Parental conflict does not necessarily lead to the evolution of imprinting

For some genes, the epigenetic state (whether they are expressed) depends on whether the gene is inherited through the mother or the father. Such imprinting, or parent-specific gene expression (PSGE), occurs in mammals, including humans, and higher plants. The theory that PSGE solves genetic conflict between mother and father is widely accepted. We argue, however, that the conditions for PSGE to evolve are restricted. With respect to seed size, PSGE can only evolve when the developing offspring has a strong effect on its own resource acquisition. When seed size is close to the optimum for the maternal parent, there is no internal conflict in the offspring because maternally and paternally derived genes both favour increased seed size. Although the literature generally suggests that the maternal parent controls seed size, a number of observations suggest an additional role for the paternal parent. Here, we critically evaluate these studies and suggest a rigorous methodology for establishing paternal effects on seed size, which can be applied to the model species Arabidopsis thaliana.