The Origins of Genomic Imprinting in Mammals

Abstract

Genomic imprinting is a process that causes genes to be expressed according to their parental-origin. Imprinting appears to have evolved gradually in two of the three mammalian subclasses with no imprinted genes yet identified in prototheria and only six found to be imprinted in marsupials to date. By interrogating the genomes of eutherian suborders, we determine that imprinting evolved at the majority of eutherian specific genes prior to the eutherian radiation. Theories considering the evolution of imprinting often relate to resource allocation and recently consider maternal-offspring interactions more generally, which in marsupials places a greater emphasis on lactation. In eutherians, the imprint memory is retained at least in part by ZFP57, a KRAB-Zinc Finger Protein that binds specifically to methylated imprinting control regions. Some imprints are less dependent on ZFP57 in vivo and it may be no coincidence that these are the imprints that are found in marsupials. Since marsupials lack ZFP57, this suggests another more ancestral protein evolved to regulate imprints in non-eutherian subclasses, and contributes to imprinting control in eutherians. Hence understanding the mechanisms acting at imprinting control regions across mammals has the potential to provide valuable insights into our understanding of the origins and evolution of genomic imprinting.