Epigenetic Inheritance And Variation Of DNA Methylation Level And Pattern In Maize Intra-specific Hybrids

DNA methylation as an important epigenetic modification plays an important role incontrolling gene expression and maintaining genomic stability in higher animals and plants.In many studies it is found that disturbance of intrinsic DNA methylation patterns may lead tofunctional and phenotypic abnormality, or evolutionary opportunity. On the other hand theinheritance of specific methylation pattern in animals and plants, and variation of methylationinduced by biotic or abiotic factors may provide novel insight into the biological significanceof this epigenetic marker.Maize (Zea mays) is among the main crops where the phenomenon of heterosis or hybridvigor is used in grain production. Nonetheless, the mechanism of heterosis in hybrids remainslargely mysterious.. In this study, three elite maize inbred lines were selected and werecrossed with each other to produce three sets of reciprocal hybrids. Inheritance and variationof methylation level and pattern in these hybrids relative to their corresponding inbredparental lines were investigated by the methylation-sensitive amplified polymorphism or(MSAP), followd by methylation-sensitive Southern blot analysis. The main results weresummeized as following.1. We used MSAP method to analyze the levels of DNA methylation in leaves, embryosand endosperm of maize inbred lines and hybrids. In leaves the two sets of reciprocal hybrids(MD,DM,8D,D8) showed higher values of relative level of cytosine methylation at CCGGsites than those of their corresponding mid-parent While the other set of reciprocal hybrids(8M and M8) showed values slightly lower than the mid-parent value, and no differencewas observed between the cross directions within a given hybrid combination. In embryos thesame two reciprocal hybrids showed higher level of methylation than that of their parents. Inendosperm, the two combinations were different in that while one set of reciprocal hybridsshowed higher levels of methylation than their parents, the other set showed the oppositetrend.2. Different organs showed different levels of DNA methylation in maize. The relativelevels of DNA methylation from high to low are: seedlings, embryos and endosperm.3. Although in the majority of CCGG sites, DNA methylation showed Mendelianinheritance from parental lines to hybrids, alterations in methylation pattern were detected inall hybrids studied at certain frequencies, ranging from 6.59% to 11.92%. Thehybridization-induced alteration frequencies also varied among the organs, with endospermshowed the greatest alteration. In addition, there were more alterations at loci inherited fromthe paternal than from the maternal origin.4. The hybridization-induced methylation alterations seemed to include both directedand random events, as while some alterations occurred in all F1 hybrid individual plantstested, others occurred only in some individuals.5. Preexisting heterozygosity was ruled out as all tested individual plants from a giveninbred line always showed the same methylation pattern.6. Hybridization-induced DNA methylation alteration occurred in diverse sequencesincluding cellular genes, transposons and sequences of unknown function.The above results suggest that, in maize, whereas cytosine methylation at the majority ofgenomic loci exhibited faithful transmition from parents to intra-specific hybrids, aleration inboth level and pattern occurred in all studied crosses, and the range of alteration appered tovary among the studied organs, leaves, embryos and endosperm. These alterations might playa role in regulating differential gene expression in hybrids vs. inbred parental lines, and byextension, might contribute, in part, to heterosis or hybrid vigor.