Abstract
Meiotic recombination by crossovers (COs) is tightly regulated, limiting its key role in producing genetic diversity. However, while COs are usually restricted in number and not homogenously distributed along chromosomes, we show here how to disrupt these rules in Brassica species by using allotriploid hybrids (AAC, 2n = 3x = 29), resulting from the cross between the allotetraploid rapeseed (B. napus, AACC, 2n = 4x = 38) and one of its diploid progenitors (B. rapa, AA, 2n = 2x = 20). We produced mapping populations from different genotypes of both diploid AA and triploid AAC hybrids, used as female and/or as male. Each population revealed nearly 3,000 COs that we studied with SNP markers well distributed along the A genome (on average 1 SNP per 1.25 Mbp). Compared to the case of diploids, allotriploid hybrids showed 1.7 to 3.4 times more overall COs depending on the sex of meiosis and the genetic background. Most surprisingly, we found that such a rise was always associated with (i) dramatic changes in the shape of recombination landscapes and (ii) a strong decrease of CO interference. Hybrids carrying an additional C genome exhibited COs all along the A chromosomes, even in the vicinity of centromeres that are deprived of COs in diploids as well as in most studied species. Moreover, in male allotriploid hybrids we found that Class I COs are mostly responsible for the changes of CO rates, landscapes and interference. These results offer the opportunity for geneticists and plant breeders to dramatically enhance the generation of diversity in Brassica species by disrupting the linkage drag coming from limits on number and distribution of COs.
Highlights
Meiotic recombination through crossovers (COs) is the key mechanism ensuring both the proper segregation of homologous chromosomes during meiosis and the generation of diversity in all sexual organisms
In Arabidopsis thaliana, of the 150 to 250 Double Strand Breaks (DSBs) generated per meiosis, on average only ~11.5 are repaired in the form of COs, the others giving rise to Non-Crossovers (NCOs) or possibly COs between sister chromatids [3, 5,6,7]
Several proteins were recently highlighted to promote the repair of DSBs into NCOs in A. thaliana (e.g. FANCM, RECQ4, FIGL1) [8,9,10,11], thereby limiting the overall number of COs formed in a meiosis
Summary
Meiotic recombination through crossovers (COs) is the key mechanism ensuring both the proper segregation of homologous chromosomes during meiosis and the generation of diversity in all sexual organisms. A particular pattern for the COs distribution is observed in some plants (e.g. Triticum turgidum, Triticum aestivum and Zea mays), with a gradual increase of the COs frequency away from centromeres [32,33,34]. These last observations could be in link with different features of genome architecture such as content in genes and transposable elements (TEs). Genes are mostly located on chromosomal extremities while TEs preferentially concentrate in the vicinity of centromeres [35]
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