Parental epigenetic asymmetry in mammals.

The early mammalian embryo is marked by genome-wide parental epigenetic asymmetries, which are directly inherited from the sperm and the oocyte, but are also amplified a few hours after fertilization. The yin-yang of these complementary parental programs is essential for proper development, as uniparental embryos are not viable. The majority of these parental asymmetries are erased, as the embryonic genome assumes its own chromatin signature toward pluripotency and then differentiation, reducing the risk for haploinsufficiency. At a few loci, however, parent-of-origin information persists through development, via maintenance and protective complexes. In this review, we discuss the parental asymmetries that are inherited from the gametes, the forces involved in their elimination, reinforcement or protection, and how this influences the embryonic program. We highlight the gradual loss of all parental asymmetries occurring throughout development, except at imprinted loci, which maintain distinct parent-of-origin chromatin and transcriptional characteristics for life. A deeper understanding of the nongenetic contributions of each germline is important to provide insight into the origin of non-Mendelian inheritance of phenotypic traits, as well as the risk of incompatibilities between parental genomes.