Divergence among rice cultivars reveals roles for transposition and epimutation in ongoing evolution of genomic imprinting

To understand the evolution of imprinting mechanisms in the rice species Oryza sativa, we analyzed DNA methylation, transcription and small RNA expression in embryo and endosperm. Cultivars chosen to represent the genetic diversity of cultivated Oryza sativa comprised Nipponbare and Kitaake of the japonica subspecies, and IR64 and 93-11 cultivars of the indica subspecies. While imprinted expression is generally conserved among rice cultivars, approximately 10% of imprinted genes show imprinting divergence across the four cultivars. Analyses of DNA methylation and small RNAs revealed that small RNA producing loci are closely associated with genic regions and four times more likely to be imprinted than genes. However, imprinting divergence most often correlated with DNA methylation epimutations. Correlative epialleles were largely stable within subspecies. Small indels and transposable element insertions were found at half of the epimutated loci and associated with imprinting divergence at half of the remaining loci. Correlating epigenetic and genetic variation occurred at key regulatory regions – the promoter and transcription start site of maternally-biased genes, and the promoter and gene body of paternally-biased genes. Our results reinforce models for the role of maternal-specific DNA hypomethylation in imprint formation at both maternally and paternally biased genes, and highlight the role of transposition events and epimutation in rice imprinting evolution. Examination of DNA methylation, total RNA and small RNA populations in dissected 7 to 8 DAF endosperm and embryo of seeds obtained from four cultivars of Oryza sativa L. and select reciprocal crosses