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.