Coadaptation and conflict, misconception and muddle, in the evolution of genomic imprinting

Abstract

Common misconceptions of the 'parental conflict' theory of genomic imprinting are addressed. Contrary to widespread belief, the theory defines conditions for cooperation as well as conflict in mother-offspring relations. Moreover, conflict between genes of maternal and paternal origin is not the same as conflict between mothers and fathers. In theory, imprinting can evolve either because genes of maternal and paternal origin have divergent interests or because offspring benefit from a phenotypic match, or mismatch, to one or other parent. The latter class of models usually require maintenance of polymorphism at imprinted loci for the maintenance of imprinted expression. The conflict hypothesis does not require maintenance of polymorphism and is therefore a more plausible explanation of evolutionarily conserved imprinting.

Figure 1

An allele (black dot, B) at an ‘imprinting' locus expressed in a parental germline epigenetically modifies sequences at an ‘imprinted' locus. The imprint (white dot) subsequently determines expression in offspring. (a) B acts in cis to establish an imprint. B and the imprint are co-inherited. (b) B acts in cis to block an imprint. B and the absence of imprint are co-inherited. (c) B acts in trans to establish an imprint. All gametes receive the same imprint. (d) B acts in trans to establish an allele-specific imprint at an unlinked locus. B and the imprint segregate independently to offspring.