Genomic imprinting resulting in the differential expression of maternal and paternal alleles in the fertilization products has evolved independently in placental mammals and flowering plants. In most cases, silenced alleles carry DNA methylation [1]. Whereas these methylation marks of imprinted genes are generally erased and reestablished in each generation in mammals [2], imprinting marks persist in endosperms [3], the sole tissue of reported imprinted gene expression in plants. Here we show that the maternally expressed in embryo 1 (mee1) gene of maize is imprinted in both the embryo and endosperm and that parent-of-origin-specific expression correlates with differential allelic methylation. This epigenetic asymmetry is maintained in the endosperm, whereas the embryonic maternal allele is demethylated on fertilization and remethylated later in embryogenesis. This report of imprinting in the plant embryo confirms that, as in mammals, epigenetic mechanisms operate to regulate allelic gene expression in both embryonic and extraembryonic structures. The embryonic methylation profile demonstrates that plants evolved a mechanism for resetting parent-specific imprinting marks, a necessary prerequisite for parent-of-origin-dependent gene expression in consecutive generations. The striking difference between the regulation of imprinting in the embryo and endosperm suggests that imprinting mechanisms might have evolved independently in both fertilization products of flowering plants.
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