, much of the duplicated DNA is gradually lost from the genome over time. This is usually
the case if it’s not doing anything important or is causing trouble.
But if that duplicated DNA does provide some kind of advantage, like gaining a helpful
new function, then evolution is more likely to keep it around.
And what they found was pretty amazing.
Multiple lineages of plants still had duplicates of similar genes that had come from that wave
of polyploids that appeared at the end of the Cretaceous.
This suggests that they experienced similar environmental challenges at that time and
they responded by evolving the same solutions from their pool of extra DNA.
And many of those genes were involved with two processes: shade avoidance and cold tolerance.
Now, the long period of darkness and cold that immediately followed the asteroid impact
would have been perhaps the biggest challenge for plants around that time.
So this makes sense. Polyploids that could quickly adapt their genetic stress responses
to darkness and cold, using their duplicated genes, may have gained a survival advantage.
And the researchers found that those independently duplicated genes are actually still active.
They can be found in the cold-tolerance and shade-avoidance pathways of many living species.
The genetic tools that polyploids forged to cope with that cataclysm are still contributing
to the ways their descendants respond to times of stress today.
And it doesn't seem to have been the only time this happened in plants, either.
The scientists saw two other, smaller waves of whole genome duplication in flowering plants
on either side of that mass extinction.
One occurred around much earlier, around 120 million years ago in the Cretaceous,
during the early evolution of two main groups of angiosperms.
Many of the duplicated genes retained from
this ancient wave were involved in water deprivation
and salt stress, a possible consequence of the arid climate around that time.
And the other wave was much more recent, within the last 20 million years. The climate cooled
during this period as carbon dioxide levels dropped.
Duplicated genes from this event included ones for cold, salt, water, and wounding.
We spend so much time talking about animals here on Eons that sometimes we forget how
fascinating plant evolution can be.
And we’re still just beginning to understand
the role that whole genome duplication played in it.
Plant genomes are often massive, messy, and hard to make sense of.
But it seems more and more likely that, rather than always being a dead end, whole genome
duplication can actually be a shortcut to success under the right conditions…
One that may have been key to the survival of many angiosperms during some especially
plant-phobic periods of the planet’s history.
Perhaps for plants in times of great stress and ecological upheaval, the more DNA the better.
Want more plant content? Check out our episode, “How Pollination Got Going Twice”, to
learn about the long running relationship between insects and plants.
And thanks to this month’s Eontologists who double the fun!
Gale Brown, Juan M, Jacksy Weiss, Melanie Lam Carnevale, Raphael Haase, Annie & Eric
Higgins, John Davison Ng, Jake Hart, and Colton.
By becoming an Eonite at patreon.com/eons, you can get fun perks, like submitting a joke
for us to read. Here’s one from Kassy W.
What did Sherlock Holmes say when Watson asked
what kind of a rock he was holding? “Sedimentary, Watson!”
Ah, that's so great! I'm reading Sherlock Holmes right now, so this is perfect.
Perfect!
"Sedimentary, my dear Watson."
...love it.
And as always thank you for joining me in
the Adam Lowe studio. Subscribe at youtube.com/eons
for more evolutionary escapades.
...and it often leaves plants with issues that make them a worse fit for their niche.
[trying different pronunciations] Kneech, kneech, knitch?
Knitch? Knitch. I'll do that one more time.