A new generation of ash trees is showing resistance to the disease (CREDIT: Paul Figg, RBG Kew)
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, is showing greater resistance to the disease than the older trees.
Natural selection
They found that natural selection is working in the ash DNA across thousands of locations, 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.
"A tragedy for the trees has been a revelation for scientists"
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."
A real example of DNA-level evolution
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."
