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

Abstract

Societal Impact Statement: The 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 preliminary genetic barcoding system for ash, and a series of hybridisation experiments between European ash and other species. Two of the hybrids show early promise against ash dieback. Summary: 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 initiate development of a genetic barcode system for the identification of Fraxinus species based on low-copy-number protein coding genes. We also conduct experimental crosses among ash species in different sections. Three genes are effective in identifying ash samples to sectional level but only in some cases to species level. They highlight that Fraxinus mandshurica, Fraxinus platypoda and Fraxinus chiisanensis may be frequently mistaken for one another in living collections. We succeeded in generating 10 wide hybrid plants: two of section Melioides (species: Fraxinus pennsylvanica) × section Fraxinus (species: Fraxinus excelsior) and eight of section Ornus (species unclear) × 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) × section Fraxinus (species: F. excelsior) formed spontaneously in the ash collection at Kew. Our findings facilitate the utilisation of global ash species diversity in response to novel threats and highlight the difficulty of designing a barcoding system capable of distinguishing all species of a genus.