Bisulfite sequencing libraries from opossum male immortalised fibroblasts
Decades of work in placental (eutherian) species have constructed a paradigm of mammalian development, wherein the genome-wide erasure of parental DNA methylation is required for embryogenesis1-10. Whether such DNA methylation reprogramming is, in fact, conserved in other mammals is unknown. To resolve this point, we generated base-resolution DNA methylation maps in gametes, embryos and adult tissues of the opossum marsupial Monodelphis domestica, revealing extensive variations from the eutherian-derived model. In stark contrast with eutherians, the marsupial genome remains hypermethylated during the cleavage stages and in the embryo proper of the blastocyst. In the extra-embryonic trophectoderm DNA methylation is reduced, suggesting an important evolutionary conserved function for DNA hypomethylation in formation of the mammalian placenta. Furthermore, unlike in eutherians, the inactive X chromosome becomes globally DNA hypomethylated during embryogenesis. Using our DNA methylation profiles, we identify a candidate mechanism for imprinted X-inactivation in marsupials, via maternal promoter DNA methylation of the Xist-like non-coding RNA RSX11. How mammalian embryos employ DNA methylation to regulate their development is therefore more mechanistically diverse than current models can accommodate. Overall design: BS-seq was performed on three replicates of WT and DNMT1-KO (mosaic) opossum male immortalised fibroblasts at day4 after the DNMT1-KO.