A Modified Meiotic Recombination in Brassica napus Largely Improves Its Breeding Efficiency

Abstract

Meiotic recombination is the main tool used by breeders to generate biodiversity, allowing genetic reshuffling at each generation. It enables the accumulation of favorable alleles while purging deleterious mutations. However, this mechanism is highly regulated with the formation of one to rarely more than three crossovers, which are not randomly distributed. In this study, we showed that it is possible to modify these controls in oilseed rape (Brassica napus, AACC, 2n = 4x = 38) and that it is linked to AAC allotriploidy and not to polyploidy per se. To that purpose, we compared the frequency and the distribution of crossovers along A chromosomes from hybrids carrying exactly the same A nucleotide sequence, but presenting three different ploidy levels: AA, AAC and AACC. Genetic maps established with 202 SNPs anchored on reference genomes revealed that the crossover rate is 3.6-fold higher in the AAC allotriploid hybrids compared to AA and AACC hybrids. Using a higher SNP density, we demonstrated that smaller and numerous introgressions of B. rapa were present in AAC hybrids compared to AACC allotetraploid hybrids, with 7.6 Mb vs. 16.9 Mb on average and 21 B. rapa regions per plant vs. nine regions, respectively. Therefore, this boost of recombination is highly efficient to reduce the size of QTL carried in cold regions of the oilseed rape genome, as exemplified here for a QTL conferring blackleg resistance.

Fine mapping of meiotic crossovers inBrassica oleraceareveals patterns and variations depending on direction and combination of crosses

“…The most striking example arises in Brassica AAC allotriploids resulting from the cross between B. napus and its B. rapa progenitor. In these plants, an unprecedented boost of CO number was observed between A genomes and associated with the formation of COs in pericentromeric regions that are totally deprived of any recombination event in B. rapa and B. napus (Boideau et al., 2021; Pelé et al., 2017).…”

Playing with the ploidy level enables to switch on and off the strict recombination control even in the vicinity ofBrassicacentromeres

“…Bars = 5µm. Original pictures from the 3x-F1 and 4x-F1 (a, b, g, h) derived from Boideau et al 2021.…”

“…In Brassica , it has been found that allotriploidy increased by 3.5-fold the homologous recombination compared to its diploid and even its allotetraploid counterpart, with also 1.7 times more recombination in female meiosis compared to the male meiosis (Pelé et al 2017). Interestingly, allotriploids present a modified recombination pattern, independently of the sex of meiosis (Leflon et al 2010, Pelé et al 2017, Boideau et al 2021). Within such allotriploid Brassica hybrids (2 n =3 x =29; AAC) deriving from a cross between B. napus (2 n =4 x =38; AACC) and B. rapa (2 n =2 x =20; AA), the A chromosomes pair as ten bivalents, whereas the remaining nine C chromosomes remain as univalent at metaphase I and segregate randomly within gametes (Leflon et al 2006).…”

Manipulation of Meiotic Recombination to Hasten Crop Improvement

“…Smaller (7.6 Mb vs. 16.9 Mb) and more numerous (21 vs. 9) introgressions of B. rapa occurred in AAC hybrids compared to AACC allotetraploid hybrids, indicating that the stimulation of recombination is also efficient to precisely map QTL carried in cold regions of the oilseed rape genome. Allotriploid AAC hybrids are therefore highly efficient to introduce novel variations within oilseed rape varieties [66].…”