What good is genomic imprinting: the function of parent-specific gene expression

Abstract

Genomic imprinting describes a form of non-Mendelian gene expression, in which the expression of an allele depends on its parent of origin. In most cases, one allele (either the one inherited from mother or the one inherited from father) is expressed and the other is transcriptionally inactive, although more complex patterns are also observed. So far, more than 60 imprinted genes have been identified in mice. In most cases, these genes are also imprinted in humans. Among the vertebrates, imprinting seems to be restricted to the marsupials and the placental mammals. Many imprinted genes influence fetal growth, although some affect behavioural phenotypes that persist into adulthood. The most commonly accepted explanation for the prevalence of diploidy is that it shields the individual from the effects of recessive deleterious mutations. Monoallelic expression exposes these mutations to selection, resulting in a 'cost of imprinting'. Therefore, there must be selective forces that overcome these costs at imprinted loci. This review discusses three theories that attempt to explain the selective advantage of genomic imprinting: 'evolvability' models propose that imprinting enhances the adaptive evolution of a species in the face of a changing environment; the ovarian timebomb hypothesis proposes that imprinting protects female mammals from the ravages of trophoblastic disease; and the kinship theory proposes that imprinting arises as a result of an evolutionary conflict in organisms between genes of maternal and paternal origin. Although the kinship theory has been the most successful theory in explaining the observed patterns of imprinting, it is still unclear whether it can account for all instances of imprinting. This determination will have to await a more detailed understanding of the phenotypic effects of imprinted genes.