Homeologous Non-Reciprocal Translocations (HNRT) Induce Selectable Genetic Variation in Brassica napus

Allopolyploidization leads to the formation of new species with the potential of fast adaptation to rapidly changing environmental conditions and plays therefore an important role in plant diversification. The genome of the allopolyploid plant species Brassica napus (AACC, 2n=38; oilseed rape, canola) derived from the interspecific hybridization of the ancestral hexaploid genomes of B. rapa (AA, 2n=20) and B. oleracea(CC, 2n=18). Synthetic combination of the two diploid genomes leads to chromosomal rearrangements induced by meiotic errors during the first generations after hybridization, potentially creating variation which is relevant for both natural and artificial selection.

In this study we resequenced 27 adapted and 17 resynthesized Brassica napus lines with highly diverse A and C genome donors to around 15x coverage (Illumina 100bp paired-end sequencing). Sequences were mapped to the Brassica napusreference genome from the winter oilseed rape cultivar Darmor-Bzh. We specifically investigated polyploidization-associated genome-wide sequence polymorphisms and structural genome variations. Applying a bioinformatic pipeline we analyzed each genotype for genomic rearrangements, particularly homeologous nonreciprocal translocations (HNRT) revealed by duplications in one chromosome segment accompanied by a corresponding deletion on the homeologous chromosome.

We were able to show that synthetic B. napus accessions have many HNRT, sometimes covering significant segments of chromosomes and mainly occuring in conserved regions, preferentially at the distal ends of the chromosomes. Natural B. napus accessions retain smaller HNRT, in corresponding areas of the genome, that appear to have been selected during domestication and breeding. Genes located within HNRTs of the adapted lines are often associated with key traits for breeding, for example flowering time and glycosinolate biosynthesis. These findings indicate that HNRTs can influence important agronomic phenotypes of Brassica napus and that they are therefore highly relevant for breeding.

This work was performed within the BMBF funded consortium Pre-Breed Yield "Zielgerichtete Züchtung zur Ertragssteigerung bei Raps" (http://www.pflanzenforschung.de/de/plant-2030/fachinformationen/projektdatenbank/zielgerichtete-zuumlchtung-zur-ertragssteigerung-bei-ra-177).