4 Conclusion
In this study, we assembled a chromosome-level Asian Clam genome using a combination of PacBio and Hi-C technology. The results suggested the high quality of the genome in several ways. The data filled the gap in our knowledge of C. fluminea and provide a reference for future research. We assembled 1.52 Gb of genome data distributed across 18 chromosomes, with a contig N50 of 521.06 Kb and a scaffold N50 of 70.62 Mb. We assigned 99.17% of contig sequences to the 18 chromosomes, thus ensuring the integrity of genetic information for each chromosome as much as possible. Additionally, the comparative genomics studies offer evidences for the evolution and characteristics of C. fluminea , and showed that C. fluminea and its closest relatives, the ancestors of marine bivalves, diverged ~ 492.00 million years ago. MITF gene family analysis identified two genes in the Asian Clam genome. The genic tree comprising all MITF family genes from involved species showed that the relationship of MITF family genes was similar to that shown by the phylogenetic tree. All lines of evidence suggest that C. fluminea is clearly different from other marine bivalves, and its migration from marine to freshwater is an ancient event. In addition, we examined expanded gene families and found 9 significantly enriched pathways associated with metabolite synthesis by KEGG analysis. The vitamin B6 metabolic pathway revealed relatively few genes involved in pyridoxal, pyridoxine, or pyridoxamine synthesis, providing reference for explaining the lower content of VB6 in C. fluminea . The genomic information presented in our analysis will help to better understand, develop, and improve C. fluminea as well as establish a strong foundation for genome-assisted breeding programs in the future.