Background: Dnmt3a , a de novo methylatransferase, is essential for both male and female germ line DNA methylation. Only one Dnmt3a is identified in mammals, and homozygous mutation of Dnmt3a is lethal, while two Dnmt3a , dnmt3aa and dnmt3ab , are identified in teleosts due to the third round of genome duplication, and homozygous mutation of dnmt3aa and dnmt3ab is viable in zebrafish. Dnmt3aa and dnmt3ab were demonstrated to have essential and non-overlapped functions on modulating behavioral control, however, their function in gonadal development is unclear in fish.
Results: In this study, the expression patterns of dnmt3aa and dnmt3ab in developing gonads of Nile tilapia was analyzed by quantitative real time PCR and fluorescence in situ hybridization. Both dnmt3aa and dnmt3ab displayed sexually dimorphic expression in developing gonads. Dnmt3aa was widely expressed in gonadal germ cells and somatic cells, highly expressed in oogonia, phase I and II oocytes and granulosa cells in ovaries and spermatogonia and spermatocytes in testes, while dnmt3ab was mainly expressed in ovarian granulosa cells and testicular spermatocytes. Mutation of dnmt3aa and dnmt3ab was achieved by CRISPR/Cas9 in tilapia. Lower GSI (Gonadosomatic index), increased apoptosis of oocytes and spermatocytes and significantly reduced sperm quality were observed in dnmt3aa −/− mutants, while no obvious phenotype was observed in dnmt3ab −/− mutants. Consistently, the expression of apoptotic genes was significantly increased in dnmt3aa −/− mutants. In addition, dnmt3aa and dnmt3ab were found to have certain compensatory effects in the gonads. The global DNA methylation level in ovaries and testes of dnmt3aa −/− mutants was decreased significantly, compared with that of dnmt3ab −/− mutants and WT.
Conclusions: Taken together, our results suggest that dnmt3aa , not dnmt3ab , plays important roles in maintaining gametogenesis in teleost. Our results enrich the understanding of the function of DNA methyltransferases in gonads of non-mammalian vertebrates.