Surveying CpG methylation at 5'-CCGG in the genomes of rice cultivars.

We investigated the CpG methylation status of the sequence CCGG in the rice genome by using methylation-sensitive AFLP and subsequent Southern analyses with the isolated AFLP fragments as probes. CpGs located in single- or low-copy-sequence regions could be grouped into two classes on the basis of their methylation status: methylation status at the class 1 CpG sites was conserved among genetically diverse rice cultivars, whereas cultivar-specific differential methylation was frequently detected among the cultivars at the class 2 CpG sites. The frequency of occurrence of methylation polymorphism between a pair of cultivars was not related to the genetic distance between the two. Through mapping, five class 2 CpG sites were localized on different chromosomes and were not clustered together in the genome. Segregation analysis of the cultivar-specific methylations with their target sites indicated that the differential methylation was stably inherited in a Mendelian fashion over 6 generations, although alterations in the methylation status at the class 2 CpG sites were observed with a low frequency.