Genetic barcodes for ash (Fraxinus) species and generation of new wide hybrids

Native ash tree species in Europe and North America are being devastated by ash dieback and the emerald ash borer, respectively. As worldwide ash species differ in their level of susceptibility to these threats, hybrid breeding may allow resistance to be transferred among species. However, we do not know the extent to which distantly related ash species can be crossed, and many ash species are difficult to identify from morphology alone leading to some mislabelling in living collections. Here, we develop a genetic barcode system for the identification of Fraxinus species based on three low-copy-number protein coding genes. We also conduct experimental crosses among ash species in different sections. Our barcodes are effective in identifying ash samples to sectional level and in some cases to species level, and can also identify hybrids. They highlight that F. mandshurica, F. platypoda and F. chiisanensis may be frequently mistaken for one another in living collections. We succeeded in generating ten wide hybrid plants: two of section Melioides (species: F. pennsylvanica) {square} section Fraxinus (species: F. excelsior) and eight of section Ornus (species unclear) {square} section Fraxinus (species: F. excelsior). One hybrid from each of our crosses has survived natural infection with the ash dieback pathogen in Ireland. We also discovered a hybrid between section Melioides (species: F. latifolia) {square} section Fraxinus (species: F. excelsior) formed spontaneously in the ash collection at Kew. Our findings facilitate the deployment of global ash species diversity in response to alien pests and pathogens. Societal impact statementThe world-wide diversity of ash trees includes genetic information encoding resistance to the ash dieback fungus and the emerald ash borer beetle, which are currently devastating ash populations in Europe and North America. In order to mobilise this genetic diversity in conventional breeding programmes we need to be able to accurately identify ash species from around the world, and cross them with one another. Here, we present a genetic barcoding system for ash species, and a series of hybridisation experiments between European ash and other species. Two of the hybrids show early promise against ash dieback.