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In both mammals and plants, Polycomb Repressive Complexes 2 (PRC2) are conserved and appear to be involved in the transition between vegetative or somatic and reproductive state in plants and mammals. In plants at least three different PRC2 control temporal aspects of development, and mutations in PcG cause heterochronies. Such heterochronic mutations affect the transition to flowering. During gametogenesis the Fertilization-Independent Endosperm-MEDEA-PRC2 (FIE-MEA PRC2) complex controls gametogenesis in synergy with a Retinoblastoma-dependent pathway. Several genes of the FIE-MEA pathway are imprinted as shown by their uniparental allele expression in the endosperm, the interface controlling maternal nutrition of the embryo in the seed. Imprinting is also a major feature for genes expressed in the placenta in mammals. Recent data have shown that imprinting in both placenta and endosperm likely share similar mechanisms involving cooperation between the PRC2 complexes and DNA methylation. KEY WORDS: polycomb, endosperm, flowering, reproduction, placentaFlowering plants and mammals are characterised by full maternal control of their sexual reproduction. Fertilisation occurs inside the maternal reproductive tract and the mother nurtures the embryo through a specialised interface, the placenta in mammals and the endosperm in flowering plants. The placenta develops from a specific mass of cell, the trophoblast, set apart after the first divisions of the embryo. The endosperm is the product of fertilisation of a secondary female gamete, the central cell. The central cell is genetically identical to the egg cell. In most plant species the central cell contains two copies of the maternal genome, leading to triploid endosperm. The fertilised egg cell gives rise to the embryo. As the two male gametes of a given pollen tube originate from a single mitosis, they are genetically identical and their zygotic fusion products are also genetically identical. In spite of their unique genetic identity, the embryo and the specialised maternal interface structure (endosperm or placenta) follow very divergent developmental pathways. Such divergence can only be accounted for by different controls of the genome expression potential. Epigenetic regulation involving changes in DNA methylation and in the histone code are likely mechanisms responsible for large-scale expression changes. Histone 3 modifications are mediated in part by Polycomb Group (PcG) class complexes. In plants, PcG genes have a strong impact on the control of the transition to reproductive phase and in endosperm development. In mammals there has been recent evidence that Polycomb Repressive Complexes 2 (PRC2) class of chromatin remodelling Int. J. Dev. Biol. 49: 707-716 (2005) complexes control placenta development. These recent findings will be reviewed and put in perspective with recent advances obtained in plants. PRC2 type PcG complexes control H3 methylationDNA in eukaryotic nuclei is compacted into a structure called chromatin. The basic unit of chro...