Reproduction Abstracts

Dynamic epigenetic reprogramming occurs during mammalian germ cell development, whereas the targets of this process including DNA methylation/demethylation remain poorly understood. Here, we examined genome-wide methylation profiles in developing and fully developed germ cells of mice using whole-genome bisulfite sequencing (WGBS). We scaled down the construction and analysis to nanogram quantities of DNA by generating a new WGBS library, termed the post-bisulfite adapter tagging (PBAT) method. Thus, we could provide complete maps of cytosine methylation in mature sperm cells, fully-grown oocytes, and developing male/female primordial germ cells (PGCs). Our methylome study demonstrated genome-wide DNA demethylation, with erasure of genomic imprinting and X-inactivation during gonadal sex determination, and gender-specific differences in genome-wide and gene-specific DNA methylation levels in developing germ cells. Some of these global/local changes in DNA methylation during germ cell progression were consistent with previous studies. However, our complete DNA methylome maps revealed important and novel details of DNA methylation/demethylation processes. Some of the new findings from this study include the following: i) DNA methylomes exhibited sex- and chromosome-specific differences in genome-wide CpG methylation during early to late PGC development; ii) LINE and LTR retrotransposons were resistant to DNA demethylation during PGC migration; iii) non-CpG methylation occurred in male gonocytes during mitotic arrest and fully-grown oocytes; iv) identification of over a thousand germline differentially methylated CpG islands; and v) strong gene-body methylation in fully-grown oocytes. Our present data establish the basis for future studies on the role of epigenetic modifications in germline development and other biological processes.