Intersexual conflict over seed size is stronger in more outcrossed populations of a mixed-mating plant

Intersexual conflict over maternal resource allocation to offspring can lead to the evolution of imprinted genes with parentof- origin–specific expression. However, the precise mechanism involved in the evolution of such imprinted genes is less well understood, and few clear predictions have been presented.We resolve this issue, and, using different populations of a mixedmating plant, we demonstrate that more outcrossed paternal populations produce larger seeds when crossed with less outcrossed maternal populations, and vice versa. This provides clear support for a “tug-of-war” mechanism operating between maternally and paternally imprinted genes. Such a mechanism can have important consequences for local adaptation in offspring size in the presence of gene flow between populations with different mating systems.acceptedVersio