Fertility, genome stability, and homozygosity in a diverse set of resynthesized rapeseed lines

Abstract

Rapeseed (Brassica napus, AACC) was formed by hybridization between progenitor species Brassica rapa (AA) and Brassica oleracea (CC). As a result of a limited number of hybridization events between specific progenitor genotypes and strong breeding selection for oil quality traits, rapeseed has limited genetic diversity. The production of resynthesized B. napus lines via interspecific hybridization of the diploid progenitor species B. rapa and B. oleracea is one possible way to increase genetic variation in rapeseed. However, most resynthesized lines produced so far have been reported to be meiotically unstable and infertile, in contrast to established B. napus cultivars. This hinders both maintenance and use of this germplasm in breeding programs. We characterized a large set of 140 resynthesized lines produced by crosses between B. rapa and B. oleracea, as well as between B. rapa and wild C genome species (B. incana, B. hilarionis, B. montana, B. Bourgeaui, B. villosa and B. cretica) for purity (homozygosity), fertility, and genome stability. Self-pollinated seed set, seeds per ten pods as well as percentage pollen viability were used to estimate fertility. SNP genotyping was performed using the Illumina Infinium Brassica 60K array for 116 genotypes, with at least three individuals per line. Most of the material which had been advanced through multiple generations was no longer pure, with heterozygosity detected corresponding to unknown parental contributions via outcrossing. Fertility and genome stability were both genotype-dependent. Most lines had high numbers of copy number variants (CNVs), indicative of meiotic instability, and high numbers of CNVs were significantly associated with reduced fertility. Eight putatively stable resynthesized B. napus lines were observed. Further investigation of these lines may reveal the mechanisms underlying this effect. Our results suggest that selection of stable resynthesized lines for breeding purposes is possible.