Chromosome-level analysis of the Crassostrea hongkongensis genome reveals extensive duplication of immune-related genes in bivalves

Crassostrea hongkongensis is a popular and important native oyster species that is cultured mainly along the coast of the South China Sea. However, the absence of a reference genome has restricted genetic studies and the development of molecular breeding schemes for this species. Here, we combined PacBio and 10 x Genomics technologies to create a C. hongkongensis genome assembly, which has a size of 610 Mb, and is close to that estimated by flow cytometry (similar to 650 Mb). Contig and scaffold N50 are 2.57 and 4.99 Mb, respectively, and BUSCO analysis indicates that 95.8% of metazoan conserved genes are completely represented. Using a high-density linkage map of its closest related species, C. gigas, a total of 521 Mb (85.4%) was anchored to 10 haploid chromosomes. Comparative genomic analyses with other molluscs reveal that several immune- or stress response-related genes extensively expanded in bivalves by tandem duplication, including C1q, Toll-like receptors and Hsp70, which are associated with their adaptation to filter-feeding and sessile lifestyles in shallow sea and/or deep-sea ecosystems. Through transcriptome sequencing, potential genes and pathways related to sex determination and gonad development were identified. The genome and transcriptome of C. hongkongensis provide valuable resources for future molecular studies, genetic improvement and genome-assisted breeding of oysters.