Dynamics of DNA Methylation and DNMT Expression During Gametogenesis and Early Development of Scallop Patinopecten yessoensis.

DNA methylation reprograms during gametogenesis and embryo development, which is essential for germ cell specification and genomic imprinting in mammals. Corresponding process remains poorly investigated in molluscs. Here, we examined global DNA methylation level in the gonads of scallop Patinopecten yessoensis during gametogenesis and in embryos/larvae at different stages. DNA methylation level fluctuates during gametogenesis and early development, peaking at proliferative stage of ovary, growing stage of testis, and in blastulae. To understand the mechanisms underlying these changes, we conducted genome-wide characterization of DNMT family and investigated their expression profiles based on transcriptomes and in situ hybridization. Three genes were identified, namely PyDNMT1, PyDNMT2, and PyDNMT3. Expression of PyDnmt3 agrees with DNA methylation level during oogenesis and early development, suggesting PyDNMT3 may participate in de novo DNA methylation that occurs mainly at proliferative stage of ovary and testis, and in blastulae and gastrulae. PyDnmt1 expression is positively correlated with DNA methylation level during spermatogenesis, and is higher at maturation stage of ovary and in 2-8 cell embryos than other stages, implying possible involvement of PyDNMT1 in DNA methylation maintenance during meiosis and embryonic development. This study will facilitate better understanding of the developmental epigenetic reprogramming in bivalve molluscs.