Retrotransposon activation followed by rapid repression in introgressed rice plants

Plant retrotransposons are largely inactive during normal development, but may be activated by stresses. Both copia-like and gypsy-like retrotransposo ns of rice were activated by introgression of DNA from the wild species Ziz ania latifolia Griseb. The copy number increase was associated with cytosin e methylation changes of the elements. Activity of the elements was ephemer al, as evidenced by nearly identical genomic Southern hybridization pattern s among randomly chosen individuals both within and between generations for a given line, and the absence of transcripts based on Northern analysis. D NA hypermethylation, internal sequence deletion, and possibly other mechani sms are likely responsible for the rapid element repression. Implications o f the retroelement dynamics on plant genome evolution are discussed.