Seed development in flowering plants is initiated by the fusion of two male gametes with two female gametes--the egg cell and the central cell--which leads to the formation of an embryo and an endosperm, respectively. Fertilization-independent seed formation is actively repressed by the FERTILIZATION-INDEPENDENT SEED (FIS) Polycomb group (PcG) proteins, an evolutionarily conserved class of proteins that ensures the stable transmission of developmental decisions. The FIS proteins act together in a complex and modify their target genes by applying repressive methylation on histone H3 lysine 27. In addition to its function before fertilization, the FIS complex restricts endosperm proliferation. This function is likely to be achieved by imprinting the maternal alleles of FIS target genes. However, imprinting in the endosperm is controlled not only by the FIS complex but also by DNA methylation, and the interconnections between these two processes are now being investigated.