GENOMIC IMPRINTING IS DISRUPTED IN INTERSPECIFIC PEROMYSCUS HYBRIDS

Genomic imprinting, the unequal expression of gene alleles on the basi s of parent of origin, is a major exception to mendelian laws of inher itance(1). By maintaining one allele of a gene in a silent state, impr inted genes discard the advantages of diploidy. and for this reason th e rationale for the evolution of imprinting has been debated(2). One e xplanation is the parent-offspring conflict model, which proposes that imprinting arose in polyandrous mammals as the result of a parental c onflict over the allocation of maternal resources to embryos(3). This theory predicts that there should be no selection for imprinting in a monogamous species. Crosses between the monogamous rodent species Pero myscus polionotus and the polyandrous Peromyscus maniculatus yield pro geny with parent-of-origin growth defects that could be explained if i mprinting was absent in the monogamous species(4). We find. however, t hat imprinting is maintained in P. polionotus, but there is widespread disruption of imprinting in the hybrids. We suggest that the signals governing genomic imprinting are rapidly evolving and that disruptions in the process may contribute to mammalian speciation.