Host defenses to transposable elements and the evolution of genomic imprinting.

Genomic imprinting is the differential expression of maternally and paternally inherited alleles of specific genes. Several organismic level hypotheses have been offered to explain the evolution of genomic imprinting. We argue that evolutionary explanations of the origin of imprinting that focus exclusively on the organismic level are incomplete. We propose that the complex molecular mechanisms that underlie genomic imprinting originally evolved as an adaptive response to the mutagenic potential of transposable elements (TEs). We also present a model of how these mechanisms may have been co-opted by natural selection to evolve molecular features characteristic of genomic imprinting.