Effects of salinity stress on methylation of the liver genome and complement gene in large yellow croaker (Larimichthys crocea)

Salinity is an important environmental factor that affects the yield and quality of large yellow croaker (Lar-imichthys crocea) during aquaculture. Here, whole-genome bisulfite sequencing (WGBS), RNA-seq, bisulfite sequencing PCR (BSP), quantitative real-time PCR (qPCR), and dual luciferase reporter gene detection tech-nologies were used to analyze the DNA methylation characteristics and patterns of the liver genome, the expression and methylation levels of important immune genes in large yellow croaker in response to salinity stress. The results of WGBS showed that the cytosine methylation of CG type was dominant, CpGIsland and repeat regions were important regions where DNA methylation occurred, and the DNA methylation in upstream 2k (2000bp upstream of the promoter) and repeat regions had different changes in the liver tissue of large yellow croaker in the response to the 12%o, 24%o, 36%o salinity stress of 4 w (weeks). In the combined analysis of WGBS and transcriptome, the complement and coagulation cascade pathways were significantly enriched, in which the complement-related genes C7, C3, C5, C4, C1R, MASP1, and CD59 were mainly changed in response to salinity stress. In the studied area of MASP1 gene promoter, the methylation levels of many CpG sites as well as total cytosine were strongly negatively correlated with mRNA expression level. Methylation function analysis of MASP1 promoter further proved that DNA methylation could inhibit the activity of MASP1 promoter, indicating that salinity may affect the expressions of complement-related genes by DNA methylation of gene promoter region.