Structural Evolutionary Dynamics in Allopolyploid Synthetic Oilseed Rape (Brassica napus L.)

Allopolyploidy, which results from the merger and duplication of two divergent genomes, has played a major role in the evolution and diversification of flowering plants. This evolutionary success of allopolyploid species results from their higher structural and functional evolutionary dynamic compared to their diploid parents, which begins immediately after the allopolyploidization event. Presently, a model system to study this phenomenon is Brassica napus (oilseed rape) since its whole genome sequence has been sequenced recently. In order to detect the immediate impact of allopolyploidy on the structural evolutionary dynamic of B. napus (2n=4x=38), two synthetic B. napus populations were created. By comparing the genome structure of 30 synthetic B. napus individuals to their parental diploid genomes using a Illumina SNP 60k array, we were able to identify important deletions in each synthetic plant, which ranged from 0.1 to more than 20 Mb (whole chromosome arm). In order to determine the type of rearrangements involved (either deletion or non reciprocal translocation), BAC-FISH experiments (Fluorescence In Situ Hybridization) were performed using BACs specific to the rearranged regions. Compared to what was previously known, we showed statistically that some chromosomes and even one of the subgenomes were more prone to structural modifications. In addition, our findings showed that the B. napus genome may be deeply shuffled only two generations after the allopolyplodization event. In some individuals, these structural rearrangements may cause the loss of one homoeologous copy for few thousands of genes.