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species have provided valuable insights into angiosperm (flowering plant) genome structure, function and evolution. For example, A. thaliana has experienced two genome duplications since its divergence from Carica, with rapid DNA sequence divergence, extensive gene loss and fractionation of ancestral gene order eroding the resemblance of A. thaliana to ancestral Brassicales1. Compared with an ancestor at just a few million years ago, A. thaliana has undergone a ~30% reduction in genome size2 and 9-10 chromosomal rearrangements3,4 that differentiate it from its sister species Arabidopsis lyrata. Whole-genome duplication has been observed in all plant genomes sequenced to date. A. thaliana has undergone three paleo-polyploidy events5: a paleohexaploidy (γ) event shared with most dicots (asterids and rosids) and two paleotetraploidy events (β then α) shared with other members of the order Brassicales. B. rapa shares this complex history but with the addition of a wholegenome triplication (WGT) thought to have occurred between 13 and 17 million years ago (MYA)6,7, making 'mesohexaploidy' a characteristic of the Brassiceae tribe of the Brassicaceae8.

Brassica crops are used for human nutrition and provide opportunities for the study of genome evolution. These crops include important vegetables (B. rapa (Chinese cabbage, pak choi and turnip) and Brassica oleracea (broccoli, cabbage and cauliflower)) as well as oilseed crops (Brassica napus, B. rapa, Brassica juncea and Brassica carinata), which provide collectively 12% of the world's edible vegetable oil production9. The six widely cultivated Brassica species are also a classical example of the importance of polyploidy in botanical evolution, described by 'U's triangle'10, with the three diploid species B. rapa (A genome), Brassica nigra (B genome) and B. oleracea (C genome) having formed the amphidiploid species B. juncea (A and B genomes), B. napus (A and C genomes) and B. carinata (B and C genomes) by