A chromosome-level genome assembly of a model conifer plant, the Japanese cedar,Cryptomeria japonicaD. Don

Abstract

AbstractJapanese cedar (Cryptomeria japonicaD. Don) is the most important Japanese forest tree, occupying about 44% of artificial forests in Japan, and planted in East Asia, Azores Archipelago, and some islands in the Indian Ocean. Although the huge genome of the species (ca. 11 Gb) with abundant repeat elements might have been an obstacle for genetic analysis, the species is easily propagated by cutting, flowered by plant hormones like gibberellic acid, transformed by agrobacterium, and edited by CRISPR/Cas9. These characteristics ofC. japonicaare preferable to make the species a model conifer for which reference genome sequences are necessary. In this study, we report the first chromosome-level assembly forC. japonica(2n = 22) using a third generation selfed progeny with an estimated homozygosity of 0.96. Young leaf tissue was used to extract high-molecular-weight DNA (>50 kb) for HiFi PacBio long read sequencing and to construct Hi-C/Omni-C library for Illumina short read sequencing. Using the 29× and 26× genome coverage of HiFi and Illumina reads, respectively, de novo assembly resulted in 2,650 contigs (9.1 Gb in total) with N50 contig size of 12.0 Mb. The Hi-C analysis mapped 97% of the nucleotides on the 11 chromosomes. The assembly was verified by comparing with a consensus linkage map of 7,785 markers. The BUSCO analysis confirmed ~91% of conserved genes. Annotations of genes, repeat elements and synteny with other Cupressaceae and Pinaceae species were performed, providing fundamental resources for genomic research of conifers.