FORMAL COMMENT Coadaptation between Mother and Offspring: Why Not?

The Kinship Theory provides a compelling explanation for the evolution of genomic imprinting at many loci. However, it need not explain the appearance of imprinting at all loci to be a valid theory, given that other processes can potentially favour imprinting [1]. Therefore, despite its predominance in the literature, we do not believe that we must necessarily consider the Kinship Theory as the default theory, even if we can construct a compatible scenario. Consequently, when hypothesizing about the evolution of imprinting of Grb10 [2], we have suggested that the appearance of complementary effects of Grb10 in mothers and pups is consistent with the expec-tation of the coadaptation theory [3]; when mothers and pups share the same expression status, pups show the “normal ” phenotype, which could potentially favour maternal expression in pups. We agree that the Kinship Theory can likewise potentially accommodate evolution ofGrb10 as a maternally expressed inhibitor of early growth. Therefore, the two theories provide contradictory scenarios for Grb10 imprinting in pups that cannot be readily differentiated using existing data. In our discussion of imprinting at Grb10 in mothers and pups [2], we logically focused particularly on explanations for imprinting arising as a consequence of parent–offspring in-teractions and hypothesized that maternal Grb10 expression in the mother’s mammary glan