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.

DNA甲基化和组蛋白甲基化是两种最重要的表观遗传修饰。然而，关于它们在雄性和雌性鱼性腺中差异表达的研究是有限的。在这项研究中，我们检查了野生型和减数雌核橄榄比目鱼Paralichthys olivaceus性腺中组蛋白 H3 (H3K4me3) 修饰谱的 DNA 甲基化和赖氨酸 4 的三甲基化特征。酶联免疫吸附试验 (ELISA) 分析显示，睾丸中的整体 DNA 甲基化水平高于卵巢。实时定量 PCR (qPCR) 结果表明维持 DNA 甲基转移酶基因dnmt1和从头 DNA 甲基转移酶基因dnmt3a在卵巢中高度表达，而 DNA 去甲基转移酶基因tets在睾丸中高度表达。性腺中DNA甲基化和甲基转移酶基因的不一致可能与睾丸中的差异分布有关。免疫组织化学 (IHC) 显示主要位于睾丸精子细胞中的 5-mC。此外，通过原位杂交（ISH）分析，dnmt3a和tets主要位于精母细胞和卵母细胞中。至于H3K4me3，用蛋白质印迹法检测的卵巢中总水平较高。IHC结果显示，睾丸支持细胞中H3K4me3的信号强于精母细胞和精子细胞。甲基转移酶基因kmt2b和去甲基化酶基因kdm5a和kdm5c在 qPCR 的睾丸中也表现出更高的表达，并且在精母细胞中检测到kmt2b和kdm5s的 ISH 信号更强。这些结果表明DNA甲基化和H3K4me3参与了比目鱼的性别差异和配子发生。