The Asymmetrical Evolution of the Mesopolyploid Brassica oleracea Genome

The completion of B. oleracea genome sequencing has greatly enhanced Brassica research, especially for revealing genome structure and evolution of polyploidy. Based on intraspecies syntenic analysis and comparison with B. rapa, the asymmetrical evolution of polyploid Brassica genomes were elucidated at the DNA, RNA, and epigenetic levels. Main features include asymmetrical gene loss in chromosomal blocks derived from whole-genome triplication (WGT), asymmetrical sequence, and expression divergence among WGT-derived duplicated genes, asymmetrical 24-nt sRNA regulated subgenome expression dominance, asymmetrical TE (transposable elements) accumulation, gene expansion between two Brassica genomes, and asymmetrical contribution to the novelty of agronomic traits between the different subgenomes. It was found that the multilayered asymmetrical evolution is also observed in ancient polyploids such as Arabidopsis, maize, grape, soybean, grasses as well as recent allopolyploids such as rapeseed, cotton, and wheat. These evidences support the role of asymmetrical evolution in the evolutionary success of polyploidy as well as their phenotypic novelty and adaptation. Further understanding of the mechanism of asymmetrical evolution of polyploid genomes lays a foundation for genetic breeding and enhancement of elite cultivars.