| DNA methylation as an important epigenetic modification plays an important role in controlling gene expression and maintaining genomic stability in animals and plants. Intraspecific crossing can impact the pattern of DNA melthylation in offsprings as a kind of stress, thereby change the phenotypic of offsprings. It has a great significance for the research of crop improvement and methylation mechanism. Mobile elements or transposons are genetic elements that can move, and sometimes spread, within their host genomes. Identification of specific inducible conditions whereby certain dormant elements can be activated to transpose is important to further understand the biological roles of mobile elements. Of equal importance is to investigate the transmission genetics of the active state of these elements.We found in this study that inheritance and variation of DNA methylation and transposon activity were detected in several intra-specific hybrids of Rice (Oryza sativa L.). The main results were summeized as following.1. We used MSAP method to analysed the level and the pattern of DNA methylation in rice and hybrids. The tissue assayed include two female parents and six male parents, 12 crosses in all and each portfolio for five offsprings(F1). A total of sixty-eight experimental material. We found that the average level of methylation in rice and hybrids was 12.5%～14.9%. The total level of methylation in hybrids(F1) was similar to female parents. The methylation of the internal cyto- sine was the main pattern. The variation of DNA methylation was detected in all hybrids (F1) of 12 croses. Frequency of the changes was higher in 3 croses than in other 9 croses, They are(01-124♀×Z-35♂,8.36%),(89-45♀×Z-35♂,9.59%),(89-45♀×Z-29♂, 10.7%). The frequence of other 9 croses was only(2.24%～5.94%).Frequency of hypermethylation in 10 croses was higher than hypomethylation,while 2croses were opposite.They are(89-45♀×Z-35♂),(89-45♀×Z-29♂). Homology analysis of isolated bands that showed variation in hybrids indicated that diverse sequences were involved, including known-function cellular genes, uncharacterized sequences , promoters and introns of genes. These bands extensively located on 12 chromatosomes of rice. The quantity of the bands was the most on the first, the seventh and the eleventh chromatosome, while the least on the eighth chromatosome .2. Southern blot analysis on selected 3croses with their hybrids whose methylation variation was higher indicated that some transposons were efficiently mobilized including mPing, Ping, Tos17, Osr3, Osr35, Osr42. MPing was efficiently mobilized in somatic cells of these rice plants, sometimes together without its putative transposase-encoding parter, ping. The activated state of mPing can be transmited to orgnismal progenies(F2) via meiotic divisions, and new mobilization also occurred. The same set of morphological mutants was analyzed for another active mobile element, the reptrotransposon Tos17. It was found that Tos17 was also transpositionally activate which can transmit to orgnismal progenies(F2) via meiotic divisions without new mobilization. No evidence for mPing activation was detected in the 3 corresponding reciprocal hybrids, suggesting possible effects of cross direction, which can be due to cytoplsmic effects or epigenetic phenomenon like imprinting.Based upon the above observations, it can be concluded that hybridization represents a potentially mutagen in plants, not only from the classic chromosomal recombination, but also from an epigenetic perspective by mobilizing otherwise quiescent mobile elements and by inducing heritable alterations in DNA methylation patterns, and the interlaced effects between the two.
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