Améliorer la recombinaison méiotique pour une exploitation optimale de la diversité chez le colza Brassica napus L. | Enhancing the meiotic recombination for an optimal use of genetic diversity in the rapeseed Brassica napus L.

English. Meiotic recombination through crossovers (COs) is the key mechanism for generating genetic diversity in plant breeding programs. However, the number and position of COs between the pairs of homologous chromosomes are strictly regulated, preventing to generate many allelic combinations in progenies. A major challenge is to overcome these constraints for enhancing genetic shuffling. In the case of the rapeseed B. napus, the use of allotriploids (AAC, 2n=3x=29), resulting from the cross between rapeseed (AACC, 2n=4x=38) and its B. rapa progenitor (AA, 2n=2x=20), could allow to disrupt the recombination rules. Indeed, a substantial increase of the number of COs was identified between one pair of A homologs compared to AA and AACC hybrids. The objective of this study was to extend at the whole A genome the COs boost in allotriploids AAC and to determine the consequences of such a variation on the distribution of COs along the chromosomes. Following the production and cytogenetic characterization of AA (2n=2x=20) and AAC (2n=3x=29) hybrids (Fig. 1), we assessed homologous recombination from nearly 3,000 COs obtained per progeny by using SNP markers well distributed along the A genome (1 SNP per ~1.25 Mbp). Compared to the diploid, the allotriploid showed 3.4 times more overall COs with extra COs between each pair of homologous A chromosomes (Fig. 2). Most surprisingly, we found that such a rise was associated with dramatic changes in the shape of recombination landscapes with COs all along the A chromosomes, even in the vicinity of centromeres that are normally deprived of COs in diploids and in most studied species (Fig. 3). These results offer the opportunity for geneticists and plant breeders to dramatically enhance the generation of new diversity and to optimize the introgression of agronomical traits of interest from B. rapa into B. napus (Fig. 4).

French. La recombinaison méiotique est le principal outil du sélectionneur pour brasser la diversité génétique entre deux parents mais le nombre et la distribution des Crossing-Overs (COs) sont strictement contrôlés. L’objectif de ce travail a été de préciser comment modifier ces règles tout en exploitant la diversité des espèces apparentées chez le colza (Brassica napus, AACC, 2n=4x=38), un allotétraploïde issu de l’hybridation naturelle entre la navette (B. rapa, AA, 2n=2x=20) et le chou (B. oleracea, CC, 2n=2x=18). Par des analyses génétiques conduites sur des descendances d’hybrides (Fig. 1), nous montrons qu’exploiter un intermédiaire allotriploïde AAC (2n=3x=29), résultant du croisement entre le colza et la navette, permet d’augmenter considérablement le nombre des COs entre chaque paire d’homologues (Fig. 2), et surtout que ceux-ci peuvent se former dans des régions génomiques usuellement dépourvues de COs (Fig. 3). Ces résultats démontrent qu’il est possible de modifier le déséquilibre de liaison ainsi que d’optimiser l’introgression de régions d’intérêt provenant de la navette chez le colza (Fig. 4).