Genes to resist ash dieback

What's going on with the UK's ash trees?
26 November 2019

Interview with 

Richard Nichols, QMUL


Ash tree


In 2012, the first cases of a fungal disease - dubbed “ash dieback” - that was wiping out ash trees across Europe were detected in the UK, probably after the fungus hitched a ride here on infected imported plants. The disease spreads when fungal spores blow from an infected tree onto an uninfected specimen, which they invade. Luckily, the disease is not universally fatal, and about 5% of ash trees carry genes that mean they’re naturally resistant to the infection. To find out what those genes are, UK scientists have compared the DNA sequences of susceptible and more resilient ash trees. And this has enabled them to come up with a way to spot the trees not to fell and that are best suited for breeding programmes to repopulate forests. Amalia Thomas reports...

Amalia - Ash trees are one of the most common types of trees in the UK and they are under serious threat of being wiped out by a fungal disease called ash dieback, which was introduced from Europe in 2012. There is to this day, no cure to this disease once the tree is affected, but curiously a small portion of our Ash trees are immune to dieback and in a recent study, scientists from Queen Mary University London and from the Royal Botanic Gardens in Kew found out why. I spoke with one of the authors of the study, Richard Nichols, who explained their results.

Richard - We found that the trees that survived were genetically distinct from the trees that died from ash dieback. We found evidence, not that there's a single gene involved in resistance, but genes spread all across the chromosomes. Hundreds, perhaps thousands of genes, each with a small effect contributing to the resilience of some trees.

Amalia - To put things in perspective, Richard explains the devastating effects of this disease on the UK's ash tree population.

Richard - When the disease arrives, it's catastrophic. Around about 70% of the ash trees in a woodland will die. That includes the well-established mature trees, so across the country that amounts to about 70 million trees. A major component of our woodland.

Amalia - Their study into how to stop ash dieback was based on samples collected by forest research.

Richard - There was a mass screening trial, that means 150,000 trees have been planted out in various locations around the South of Britain, and we went and found the worst effected trees and the relatively unaffected trees. All in all, we took a thousand samples.

Amalia - And from these samples they were able to extract genetic information from their DNA.

Richard - We had a whole series of millions of reads from trees which had been affected and another series of reads, which we knew had come from trees which were unaffected, and we compared the two to look for differences. So a lot of the differences that we've found may be spurious differences because there are so many variations in the genetic code. But if we look across all of the chromosomes in an ash tree, even if nothing was going on genetically, we would be bound to find some differences just by chance between the diseased trees and those which are relatively unaffected. But when we looked at those genes which had the largest, most impressive differences, they did seem to make sense. They did seem to be genes which in other species have been found to be associated with disease resistance.

Amalia - So from a small piece of an ashtree, Richard and his team are able to predict whether or not it will be susceptible to ash dieback.

Richard - What we do is we create a score. We look at the trees and say, well, of our suspect genes, the genes that we think might be involved, how many is this tree got? And then we look to see how well the tree performed. And when we did that, we found we got remarkably accurate predictions. So when a tree had a really high score, then with 80% probability it was one of the more resilient trees.

Amalia - Identifying which trees are resilient to ash dieback is a huge step forward in the battle of conservation of the ash tree population and the ecosystems they sustain, which are so significant in the UK.

Richard - What we want to be able to do is to breed trees which will be useful to to repopulate woodlands which will be resilient to the disease and we can use the genetic information to do that, and the second thing we would like to be able to do is go to a woodland which has been infected and identify the trees which are going to do relatively well because we don't want to cut down all the trees. If some trees are going to do well and would be good to re-seed the woodland. We want to be able to identify those and not to cut them down.

Richard - This is a really important example of why our conservation efforts should maintain large, diverse populations, because if our ash trees had been just one cultivar which was planted all over the country, we wouldn't have this natural reservoir of resilience.


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