Brassica Allotriploids Hybrids: New Way to Exceed Recombination Limits for COs Rate and Distribution

Meiotic homologous recombination by crossovers (COs) is the main mechanism responsible for mixing genetic diversity in plant breeding. However, loci separations are limited due to the strict regulation of the rates and distribution of recombination events along the chromosomes. Indeed, rarely more than two COs occur between homologous chromosomes per meiosis and their distribution is not homogenous in all chromosomic regions, such as the pericentromeric regions that are free of recombination. Exceptions to this rule were observed and linked to different factors such as polyploidy, genetic background or sex meiosis. Accordingly, it was shown that in allotriploid hybrids (AAC, 2n=29), resulting from crosses between Brassica napus (AACC, 2n=38) and B. rapa(AA, 2n=20), COs get a boost along the A genome compared to diploid hybrids (AA, 2n=20). However, the impact on COs distribution as well as the effects of different genetic backgrounds and sex meiosis were still unclear. To that purpose, progenies deriving from 3 diploid and 3 triploid hybrids were used, enabling to study the distribution of 3000 COs per hybrid by the genotyping of 200 SNPs well distributed along A chromosomes (one SNP each 1.2 Mb). Compared to what was previously known, we showed that the presence of the haploid C genome modifies the COs distribution, by generating new recombining regions within A chromosomes. In addition, significant variations were observed between triploid hybrids depending on the genetic background and sex meiosis. These findings may enable to combine new loci by modifying interference and linkage disequilibrium.