During germline development in eutherian (placental) mammals, gamete precursors (primordial germ cells) undergo genome-wide erasure of epigenetic marks (1,2). This process, termed epigenetic reprogramming, safe-guards against precocious germline differentiation until the gonadal niche is sufficiently developed to dictate male or female fate, at which point methylation levels are restored (3).
While this process is critical to development of the eutherian germline, its role in other vertebrate groups is largely unknown. Evidence from zebrafish (4) suggests that reprogramming is not shared by non-mammalian vertebrates and arose somewhere in the mammalian lineage. Marsupials—the sister group to eutherian mammals—are ideally placed to test this hypothesis.
Since marsupials have short pregnancies and are born highly altricial, germline development occurs post-natally (5). We used post-bisulfite adaptor tagging (PBAT; 6) and deep sequencing to assess global levels of DNA methylation in PGCs isolated from brushtail possum pouch young (Trichosurus vulpecula) across early development (2 to 80 days post-partum (dpp)). Previous work has established that at least one imprinted region is reprogrammed in a marsupial (7), but ours is the first study to investigate genome-wide methylation dynamics.
We found that global levels of DNA methylation decreased from 62 to 41% between 2 and 13 dpp, developmentally equivalent to humans and mice (1,2). PGCs remained hypomethylated for approximately 10 days and methylation was restored (65%) by 38 dpp. Methylation loss occurred primarily at CpG islands while repeat elements retained almost all methylation, as for eutherian mammals (1).
Our findings demonstrate that broad patterns of reprogramming are conserved in both mammalian groups. However, since marsupial PGCs retain substantial levels of global methylation (>40% compared with 14-7% in mice) the marsupial pattern involves global methylation reduction, not complete erasure. Like non-mammalian vertebrates, the marsupial germline may thus hold greater potential for transgenerational epigenetic inheritance (4).