The ash tree (Fraxinus excelsior) is one of Europe’s largest native deciduous trees. Image credit: Matthieu Sontag / CC-BY-SA.
Ash trees (genus Fraxinus) are widespread throughout the Northern Hemisphere, but are being devastated in Europe by the fungus Hymenoscyphus fraxineus, causing ash dieback.
The disease is aggressive, spreads quickly through the population, and has no cure, other than individual natural tolerance to the infection.
It is spread on the wind or via the transfer of infected saplings between areas. Symptoms include loss of leaves and lesions, which are a useful way to diagnose ash dieback, as they leave a characteristic diamond shape scar on the bark.
The disease was first identified in Poland, where it devastated the native ash tree population. It rapidly spread across northern Europe, and was discovered in the UK in 2012.
Now researchers report in the journal Nature that they have sequenced the genome of an ash tree (Fraxinus excelsior) from Gloucestershire, UK, and identified genetic markers for disease tolerance that suggest UK ash trees may have a fighting chance against the fungal infection.
“This ash tree genome sequence lays the foundations for accelerated breeding of ash trees with resistance to ash dieback,” said lead author Dr. Richard Buggs, from the School of Biological and Chemical Sciences at Queen Mary University of London.
The ash tree genome contains some surprises. Up to 25% of its 38,852 genes are unique to ash. Known as orphan genes, they were not found in ten other plants whose genomes have been sequenced.
“Orphan genes present a fascinating evolutionary conundrum as we have no idea how they evolved,” Dr. Dr Buggs said.
The authors had previously tested a genetic screening process on Danish trees. Using these data alongside information from the ash tree genome, they were able to improve the genetic markers for disease tolerance, and use them to predict the tolerance of a sample of trees from across the UK.
Early indications suggest that the proportion of UK trees with tolerance to ash dieback is greater than that of the Danish and Polish trees, but it is still unknown whether the UK trees have previously been infected with the disease and built tolerance or whether their genetic tolerance is yet to be tested.
“Plants use a vast range of chemicals to defend against fungal attack, and the primary objective was to identify differences which could be used to screen young ash trees and choose the best ones for replanting,” said co-author Prof. Murray Grant, from the University of Warwick.
“Our findings underline the need for further research to ensure that we select ash trees resilient to present and future threats.”
The genome sequence will also help efforts to combat the beetle Emerald Ash Borer, which has killed hundreds of millions of ash trees in North America.
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Elizabeth S. A. Sollars et al. Genome sequence and genetic diversity of European ash trees. Nature, published online December 26, 2016; doi: 10.1038/nature20786
