Stable resynthesized Brassica napus lines show similar meiotic behaviour to established B. napus

Brassica napus (rapeseed/canola) is an allotetraploid (AACC, 2n = 4x = 38) resulting from spontaneous hybridization between B. rapa (AA, 2n = 2x = 20) and B. oleracea (CC, 2n = 2x = 18). Although established B. napus is meiotically stable, resynthesized lines (2n = AACC) produced by hybridizing between progenitor species B. rapa and B. oleracea are usually meiotically unstable, and show frequent chromosomal rearrangements caused by homoeologous recombination between the A and C genomes. Previously, we identified resynthesized rapeseed lines showing contrasting levels of homoeologous recombination, as assessed by genotyping for copy number variants. Here, we aimed to characterise meiotic chromosome pairing behaviour in fifteen resynthesized lines representing putatively stable, unstable and intermediate types. Putatively stable lines showed predominantly normal meiosis (average 91% bivalent formation), while putatively unstable lines showed frequent abnormalities such as multivalent formation (average 60% bivalent formation). Univalents were unexpectedly rare in Metaphase I. A1-C1 specific probes revealed that stable lines showed approximately 18% A-C pairing (7/40 pollen mother cells), not significantly different to the 13% A-C pairing (5/40 pollen mother cells) in established B. napus, but in contrast to the unstable line with 46% A-C pairing (25/54 pollen mother cells). Surprisingly, all resynthesized lines initially classed as having intermediate stability based on copy number variation showed either stable or unstable-type meiotic behaviour, suggesting a more qualitative rather than quantitative phenotype. Our results suggest that differences in multivalent formation frequencies and homoeologous A-C pairing differentiate stable and unstable lines, confirm the production of meiotically stable synthetic B. napus, and provide a basis for further investigation of genetic factors contributing to this effect.