Chromosome-level genome assembly of grass carp (Ctenopharyngodon idella) provides insights into its genome evolution.

Background: The grass carp has great economic value and occupies an important evolutionary position. Genomic information regarding this species could help better understand its rapid growth rate as well as its unique body plan and environmental adaptation.

Results: We assembled the chromosome-level grass carp genome using the PacBio sequencing and chromosome structure capture technique. The final genome assembly has a total length of 893.2 Mb with a contig N50 of 19.3 Mb and a scaffold N50 of 35.7 Mb. About 99.85% of the assembled contigs were anchored into 24 chromosomes. Based on the prediction, this genome contained 30,342 protein-coding genes and 43.26% repetitive sequences. Furthermore, we determined that the large genome size can be attributed to the DNA-mediated transposable elements which accounted for 58.9% of the repetitive sequences in grass carp. We identified that the grass carp has only 24 pairs of chromosomes due to the fusion of two ancestral chromosomes. Enrichment analyses of significantly expanded and positively selected genes reflected evolutionary adaptation of grass carp to the feeding habits. We also detected the loss of conserved non-coding regulatory elements associated with the development of the immune system, nervous system, and digestive system, which may be critical for grass carp herbivorous traits.

Conclusions: The high-quality reference genome reported here provides a valuable resource for the genetic improvement and molecular-guided breeding of the grass carp.