Endosperm triploidy has a selective advantage during ongoing parental conflict by imprinting

The endosperm of the flowering plant mediates the supply of maternal resources for embryogenesis. An endosperm formed in sexual reproduction between diploid parents is typically triploid, with a 2 : 1 ratio of maternal genetic material (denoted as 2m : 1p). Variation from this ratio affects endosperm size, indicating parent-specific expression of genes involved in endosperm growth and development. The presence of paternally or maternally imprinted genes can be explained by parental conflict over the transfer of nutrients from maternal to offspring tissue. Genomic imprinting can, for example, provide the male parent of an embryo in a mixed-paternity seed pod, with an opportunity for expressing its preference for a disproportionate allocation of resources to its embryo. It has been argued that a diploid 1m : 1p endosperm was ancestral and the 2m : 1p endosperm evolved after parental conflict, to improve maternal control over seed provisioning. We present a population genetic model, which instead places the origin of triploidy early in the parental conflict over resource allocation. We find that there is an advantage to having a triploid endosperm as the parental conflict continues. This advantage can help to explain why the 2m : 1p endosperm prevails among flowering plants.