DNA Methylation of pacap Gene Is Involved in Growth Superiority of Female Half Smooth Tongue Sole ( Cynoglossus semilaevis ) in Different Salinities

Pituitary adenylate cyclase activating polypeptide (PACAP) and growth hormone-releasing hormone (GHRH) play important roles in the GH/IGF growth axis in fishes. To determine whether epigenetic change is involved in the regulation of pacap and ghrh responses to low salinity stress in Cynoglossus semilaevis, the correlation between growth traits, DNA methylation status and gene expression level in low salinity (15, S15) and optimal salinity (30, S30) at day 7 (D7) and day 60 (D60) were analyzed. Results showed that exposure to low salinity for 60 days attenuated C. semilaevis growth rate. Under low salinity, DNA methylation level of pacap promoter increased in females and decreased in males at day 7, but inverted at day 60. Additionally, pacap expression was up-regulated in both males and females. The pacap promoter methylation level was higher and expression level was lower in female than in male. The results suggest that pacap promoter methylation level is negatively correlated to mRNA level and positively correlated to body weight, while gene expression level is negatively related with body weight. With the decrease of salinity, DNA methylation level of ghrh promoter and exon1, as well as its gene expression displayed minor changes. Overall, pacap gene seems to play an important role in fish growth, contributing to female growth superiority, while ghrh gene seems not pertinent under salinity stress. The results indicate that low salinity potentially affects fish growth through regulating DNA methylation in pacap promoter. This study expands the understanding of the molecular mechanism of how salinity modulates fish growth from the epigenetic perspective.