Abstract
Brassica napus (2n = 4x = 38, AACC) is an important allopolyploid crop derived from interspecific crosses between Brassica rapa (2n = 2x = 20, AA) and Brassica oleracea (2n = 2x = 18, CC). However, no truly wild B. napus populations are known; its origin and improvement processes remain unclear. Here, we resequence 588 B. napus accessions. We uncover that the A subgenome may evolve from the ancestor of European turnip and the C subgenome may evolve from the common ancestor of kohlrabi, cauliflower, broccoli, and Chinese kale. Additionally, winter oilseed may be the original form of B. napus. Subgenome-specific selection of defense-response genes has contributed to environmental adaptation after formation of the species, whereas asymmetrical subgenomic selection has led to ecotype change. By integrating genome-wide association studies, selection signals, and transcriptome analyses, we identify genes associated with improved stress tolerance, oil content, seed quality, and ecotype improvement. They are candidates for further functional characterization and genetic improvement of B. napus.
Highlights
Brassica napus (2n = 4x = 38, AACC) is an important allopolyploid crop derived from interspecific crosses between Brassica rapa (2n = 2x = 20, AA) and Brassica oleracea (2n = 2x = 18, CC)
We developed a large genome variation data set for genetically diverse B. napus accessions, which provided an opportunity to finely resolve the origin and evolutionary history of B. napus
We posit that the B. napus was originated from the hybridization between domesticated B. rapa and B. oleracea ~1910–7180 years ago (Supplementary Fig. 32), which accorded with previous conclusions (~6700 and 7500 years ago) derived from Ks estimation[4,5]
Summary
By integrating genome-wide association studies, selection signals, and transcriptome analyses, we identify genes associated with improved stress tolerance, oil content, seed quality, and ecotype improvement. They are candidates for further functional characterization and genetic improvement of B. napus. As one of the earliest allopolyploid crops, B. napus was formed by hybridization of B. rapa and B. oleracea[1]. No truly wild B. napus populations are known[7], leading to the precise identities of the two progenitors that hybridized to form B. napus remain elusive, as B. rapa and B. oleracea have morphologically diverse subspecies and have commonly been cultivated throughout Europe for hundreds of years. A recent study suggested that the B. napus A subgenome might be derived from the ancestor of European turnip (B. rapa ssp. rapa)[6], more evidence to support the conclusion need to be provided, due to only 5 B. napus and 27
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