5 – The Origins of Genomic Imprinting in Mammals

Genomic imprinting is an exceptionally good tool to study epigenetic gene regulation because both the active and the silent allele are retained in the same nucleus. Genomic imprinting appears to be conserved in all placental mammals. It is also present in marsupial mammals where two autosomal genes plus X chromosome inactivation are known to be imprinted. Genomic imprinting likely arose in the vertebrate lineage, after the formation of mammals but prior to the divergence of the marsupial and placental subclasses. However, the presence of parental specific gene expression in the endosperm of angiosperm plants, for example Arabodopsis and parental specific chromosome inactivation linked to sex determination in scale insects ( Coccidea ) indicate that genomic imprinting evolved several times and it serves more than one biological function. Since the majority of genes in the modern day mammalian genome are non-imprinted and show biallelic expression, it is logical to consider the imprinting mechanism initially as one that acts to silence one of the two parental alleles. Models A and B are outlined to show two possible scenarios for acquisition of imprinting by parental-specific silencing of genes that show biallelic expression in an ancestral population.