Genomic diversity of elm trees for future treescapes

O_LIDutch elm disease has devastated the treescapes of Europe and North America, causing large elm trees to become largely absent in many areas. Several decades of work are now coming to fruition with the availability of potentially resistant cultivars for restoration. C_LIO_LIHere, using single nucleotide polymorphisms (SNPs) derived from whole genome sequences of over 200 trees, we investigate the genomic diversity of elm trees growing historically in Britain and Europe, and that of some new cultivars; including widespread native European and British elms; exotic elm species with natural resistance; and the often-complex hybrids between these and native elms available from the various breeding programmes and living collections. We also sequenced a variety of disease-resistant selections from Spain of interest for planting in Britain. C_LIO_LIWhile confirming many specimen labels and multi-generational crossing records, our results also revealed unexpected misidentifications and origins of some samples. This included past hybridisation in some resistant selections thought to be pure species. We find that within our samples, field elm (Ulmus minor) is more genetically diverse C_LIO_LIthan Wych elm (U. glabra). We show that over ten British elm morphological forms sometimes assigned species epithets fall within the genetic range of U. minor. C_LIO_LIWith their often-exotic parentage, many of the elm cultivars introduced into Britain post-epidemic inevitably represent a considerable increase in nucleotide diversity over the still numerically large, native U. minor and U. glabra populations. Currently, the cultivars are being planted mainly as ornamentals. Their long term arboricultural, adaptive and landscape suitability compared to native elms remains to be demonstrated. C_LI