Methylation, transcription, and rearrangements of transposable elements in synthetic allopolyploids

Transposable elements (TEs) constitute over 90% of the wheat genome. It was suggested that "genomic stress" such as hybridity or polyploidy might activate transposons. Intensive investigations of various polyploid systems revealed that allopolyploidization event is associated with widespread changes in genome structure, methylation, and expression involving low- and high-copy, coding and noncoding sequences. Massive demethylation and transcriptional activation of TEs were also observed in newly formed allopolyploids. Massive proliferation, however, was reported for very limited number of TE families in various polyploidy systems. The aim of this review is to summarize the accumulated data on genetic and epigenetic dynamics of TEs, particularly in synthetic allotetraploid and allohexaploid wheat species. In addition, the underlying mechanisms and the potential biological significance of TE dynamics following allopolyploidization are discussed.