Figure 1.
Illustrations of three scenarios that could favor the evolution of imprinted gene expression.
The genome of each individual is represented by two symbolic chromosomes, with the left chromosome representing maternally inherited alleles, and the right chromosome paternally inherited alleles. Different colors and patterns on the chromosomes are used to suggest different allelic variants. (A) The Kinship Theory applied to fetal growth modifiers. The focal offspring is more closely related to its litter-mates through its maternally inherited allele than through its paternally inherited allele. Paternally inherited alleles favor greater demand on maternal resources, because their inclusive fitness is less affected by the indirect costs of reducing the pool of maternal resources available to the mother's other offspring. (B) One way the Kinship Theory might explain certain imprinted gene effects in adults. If demographic processes (e.g., sex-biased migration) create groups that are more closely related through their maternally than paternally inherited alleles, imprinted expression could be favored at genes that affect the fitness of other group members (e.g., by favoring “cooperation”). (C) The central idea of the Coadaptation Theory. The mother, who carries two “red” alleles has three hypothetical offspring, each of which inherits one “red” allele from her, and one “blue” allele from the father. The offspring on the left silences its paternally inherited (blue) allele, and thus expresses the “red” phenotype, matching the mother. The offspring at the center and right show biallelic (red+blue = purple) and paternal (blue) expression, respectively, resulting in phenotypes that do not match that of the mother. Paternal silencing is favored if phenotype matching (or complementarity) leads to increased fitness.