The British Ash Tree Genome Project: Sequencing the Genome of Fraxinus excelsior (European Ash)

Fraxinus excelsior (European Ash) is a common tree in Europe with about 80 million individuals in the UK alone, and is of great ecological and economic value as a key forest species. It is currently threatened by the fungal pathogen Hymenoscyphus pseudoalbidus; the causative agent of ash dieback. In Denmark, approximately 95% of trees have been affected by the disease since 2003, and it is expected that a similar percentage will be susceptible in the UK, where dieback was only recently first identified in 2012. Natural genetic variation within ash populations includes a small percentage of low-susceptibility individuals, which could provide a basis for the selective breeding of disease resistance. The British Ash Tree Genome Project is providing a reference F. excelsior genome sequence, to make the identification of genetic variants conferring low susceptibility possible on a genome-wide scale.

We have sequenced and assembled the 880Mb genome de novo, of a low-heterozygosity British ash tree produced by self-pollination, using a combination of Illumina HiSeq2000 and Roche 454 FLX+ technologies. Comparing the results of several de novo assemblers, we have selected the best assembly, consisting of 90k scaffolds and an N50 of almost 100kb. It is available for download on the project’s website, www.ashgenome.org, along with our raw sequencing data. We are also annotating the assembly with RNA-seq data from several tissues. This reference genome may also assist scientists working on the emerald ash borer infestation of F. pennsylvanica and F. americana in North America.