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  • The Nile tilapia Oreochromis niloticus is


    The Nile tilapia (Oreochromis niloticus) is an important farmed fish with an XX/XY sex-determining system. The availability of the whole genome sequence of tilapia and tissue transcriptomes (Brawand et al., 2014), together with its gonadal transcriptomes at different developmental stages (Tao et al., 2013) made it an excellent model for genome wide identification, tissue distribution and gonadal expression profile investigation for dnmts family. Given the significance of dnmts in diverse biological processes and to better understand the role of DNA methylation in the epigenetic regulation of Nile tilapia gonads development, we: (i) identified all dnmts encoding genes in whole genome of Nile tilapia; (ii) compared the divergence expression pattern of dnmts based on transcriptome data from eight adult tilapia tissues; (iii) characterized the spatial and temporal expression pattern of dnmts during gonadal development; (iv) verified the IL-4, murine recombinant expressing population of the dnmts family members in the developing gonads; (v) evaluated the expression pattern of dnmts and the key genes in sex differentiation after the treatment of DNA methyltransferase inhibitor (5-aza-2′-deoxycytidine).
    Materials and methods
    Conclusions In this study, we identified dnmt1, dnmt2, and five dnmt3 paralogs genes in Nile tilapia. Phylogenetic analysis revealed a high conservation of dnmts family in teleosts, with dnmt3 expansion due to fish-specific duplications. Four de novo dnmts (dnmt3aa, dnmt3ab, dnmt3bb.1, dnmt3bb.2) were sexual dimorphically expressed and highly expressed in testis. Especially, their expression were significantly up-regulated in sex-reversed gonad compared to normal ovary. The treatment of the methyltransferase inhibitor significantly down-regulated the dnmts expression in gonads, which in turn, up-regulated the expression of key genes involved in sex differentiation. Taken together, these results provide a new perspective on the evolution of dnmts family and a solid foundation for uncovering the roles of dnmts in teleosts, in relation to gonadal development.
    Competing financial interests
    Author contributions statement
    Acknowledgments This work was supported by grant 31572609 and 31630082 from the National Natural Science Foundation of China; grant cstc2015jcyjB80001 from the Natural Science Foundation Project of Chongqing, Chongqing Science and Technology Commission.
    Introduction DNA methylation is a biochemical process where a methyl group is added to a nucleotide base, and the reaction is catalyzed by DNA methyltransferases. DNA methylation and histone modifications represent two key determinants in epigenetic regulation of gene expression in mammalian cells (O'Neill, 2015). DNA methylation is crucial for normal development and is involved in processes such as gene expression and genomic imprinting, X-chromosome inactivation, inhibition of recombination, aging and carcinogenesis (Bird, 2002). In mammals, DNA methylation is essential for normal development and has several functions. In general, methylation within gene regulatory elements suppresses gene expression (Razin and Riggs, 1980; Jones and Takai, 2001). Methylation of gene-deficient regions is vital for maintenance of chromosome structure and integrity (Robertson and Jones, 2000; Friso and Choi, 2002; Choi and Friso, 2009). Among the many functions of DNA methylation, the necessary connection between promoter methylation and gene silencing has yielded the most convincing evidence (Choi and Friso, 2009). DNA methylation changes during development both in gametogenesis and embryogenesis. After fertilization, parental genomes undergo demethylation with the exception of imprints (Stuppia et al., 2015). After implantation, the embryo undergoes de novo methylation (He et al., 2011). Imprints are erased and reestablished during gametogenesis. Methylation of DNA occurs at the 5-position of cytosine and is mediated by enzymatic action of DNA methyltransferases (DNMT). In mammals, the family of DNMTs comprises four members: DNMT1, DMNT3A, DNMT3B and DNMT3L. DNMT2 functions as a tRNA methyltransferase.