Evolution of genomic imprinting: Insights from marsupials and monotremes

Abstract

Parent-of-origin gene expression (genomic imprinting) is widespread among eutherian mammals and also occurs in marsupials. Most imprinted genes are expressed in the placenta, but the brain is also a favored site. Although imprinting evolved in therian mammals before the marsupial-eutherian split, the mechanisms have continued to evolve in each lineage to produce differences between the two groups in terms of the number and regulation of imprinted genes. As yet there is no evidence for genomic imprinting in the egg-laying monotreme mammals, although these mammals also form a placenta (albeit short-lived) and transfer nutrients from mother to embryo. Therefore, imprinting was not essential for..