Comparative Methylome Analysis Reveals Epigenetic Signatures Associated with Growth and Shell Color in the Pacific Oyster, Crassostrea gigas

Fast growth is one of the most important breeding goals for all economic species such as the Pacific oyster (Crassostrea gigas), an aquaculture mollusk with top global production. Although the genetic basis and molecular mechanisms of growth-related traits have been widely investigated in the oyster, the role of DNA methylation involved in growth regulation remains largely unexplored. In this study, we performed a comparative DNA methylome analysis of two selectively bred C. gigas strains with contrasted phenotypes in growth and shell color based on whole-genome bisulfite sequencing (WGBS). Genome-wide profiling of DNA methylation at the single-base resolution revealed that DNA methylations were widely spread across the genome with obvious hotspots, coinciding with the distribution of genes and repetitive elements. Higher methylation levels were observed within genic regions compared with intergenic and promoter regions. Comparative analysis of DNA methylation allowed the identification of 339,604 differentially methylated CpG sites (DMCs) clustering in 27,600 differentially methylated regions (DMRs). Functional annotation analysis identified 11,033 genes from DMRs which were enriched in biological processes including cytoskeleton system, cell cycle, signal transduction, and protein biosynthesis. Integrative analysis of methylome and transcriptome profiles revealed a positive correlation between gene expression and DNA methylation within gene-body regions. Protein-protein interaction (PPI) analysis of differentially expressed and methylated genes allowed for the identification of integrin beta-6 (homolog of human ITGB3) as a hub modulator of the PI3K/Akt signaling pathway that was involved in various growth-related processes. This work provided insights into epigenetic regulation of growth in oysters and will be valuable resources for studying DNA methylation in invertebrates.