The nearly complete genome of Ginkgo biloba illuminates gymnosperm evolution

Gymnosperms are a unique lineage of plants that currently lack a high-quality reference genome due to their large genome size and high repetitive sequence content. Here, we report a nearly complete genome assembly for Ginkgo biloba with a genome size of 9.87 Gb, an N50 contig size of 1.58 Mb and an N50 scaffold size of 775 Mb. We were able to accurately annotate 27,832 protein-coding genes in total, superseding the inaccurate annotation of 41,840 genes in a previous draft genome assembly. We found that expansion of the G. biloba genome, accompanied by the notable extension of introns, was mainly caused by the insertion of long terminal repeats rather than the recent occurrence of whole-genome duplication events, in contrast to the findings of a previous report. We also identified candidate genes in the central pair, intraflagellar transport and dynein protein families that are associated with the formation of the spermatophore flagellum, which has been lost in all seed plants except ginkgo and cycads. The newly obtained Ginkgo genome provides new insights into the evolution of the gymnosperm genome.