| DNA methylation is a process that DNA methyltransferases catalyze the transfer of a methyl group to DNA using S-adenosyl methionine (SAM) as the methyl donor. It is an epigenetic modification that does not affect the primary DNA sequence. As a general genetic modification existing in living cells in organisms, DNA methylation depends on a variety of complex and interactive cellular mechanisms to build and maintain. In plants, the 5'-mehtylcytosine is the main form of methylation, and usually occurs in strand-symmetrical sequences CpG and CpNpG. The level of DNA methylation and the ratio of 5'-mehtylcytosine are variable among plants. DNA methylation involves a variety of cellar biochemical processes, and plays important roles in epigenetic regulation, plant growth and development, as well as polyploids evolution. In allopolyploids, the interaction between duplicated genomes can change duplicated genes expression by methylating and then silencing one homolog.Oryza L. is one of important genera in the family Gramineae. In the genus Oryza, the CD genome species consists of three species:O. alta, O. grandiglumis and O. latifolia. Due to their homogeneous genome types, similar morphological characteristics, as well as overlapping distributions, the delimitation and phylogenetic relationships of the three species have long been controversial. The previous studies indicated that the three polyploids and three CC genome species and an EE species had close phylogenetic relationships. Here methylation of two specific sites in CCDD genome was compared, and methylation status of the whole genome was also analyzed to explore the possible methylation variation patterns between the CCDD polyploids and their potential natural diploids parents, and among the CCDD polyploids themselves. Additionally, by comparing the amplification fragment length polymorphic on the CCGG sites of the whole genome, the present study also provided some insight to the phylogenetic relationships among three CCDD species. The main results are summarized as follows:1. Two low copy nuclear genes relevant to granule bound starch synthase were selected in this study. Here the duplication status and sequence characteristics of the two loci were analyzed, and the levels and patterns of methylation-polymorphism diversity at CCGG sites of the duplicated homologues of a selected set of 15 accessions in Oryza species with genome CCDD were explored using a method of methylation sensitive restricted fragment length polymorphism. The enzymatic digestion results of CCGG sites in two loci indicated that, of 4 recognizable CCGG sites, no methylated difference between duplicated homologues was found, suggesting the evolutional complexity of duplicated homologues derived by polyploidization. The study provides basic data and thoughts for further studying the mechanism of differentiation expression of duplicated homologues.2. Using a methylation-sensitive amplified fragment length polymorphism method, we assessed the levels and patterns of methylation-polymorphism at CCGG sites in a selected set of 30 accessions from 10 species distributed in 6 recognized genome types (A, B, C, CD, E, G) in the genus Oryza. The results indicated that, in general, methylation diversities mirrored genetic diversities. The CD genome allotetraploid species displayed significantly high levels of methylation and methylation polymorphism, while the C genome species had intermediate levels and the E genome species displayed relatively low levels, suggesting the importance divergence of methylation and methylation polymorphism within and among populations. We speculated that the remarkable increased levels of methylation within the allotetrapolyploids in contrast to those within their possible diploid donors may be great related to the genome diplodization and stabilization after polyploidization. The study provides research basis for exploring the relationship between DNA methylation and polyploidy evolution. |
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