Sexual dimorphism of DNA and histone methylation profiles in the gonads of the olive flounder Paralichthys olivaceus.

DNA methylation and histone methylation are two types of the most important epigenetic modifications. However, research on their differential expression in gonads of male and female fish is limited. In this study, we examined the characteristics of DNA methylation and tri-methylation of lysine 4 of histone H3 (H3K4me3) modification profiles in the gonads of the wild-type and meio-gynogenetic olive flounders Paralichthys olivaceus. Enzyme-linked immunosorbent assay (ELISA) analysis revealed that the global DNA methylation level was higher in the testis than in the ovary. Real-time quantitative PCR (qPCR) results indicated that maintenance DNA methyltransferase gene dnmt1 and de novo DNA methyltransferase gene dnmt3a are highly expressed in the ovary, while DNA demethyltransferase genes tets are highly expressed in the testis. The inconsistency of DNA methylation and methyltransferase genes in the gonads might associate with the differential distribution in the testis. 5-mC mainly located in the spermatids of the testis was shown with immunohistochemistry (IHC). Furtherly, dnmt3a and tets are mainly located in spermatocytes and oocytes with in situ hybridization (ISH) analysis. As for H3K4me3, total level is higher in the ovary detected with western blot assay. IHC results showed that the signals of H3K4me3 in Sertoli cells of the testis were stronger than those in spermatocytes and spermatids. Methyltransferase gene kmt2b and demethylase genes kdm5a and kdm5c also exhibit much higher expression in the testis with qPCR, and ISH stronger signals of kmt2b and kdm5s were detected in spermatocytes. These results implied that DNA methylation and H3K4me3 are involved in the flounder sex differences and gametogenesis.