Scientists at the Royal Botanic Gardens, Kew and Queen Mary University of London have discovered that a new generation of ash trees, growing naturally in woodland, exhibits greater resistance to the disease compared to older trees. They find that natural selection is acting upon thousands of locations within the ash tree DNA, driving the evolution of resistance. The study, published in Science, offers renewed hope for the future of ash trees in the British landscape and provides compelling evidence for a long-standing prediction of Darwinian theory.
Ash dieback, caused by the fungus Hymenoscyphus fraxineus, arrived in Britain in 2012, prompting an emergency COBRA meeting. The disease has since wrought havoc on the British countryside, leaving behind skeletal remains of dying ash trees. Past predictions estimate that up to 85% of ash trees will succumb to the disease, with none displaying complete immunity.
The new study compared the DNA of ash trees established before and after the fungal invasion. Researchers observed subtle shifts in the frequencies of DNA variants associated with tree health across thousands of locations in the genome. These shifts indicate that the younger generation possesses greater resistance than their predecessors, offering hope for the survival of ash trees.
This research provides a real-world example of natural selection in action, as theorised by Charles Darwin. It also demonstrates selection on a trait influenced by numerous genes, a phenomenon that has been widely assumed but difficult to prove.
Professor Richard Nichols, Professor of Evolutionary Genetics at Queen Mary University of London, commented: "A tragedy for the trees has been a revelation for scientists: allowing us to show that thousands of genes are contributing to the ash trees' fightback against the fungus. Our detection of so many small genetic effects was possible because of the exceptional combination of circumstances: the sudden arrival of such a severe disease and the hundreds of offspring produced by a mature tree."
Dr Carey Metheringham, whose PhD research included this study, commented: "Thanks to natural selection, future generations of ash should have a better chance of withstanding infection. However, natural selection alone may not be enough to produce fully resistant trees. The existing genetic variation in the ash population may be too low, and as the trees become scarcer, the rate of selection could slow. Human intervention, such as selective breeding and the protection of young trees from deer grazing, may be required to accelerate evolutionary change."
Professor Richard Buggs, Senior Research Leader (Plant health and adaptation) at the Royal Botanic Gardens Kew, and Professor of Evolutionary Genomics at Queen Mary University of London, commented: "We are so glad that these findings suggest that ash will not go the way of the elm in Britain. Elm trees have struggled to evolve to Dutch elm disease, but ash are showing a very different dynamic because they produce an abundance of seedlings upon which natural selection can act when they are still young. Through the death of millions of ash trees, a more resistant population of ash is appearing."
Buggs added: "Lots of textbooks about evolution have hypothetical examples of natural selection driving change in quantitative traits (for example, size and speed of wolves) but these are actually hard to prove in real life cases. Here, we provide a real example which is characterised at the DNA level."
The study, which was mainly funded by the Department for Environment Food and Rural Affairs (Defra), was carried out in Marden Park wood in Surrey, which is owned and managed by the Woodland Trust. Rebecca Gosling, Lead Policy Advocate (Tree Health and Invasive Species) at the Woodland Trust said: "Ash dieback demonstrates how devastating introduced pathogens can be for our trees and the species which rely upon them. This important research gives us hope for the future of our ash populations. The findings highlight how vital it is to support natural regeneration in woodlands, furthering our understanding of how to best manage our ash woodlands."
Professor Nicola Spence, Defra's Chief Plant Health Officer, said: "Tackling the growing threat from tree pests and diseases due to climate change is critical to protect the long-term health and resilience of our trees. Defra has invested over £9 million in ash dieback related research since the confirmation of the disease in the UK in 2012. This study, funded by Defra, contributes significantly to our knowledge base by demonstrating that tolerance to ash dieback is heritable, and highlighting that breeding programmes and natural regeneration could work together to secure the future of our native ash tree."
