Figure 2.

Scenarios through which trans-acting factors may affect imprinted gene expression in hybrids. (a) Trans-acting factors involved in the somatic maintenance of allelic repression at imprinted genes. To the left, the situation in a wild-type embryo, in which the two paternal genomes are of the same genotype (‘X' genotype). A trans-acting repressor (large red sphere) maintains allelic repression at an imprinted target gene elsewhere in the genome. In embryos of the hybrid X × Y genotype, the Y genome-encoded repressor protein does not repress efficiently the imprinted gene of the X genotype, leading to loss of imprinting. (b) A trans-activator-imprinted gene encodes an activating factor (green spheres), which maintains allelic gene transcription at multiple imprinted target genes, on both the parental genomes. In embryos of hybrid genotype—shown here is the example of hybrid X × Y genotype—the factor activates imprinted genes on its own parental (X) genome, but not on the opposite parental (Y) genome. This leads to an aberrant acquisition of repressive modifications and loss of expression at these imprinted genes. Imprinted trans-acting factors can not only be proteins but also non-coding RNAs.