Genomic Imprinting in Animals: Evolutionary Conservation with Adaptive Flexibility

Imprinting is an epigenetic mechanism found in many diploid organisms that enables discrimination between genetic material inherited from one or the other parent. It has been predominantly studied in mammals, where, in the adult, it affects the expression of a limited set of genes (in most higher mammals) or leads to paternal X chromosome inactivation in marsupials. DNA CpG methylation and heterochromatic epigenetic marks play a crucial role in these processes. Imprinting also exists in other organisms, where it can cause unusual parent-of-origin-dependent effects. Two classical examples are whole paternal genome inactivation in mealybug males and selective paternal X chromosome elimination in fungus gnats. Although these manifestations of imprinting are quite exotic, they share features in common with those observed in mammals, implying that, despite the differences, the core mechanism, the H3K9me3:HP1:H4K20me3 pathway, is evolutionarily conserved across animal phyla. Here, we discuss recent advances that provide further evidence supporting this concept.