Insights on Imprinting from Beyond Mice and Men

Genomic imprinting is an epigenetic phenomenon that results in the silencing of alleles, dependent on their parent of origin. Within vertebrates, this phenomenon is restricted only to the mammals and has been identified in eutherians and marsupials but not in the egg-laying monotremes. Many hypotheses have been put forward to explain why genomic imprinting evolved, most of which are centered on the regulation of nutrient provisioning from parent to offspring. The three different mammalian lineages have adopted very different modes of reproduction and, as a result, vary widely in the amount of nutrient provisioning to the conceptus. Examining imprinting across the three mammal groups enables us to test hypotheses on the origin of this phenomenon in mammals and also to investigate changes in the genome coincident with its evolution.