Variation in mature seed size is the result of a complex array of interactions involving genes, genomes and environment that may change during development. Diggle et al. (pp. 309–319) study the dynamics of embryo and seed development in Raphanus sativus and find that there is evidence for a paternal role as well as the expected maternal influence, and the balance between the two may vary. Thus, there may be times during development when selection effects on the maternal, paternal or embryo contributions to development will also vary.