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The success or failure of interspecific crosses is vital to evolution and to agriculture, but much remains to be learned about the nature of hybridization barriers. Several mechanisms have been proposed to explain postzygotic barriers, including negative interactions between diverged sequences, global genome rearrangements, and widespread epigenetic repro-gramming. Another explanation is imbalance of paternally and maternally imprinted genes in the endosperm. Interspecific crosses between diploid Arabidopsis thalian