The chromosome-level genome assembly of Fraxinus americana provides insights into the evolution of Oleaceae plants

Abstract

White ash (Fraxinus americana linn.) originates from the southeastern United States. It is a tall and fast-growing tree species with strong salt-alkali resistance and cold tolerance, making it an important reforestation species and widely planted worldwide. Here, we completed the chromosome-level reference genome assembly of F. americana based on Illumina, PacBio, and Hi-C reads, with a genome size of 878.98 Mb, an N50 of 3.27 Mb, and a heterozygosity rate of 0.3 %. Based on de novo prediction, transcriptome prediction, and homology-based protein prediction, we obtained 39,538 genes. Approximately 843.21 Mb of the assembly genome was composed of 37,928 annotated protein-coding genes, with a gene function annotation rate of 95.93 %. 99.94 % of the overlap clusters (877.44 Mb) were anchored to 23 chromosomes. Synteny analysis of F. americana and other Oleaceae plants showed that F. americana underwent frequent chromosome rearrangements. The amplification of the Ale transposons effectively promoted the genome size of F. americana. Compared with other Oleaceae plants, the Glutathione S-transferase (GST) gene family in the F. americana genome has undergone significant expansion, which may help F. americana cope with adverse natural environments. Furthermore, we found that key enzyme-coding gene families related to lignin biosynthesis were expanded and highly expressed in F. americana leaves. These key genes drive lignin synthesis and benefit F. americana in fast-growing, as well as resisting biotic and abiotic stress. Overall, the F. americana genome assembly provides insights into the evolution of Oleaceae plants and provides abundant resources for breeding and germplasm conservation of white ash.