Genetic conflict reflected in tissue-specific maps of genomic imprinting in human and mouse

Genomic imprinting is an epigenetic process that restricts gene expression to either the maternally or paternally inherited allele(1,2). Many theories have been proposed to explain its evolutionary origin(3,4), but our understanding has been limited by a paucity of data mapping the breadth and dynamics of imprinting within any organism. We generated an atlas of imprinting spanning 33 mouse and 45 human developmental stages and tissues. Nearly all imprinted genes were imprinted in early development and either retained their parent-of-origin expression in adults, or lost it completely. Consistent with an evolutionary signature of parental conflict, imprinted genes were enriched for co-expressed pairs of maternally/paternally expressed genes, showed accelerated expression divergence between human and mouse, and were more highly expressed than their non-imprinted orthologs in other species. Our approach demonstrates a general framework for imprinting discovery in any species, and sheds light on the causes and consequences of genomic imprinting in mammals.