Brassica allotriploids, a solution to deal with hampered meiotic recombination

Meiotic recombination by crossovers is the main mechanism used by breeders to shuffle the genetic diversity. Nevertheless, loci separation is often hampered due to the strict regulation of meiotic recombination, which results in a few number of crossovers formed per pair of homologs (≤ 3) primarily located on chromosome extremities (Mercier et al.,2015).In Brassicagenus, which contains many economically important crops (from vegetables and fodders to oilseed), ploidy level of hybrids has been linked to hugely modify crossover frequency. The higher crossover frequency was observed in AAC allotriploid hybrids (2n=3x=29), resulting from the cross between the rapeseed (B. napus, AACC, 2n=4x=38) and its B. rapaprogenitor (AA, 2n=2x=20), with a two to four-fold increase compared to diploid AA and allotetraploid AACC hybrids (Leflon et al.,2010). However, thedistribution of these extra crossovers in AAC allotriploids as well as the situation in CCA allotriploids (2n=3x=28), deriving from B. napusx B. oleracea(CC, 2n=2x=18), were still unknown. To answer the first question, we developed several populations and assessed the frequency and distribution of recombination events in diploid AA and allotriploid AAC hybrids, using SNP markers well distributed along A chromosomes (one SNP each 1.2 Mb). Based on about 3.000 crossovers detected per progeny, we showed that the presence of the C genome always leads to a very substantial increase of crossovers all along the A chromosomes, especially in the vicinity of centromeres that are normally deprived of crossovers (Pelé et al.,2017). Our preliminary analysis of CCA hybrids seems to indicate similar modifications between homologous CC chromosomes. Altogether, these findings provide new insights on the crossovers reshaping occurring in Brassicaallotriploids and offer a unique opportunity for Brassicabreeders to efficiently combine new loci of interests by modifying the linkage disequilibrium.