Some plants have a genetic superpower that may help them survive a cataclysm : NPR

Most people are diploid. That is, we have two sets of chromosomes – one set from each parent. But this is not always the case with other species, especially plants.

“Strawberries, for example, have eight sets of chromosomes,” says plant biologist Yves Van de Peer of Ghent University in Belgium.

This phenomenon, called polyploidy, occurs when each cell of an organism is filled with more than two sets of chromosomes – in other words, a whole genome duplication. And it appears to allow some plant species to survive episodes of extreme environmental stress, such as changes in climate.

It was not clear to biologists that polyploidy would necessarily be a good thing. In fact, having double the number of chromosomes can ultimately impact a species’ survival, leading to its extinction. And yet, it is very common today, especially in plants. This creates what Van de Peyer calls the polyploidy paradox. Why do so many plants have a trait that makes them evolutionarily vulnerable?

In new work published in the journal roomVan de Peer and his colleagues suggested an answer to the puzzle. When they looked at ancient genome duplication events across several hundred plant species, they found that they occurred during periods of upheaval over the past 150 million years – times of dramatic cooling, warming or mass extinction. Polyploidy can help species survive such upheavals.

Van de Peer believes he has finally solved the paradox. “I think I can retire now because this is the culmination of 25 years of work,” he says with a laugh.

cluster in time

You can think of polyploidy as a massive mutational phenomenon, says Van de Peer. “Sometimes something goes wrong, and you basically end up with a new cell with twice the amount of DNA than a normal plant cell.”

The species may survive fine for a while, but this wholesale duplication of the genome has its own effects. Having more chromosomes can inhibit cell division, leaving more opportunities for errors and mutations. And this can cause other plants – with less genetic load – to compete with them, causing the polyploid plant to become extinct.

This is why most genome duplications disappear over time. Van de Peyer and his colleagues wanted to understand what caused the abundance of modern polyploidy and its relative scarcity long ago.

Their first step involved collecting the genomes of all 470 flowering plants that have been sequenced, a mix of wild species and agricultural crops from around the world. The research team tested the DNA of those plants for suites of duplicated genes – evidence of whole genome duplication events that occurred long ago.

“Not all plants showed it, but some did,” says Van de Peer. And when different plants first evolved, researchers used the fossil record to determine when each duplication event occurred.

The results were clear. “These whole-genome duplications do not occur randomly. They are clustered in time,” says Van de Peer.

A Polyploid’s Superpower

In particular, the cluster of duplications formed during episodes characterized by environmental upheaval over the past 150 million years – for example significant cooling or warming periods.

Or, perhaps most dramatically, one such event occurred about 66 million years ago when an asteroid hit Earth, darkening the skies and possibly wiping out the dinosaurs – and more than half of all plant species.

But it appears that there are not many polyploid plants. For all their trappings, polyploid plants actually excel at surviving environmental stress—they’re a bunch of “optimistic monsters,” as Van de Peer puts it.

Such stresses include prolonged changes in temperature or light levels.

“They may be better at photosynthesis, for example, because they have more genes to capture what little light is still present,” he explains. “And so they have an advantage over many other plant lineages where there was no complete genome duplication and they all went extinct.”

In other words, polyploidy is like an insurance policy. Most of the time plants with extra sets of chromosomes are destroyed. But in rare times of extreme turmoil, they prevail. And their descendants, who often lose those extra chromosome copies, retain in their DNA evidence of the ancient duplication event that helped their lineage survive.

“The paper is really very rigorous,” says Sandra Pitta, a plant biotechnologist at CONICET, Argentina’s national scientific and technological research council, who was not involved in the study. “And it kind of gives us a lot of hope.”

This hope is due to the fact that our planet is again facing a changing climate, which polyploid plants can tolerate well.

These findings will also help plant breeders like Pitta. “If polyploidism helps them resist different types of stresses, then fine, that’s really useful to me,” she says.

Sometimes a seeming dead end can actually pay off in the future – there’s a trick these plants have up their green sleeves.