Understanding Polyploidy and Chromosome Evolution in Resynthesized Brassica napus

Polyploidy has contributed to the evolution of eukaryotes, particularly flowering plants. The genomic consequences of polyploidy have been extensively studied, but the mechanisms for chromosome stability and diploidization in polyploids remain largely unknown. By using new cytogenetic tools to identify all of the homoeologous chromosomes, we conducted mitotic and meiotic cytological investigations of resynthesized and natural Brassica napus allopolyploids. We found that early generations of resynthesized B. napus involved aneuploidy and gross chromosomal rearrangements, and that dosage balance mechanisms enforced chromosome number stability. In comparison, the genomes of natural cultivars are considered stable due to the rarity of such genetic changes.  Meiosis studies in resynthesized and natural cultivars showed that non-homologous chromosome associations and abnormal chromosome segregation among resynthesized lines were significantly greater than among all four cultivars. We detected chromosome exchanges that resulted in homoeologous reciprocal translocations in resynthesized lines, but not among natural cultivars.  We observed early sister chromatid separation during meiosis I, homoeolog replacement, and chromosome fragments leading to unbalanced daughter cells in both resynthesized lines and natural cultivars. Early meiotic pairing issues in natural cultivars appeared to be sorted out by late meiosis, as evidenced by higher rates of normal daughter cell production.  These data suggest that pairing control mechanisms in natural cultivars resolved homoeologous pairing issues observed early in meiosis.