Genome-Wide Duplication of AllotetraploidBrassica napusProduces Novel Characteristics and Extensive Ploidy Variation in Self-Pollinated Progeny

Whole genome duplications (WGDs) have played a major role in angiosperm species evolution. Polyploid plants have undergone multiple cycles of ancient WGD events during their evolutionary history. However, little attention has been paid to the additional WGD of the existing allopolyploids. In this study, we explored the influences of additional WGD on the allopolyploidBrassica napus. Compared to tetraploidB. napus, octoploidB. napus(AAAACCCC, 2n= 8x=76) showed significant differences in phenotype, reproductive ability and the ploidy of self-pollinated progeny. Genome duplication also altered a key reproductive organ feature inB. napus, that is, increased the number of pollen apertures. Unlike autopolyploids produced from the diploidBrassicaspecies, the octoploidB. napusproduced from allotetraploidB. napushad a relatively stable meiotic process, high pollen viability and moderate fertility under self-pollination conditions, indicating that sub-genomic interactions may be important for the successful establishment of higher-order polyploids. Doubling the genome ofB. napusprovided us with an opportunity to gain insight into the flexibility of theBrassicagenomes. The genome size of self-pollinated progeny of octoploidB. napusvaried greatly, and was accompanied by extensive genomic instability, such as aneuploidy, mixed-ploidy and mitotic abnormality. The octoploidB. napuscould go through any of genome reduction, equilibrium or expansion in the short-term, thus providing a novel karyotype library for theBrassicagenus. Our results reveal the short-term evolutionary consequences of recurrent polyploidization events, and help to deepen our understanding of polyploid plant evolution.