Origins of the amphiploid species Brassica napus L. investigated by chloroplast and nuclear molecular markers

Abstract

BackgroundThe amphiploid species Brassica napus (oilseed rape, Canola) is a globally important oil crop yielding food, biofuels and industrial compounds such as lubricants and surfactants. Identification of the likely ancestors of each of the two genomes (designated A and C) found in B. napus would facilitate incorporation of novel alleles from the wider Brassica genepool in oilseed rape crop genetic improvement programmes. Knowledge of the closest extant relatives of the genotypes involved in the initial formation of B. napus would also allow further investigation of the genetic factors required for the formation of a stable amphiploid and permit the more efficient creation of fully fertile re-synthesised B. napus. We have used a combination of chloroplast and nuclear genetic markers to investigate the closest extant relatives of the original maternal progenitors of B. napus. This was based on a comprehensive sampling of the relevant genepools, including 83 accessions of A genome B. rapa L. (both wild and cultivated types), 94 accessions of B. napus and 181 accessions of C genome wild and cultivated B. oleracea L. and related species.ResultsThree chloroplast haplotypes occurred in B. napus. The most prevalent haplotype (found in 79% of accessions) was not present within the C genome accessions but was found at low frequencies in B. rapa. Chloroplast haplotypes characteristic of B. napus were found in a small number of wild and weedy B. rapa populations, and also in two accessions of cultivated B. rapa 'brocoletto'. Whilst introgression of the B. napus chloroplast type in the wild and weedy B. rapa populations has been proposed by other studies, the presence of this haplotype within the two brocoletto accessions is unexplained.ConclusionsThe distribution of chloroplast haplotypes eliminate any of the C genome species as being the maternal ancestor of the majority of the B. napus accessions. The presence of multiple chloroplast haplotypes in B. napus and B. rapa accessions was not correlated with nuclear genetic diversity as determined by AFLPs, indicating that such accessions do not represent recent hybrids. Whilst some chloroplast diversity observed within B. napus can be explained by introgression from inter-specific crosses made during crop improvement programmes, there is evidence that the original hybridisation event resulting in to B. napus occurred on more than one occasion, and involved different maternal genotypes.

Repeated Polyploidy Drove Different Levels of Crossover Suppression between Homoeologous Chromosomes inBrassica napusAllohaploids

“…We also confirmed that rutabagas had the main ptDNA haplotype (ptDNA haplotype 1), whereas asparagus kale had ptDNA haplotype 2. These results are also in agreement with the recent work of Allender and King (2010).…”

“…We analyzed different plastid regions than did Song and Osborn (1992), so this difference could reflect the idiosyncrasy of the B. montana chloroplast genome, which has local similarities with the other B. napus ptDNAs (Song and Osborn, 1992). Second, none of the accessions in our study had a B. oleracea ptDNA haplotype, which was represented only by the New Zealand Rawara accession in the Song and Osborn study; however, we found that this accession had only 18 chromosomes, suggesting that it belongs to the B. oleracea and not the B. napus germplasm (confirmed in Allender and King, 2010). Apart from these two points, our results confirmed the conclusions of Song and Osborn (1992).…”

“…Although phenotyping other B. napus accessions could provide supplementary information, the dichotomy of meiotic behaviors found here certainly highlights the overall variation present in this species, which contains germplasm of shared recent common ancestry (Prakash and Hinata, 1980;Allender and King, 2010). This observation and the fact that no other meiotic behavior was found in a handful of other accessions (Olsson and Hagberg, 1955;Renard and Dosba, 1980;Attia and Rö bbelen, 1986;Tai and Ikonen, 1988) suggest that most B. napus allohaploids display either a high (MU phenotype) or a low (fu phenotype) number of univalents, with only slight variations within these two canonical phenotypes (Figure 2).…”

“…B. napus (AACC; 2n = 38) is a young allopolyploid species with multiple origins (i.e., a polyphyletic species) that formed by repeated interspecific hybridization between ancestors of Brassica oleracea (CC, 2n = 18) and Brassica rapa (AA, 2n = 20) (U, 1935;Palmer et al, 1983;Song and Osborn, 1992;Allender and King, 2010). Natural euploid B. napus displays predominantly 19 bivalents at Metaphase I (MI) and an almost strict disomic inheritance; this shows that the vast majority of COs are formed between homologous chromosomes.…”

Genetic investigation of the origination of allopolyploid with virtually synthesized lines: Application to the C subgenome of Brassica napus

“…Our findings support the previous findings that the origin of rapeseed occurs on more than one occasion (Song and Osborn, 1992;Allender and King, 2010), and that the C subgenome of rapeseed is related to cultivated B. oleracea (Olsson, 1954;Naughton, 1976;Song and Osborn, 1992;Gó mez-Campo and Prakash, 1999) or B. montana (Song and Osborn, 1992). In addition, our data suggest that the potential progenitors of the C subgenome of rapeseed can be extended to include other wild types, such as B. incana, B. bourgeaui, B. oleracea ssp.…”