Chromosome-scale genome assembly of Castanopsis tibetana provides a powerful comparative framework to study the evolution and adaptation of Fagaceae trees.

Abstract

Fagaceae species are increasingly used as models to elucidate the process and mechanism of adaptation and speciation by integrating ecology, evolution and genomics. The genus Castanopsis belongs to the family Fagaceae and is mainly distributed across subtropical and tropical Asia. In the present study, we reported the first chromosome-scale genome assembly of Castanopsis tibetana, a common species of evergreen broadleaved forests in subtropical China. The combination of Nanopore sequencing and Hi-C technologies enabled a high-quality genome assembly. The final assembled genome size of C.tibetana was 878.6Mb (97.6% of the estimated genome size), consisting of 477 contigs with an N50length of 3.3Mb. The benchmarking universal single-copy orthologue (BUSCO) assessment indicated a completeness of 93.0%. Hi-C scaffolding generated 12pseudochromosomes, representing 98.7% of the assembled genome. Subsequently, 40,937 protein-coding genes were predicted and 90.04% of them were functionally annotated. More than 476.9Mb of repetitive sequences (54.3% of the genome) were identified, and the percentage of the genome covered by TE elements was 39.98%. Comparative genomics analysis revealed that C.tibetana was most closely related to Castanea mollissima and diverged at 18.48Ma, and that C.tibetana has undergone considerable gene family expansion and contraction. Evidence of positive selection was detected in 53genes, which showed different arrangement pattern compared to Quercus robur. The chromosome-scale genome assembly of C.tibetana will expand Fagaceae genome resources across the family and provide a powerful comparative framework to study the adaptation and evolution of Fagaceae trees.