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

Abstract

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. While sequences from the internal transcribed spacer (ITS) of nuclear ribosomal DNA can identify some Fraxinus species, this is often hindered by intragenomic variation in ITS sequences within the genus. 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. mandschurica, F. platypoda and F. chiisanensis are frequently mistaken for one another in living collections. We succeeded in generating ten wide hybrid plants: two of F. pennsylvanica X F. excelsior and eight of section Ornus X F. excelsior. One hybrid from each of our crosses has survived natural ash dieback inoculation in Ireland. Our findings facilitate the deployment of global ash species diversity in response to alien pests and pathogens.