Genomic imprinting and conflict-induced decanalization.

Genomic imprinting is the phenomenon in which the expression pattern of an allele depends on its parental origin. When maternally expressed and paternally expressed imprinted loci affect the same trait, the result is an arms race, with each locus under selection to increase its level of expression. This article develops a model of the deleterious consequences of this escalation, deriving from an increase in the variance in gene expression level, and resulting increase in phenotypic variance in the population. This phenomenon is referred to here as "conflict-induced decanalization." Modifiers that canalize gene expression are selectively favored, but these induce further escalation from both loci, resulting in a net increase in phenotypic variance and a reduction in population mean fitness. This results in a feedback loop, where increasing canalization of gene expression leads to increasing decanalization of the phenotype. This phenomenon may explain the surprisingly high frequency of certain diseases. Disorders to which this decanalization process might contribute include growth- and metabolism-related phenomena such as preterm birth, as well as certain major psychiatric disorders, including schizophrenia and autism.