Most people are diploid. That is, we have two sets of chromosomes — one set from each parent. But that's not always the case for other species, especially plants.
"Strawberries, for example, have eight sets of chromosomes," says Yves Van de Peer, a plant biologist at Ghent University in Belgium.
This phenomenon, called polyploidy, happens when an organism has more than two sets of chromosomes stuffed into every cell — in other words, a whole genome duplication. And it appears to allow some plant species to survive episodes of extreme environmental stress, like changes in the climate.
It wasn't obvious to biologists that polyploidy would necessarily be a good thing. In fact, having double the chromosomes can ultimately impact a species' survival, leading to its extinction. And yet, it's pretty common today, especially in plants. This forms what Van de Peer calls the polyploidy paradox. Why would so many plants possess a trait that makes them evolutionarily weaker?
In new work published in the journal Cell, Van de Peer and his colleagues suggest an answer to the puzzle. When they looked back at ancient genome duplication events in several hundred plant species, they found that they occurred during periods of turmoil over the last 150 million years — times of dramatic cooling, warming, or widespread extinction. Polyploidy may help species survive such upheavals.
Van de Peer believes he may have finally solved the paradox. "I feel like I can retire now because this is the culmination of 25 years of work," he says with a chuckle.
Clusters in time
You can think of polyploidy, says Van de Peer, as a large-scale mutational event. "Once in a while something goes wrong, and you end up basically with a new cell with twice the amount of DNA than a normal plant cell."
The species may do OK for a while, but this wholesale duplication of the genome has its baggage. More chromosomes can bog down cell division, introducing more opportunity for errors and mutations. And that can lead to other plants — with slimmer genetic loads — to outcompete them, leading to the polyploid plant's extinction.
This is why most genome duplications get lost to time. Van de Peer and his colleagues wanted to understand what accounts for the abundance of modern polyploidy and the seeming relative scarcity of it long ago.
Their first step involved gathering all 470 flowering plant genomes that have been sequenced, a mix of wild species and agricultural crops from all over the world. The research team scoured the DNA of those plants for suites of repeated genes — evidence of whole genome duplication events that occurred long ago.
"Not all the plants showed it," says Van de Peer, but some did. And the researchers used the fossil record to anchor when different plants first evolved to determine when each duplication event happened.
The results were clear. "These whole genome duplications," says Van de Peer, "they do not occur randomly. They are clustered in time."
A polyploid's superpower
In particular, the duplications clustered during episodes characterized by environmental upheaval over the last 150 million years — important cooling or warming periods, for instance.
Or, perhaps most dramatically, one such clustering happened some 66 million years ago when an asteroid collided with Earth, darkening the skies and likely wiping out the dinosaurs — and over half of all plant species.
But not, it appears, many of the polyploid plants. For all their baggage, polyploid plants actually excel at surviving environmental stress — they're a set of "hopeful monsters," as Van de Peer puts it.
Such stressors include prolonged changes in temperature or light level.
"They might be better in doing photosynthesis, for example, because they have more genes to capture the little light that is still there," he explains. "And so they have an advantage over a lot of other plant lineages where there was no whole 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 fade away. But in rare times of extreme turmoil, they win out. And their descendants, which often go on to lose those extra chromosome copies, retain evidence in their DNA of the ancient duplication event that helped their lineage survive.
"The paper really is very rigorous," says Sandra Pitta, a plant biotechnologist at CONICET, Argentina's National Scientific and Technical Research Council who wasn't involved in the study. "And it gives us a lot of hope, in a way."
That hope is due to the fact that our planet is again facing a changing climate, one that polyploid plants may well endure
These findings will also help plant breeders like Pitta. "If polyploidism helps them resist more different types of stresses," she says, "well, that is really useful to me."
Sometimes a seeming dead end can actually pay off in the future — a trick that these plants have up their green sleeves.
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