Asymmetric Evolution of Transposable Elements in Brassica oleracea

The reference sequences of the two Brassica diploids, Brassica rapa and Brassica oleracea, and their allotetroploid, Brassica napus, demonstrate that transposable elements (TEs) are the major constituent of these three Brassica species and function as the main contributor to the genome size variation. Although TEs have the ability to proliferate to high copies in the genome, most of them are suppressed by epigenetic pathways and are removed by DNA removal mechanisms, such as unequal homologous recombination and illegitimate recombination. Recent advances indicate the accumulation, elimination, and evolutionary rates of different types of TEs are dynamically distinct within the B. oleracea genome and among different Brassica species. The responses of TEs to polyploidy or interspecific hybridization remain largely unknown in Brassica. Nevertheless, TEs have been known to produce varieties of changes in gene expression and evolution, suggesting the importance of TEs in the regulation of Brassica species.