Nature
PUBLISHED
The impact that ended the Cretaceous Period could have been much worse for life on Earth, were it not that many flowering plants carried duplicated genomes, a new paper proposes. If this is true, it could prove useful in limiting the damage caused by rapidly rising global temperatures today.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.When the extinction event that wiped out the non-avian dinosaurs is discussed, it’s common to ponder how various lines of animals survived. Birds, the surviving dinosaurs, are the most obvious example, but puzzlement abounds as to how amphibians or even crocodiles made it through. Plenty of plant species went extinct at the same time, but the secret of the survivors’ success seems less troubling – we know many seeds can remain dormant for years or even centuries and eventually sprout successfully.
However, Professor Yves Van de Peer of Ghent University thinks there is more to it than that. Van de Peer and co-authors point to whole genome duplication as a key survival strategy. It’s common for individual genes to be replicated and find a new home in the genome, sometimes leading to a slightly different role.
Entire chromosomes can also be duplicated. Whole genome duplication, known as polyploidy, is rare in animals, but more common in flowering plants. Most commercial bananas contain three sets of chromosomes, rather than the traditional two for example, and wheat can carry up to six.
“Whole-genome duplication is often seen as an evolutionary dead end in stable environments,” Van de Peer said in a statement. “But in harsh situations, it can provide unexpected advantages.” The authors give the example of cultivars of rice with four copies of their genome, which have been found to cope with more salt and less water than their 2-copy counterparts.
It’s hard to imagine circumstances harsher than a space rock the size of a mountain slamming into the planet, sending a wall of heat around the globe, followed by tsunamis and years of ash blocking the Sun.
DNA is expensive, energetically speaking, to make and requires nutrients that are often scarce. An extra gene or two matters little, but an entire spare genome is a drag that works against survival.
All that extra DNA also means more mutations, and while many will be overridden by the unmutated copies of the genome, some will do real damage. Consequently, most duplications are not passed on for long, if at all.
However, just as most mutations are harmful or neutral, but a few are the fuel on which evolution feeds, the extra opportunities for mutation can come into their own at times. The more copies of its genome a plant has, the more chance of a beneficial mutation that could be just what is needed to survive under pressure.
Using a database of 470 flowering plant genomes, Van de Peer and co-authors looked for pairs of very similar genes, representing a legacy of a past whole genome duplication event. They found 132 examples and investigated the timing of when they appeared, using the fossil record to discover when these species evolved.
The authors concluded that most surviving pairs are the product of duplications that cluster at nine specific points in time. One of the points is a mystery, but the other eight all match with times when the planet underwent relatively sharp change. The biggest clusters appear shortly after the Cretaceous ended, and around the Paleocene-Eocene Thermal Maximum (PETM). The PETM refers to a period when temperatures rose by around 5 to 9°C (9 to 16°F) over approximately 100,000 years and stayed that way for about twice as long.
All that extra DNA may have been even harder to support under the extreme conditions the plants faced during these two events, but apparently the flexibility more than made up for it. Once the crisis passed, most plants dropped the duplication of the whole genome, but kept the matched genes that tell us it occurred.
The same team had found similar clustering in a previous study around the end of the Cretaceous, but drew on a much smaller sample size. The authors acknowledge, however, that some previous estimates of the timing of whole genome duplication events have turned out to be wrong, and some of theirs could also be in error.
Although it is very unlikely human influence will heat the world by as much as during the PETM, the temperature rise is now hundreds of times more rapid. “What we see from the past suggests that polyploidy may help plants cope with these stressful conditions,” Van de Peer said.
The study is published in Cell.
