Figure 1.

Scenarios of imprinted gene perturbation in hybrids. (a) A hypothetical imprinted gene (rectangles) has a different level of expression in one species (‘genotype X') than in another (genotype Y). In the reciprocal X × Y and Y × X hybrids, this leads to different expression levels, with possible phenotypic consequences. A similar differential expression arises in hybrid offspring in case a gene is imprinted in one species, but not in the other. (b) Some imprinted genes are present in certain analysed species only. Shown is a recently evolved imprinted gene present in species X (top of the figure) but not in species Y (not shown). In the X × Y and Y × X reciprocal hybrids, this leads to either normal expression or a complete lack of expression. Small red spheres indicate DNA methylation. Arrows depict RNA expression; their thickness reflects levels of transcription.