Genomic Imprinting and the X Chromosome

David Haig
Department of Organismic and Evolutionary Biology; Harvard University

A gene with sexually-antagonistic effects confers fitness benefits in one sex but fitness costs in the other. Therefore, sexual antagonism can favor sex-limited expression in which a gene is silenced in one sex but active in the other. By analogy, a gene can be said to have parentally-antagonistic effects if it confers fitness benefits when inherited from one sex but fitness costs when inherited from the other. Therefore, parental antagonism can favor imprinted expression in which the gene is silenced when inherited from one sex but not the other. Sexual and parental antagonism are confounded for X-linked loci because a gene that is only expressed when paternally-derived (imprinted expression) is only expressed in females (sex-limited expression). Both sexual and parental antagonism have been invoked to explain imprinting of X-linked loci. These theories will be discussed.