Alteration In DNA Methylation And Mobilization Of Transposons MPing And Pong In Rice (Oryza Sativa L.) By Introgression From Pennisetum Sp. Or Zea Mays L.

Hybridization between different species plays an important role in plant genome evolution, and is also a widely used approach for crop improvement. McClintock has predicted decades ago that plant wide hybridization constitutes a''genomic shock''whereby cryptic transposable elements may be activated to respond and to restructure the genome. However, direct experimental evidence showing a causal relationship between plant wide hybridization and transposon mobilization has been reported only in limited cases.The miniature-Ping (mPing) is a recently isolated active miniature inverted-repeat transposable element transposon from rice, which is mobilized by tissue culture andγ-ray irradiation. It has been shown that mPing, together with its putative transposase-encoding partner, Pong, is mobilized in several introgression lines derived from hybridization between rice and Zizania latifolia Griseb., We found here that mPing is also mobilized in in rice by introgression from Pennisetum sp.or Zea mays L. Thus, we have presented an additional line of evidence that is consistent with McClintock's insight.Genome-wide analysis by MSAP markers on rice plants by introgression from Pennisetum sp.or Zea mays L indicated the occurrence of cytosine methylation alteration at multiple genomic loci, with transgenerational instability being prevalent. Given simplicity of this cross-pollination experiment, it can be envisioned that similar incidents may occur under natural conditions. Thus, our results may implicate another role of hybridization in plant evolution, namely,"accidental"cross-pollinationby a certain unrelated species may be stochastically mutagenic and elicit rampant genetic and epigenetic instabilities, which may be subject to natural selection and contribute to changes in gene expression and phenotypes.