Abstract
Turnip yellows virus (TuYV) is aphid-transmitted and causes considerable yield losses in oilseed rape (OSR, Brassica napus, genome: AACC) and vegetable brassicas. Insecticide control of the aphid vector is limited due to insecticide resistance and the banning of the most effective active ingredients in the EU. There is only one source of TuYV resistance in current commercial OSR varieties, which has been mapped to a single dominant quantitative trait locus (QTL) on chromosome A04. We report the identification, characterisation, and mapping of TuYV resistance in the diploid progenitor species of OSR, Brassica rapa (genome: AA), and Brassica oleracea (genome: CC). Phenotyping of F1 populations, produced from within-species crosses between resistant and susceptible individuals, revealed the resistances were quantitative and partially dominant. QTL mapping of segregating backcross populations showed that the B. rapa resistance was controlled by at least two additive QTLs, one on chromosome A02 and the other on chromosome A06. Together, they explained 40.3% of the phenotypic variation. In B. oleracea, a single QTL on chromosome C05 explained 22.1% of the phenotypic variation. The TuYV resistance QTLs detected in this study are different from those in the extant commercial resistant varieties. To exploit these resistances, an allotetraploid (genome: AACC) plant line was resynthesised from the interspecific cross between the TuYV-resistant B. rapa and B. oleracea lines. Flow cytometry confirmed that plantlets regenerated from the interspecific cross had both A and C genomes and were mixoploid. To stabilise ploidy, a fertile plantlet was self-pollinated to produce seed that had the desired resynthesised, allotetraploid genome AACC. Phenotyping of the resynthesised plants confirmed their resistance to TuYV. Genotyping with resistance-linked markers identified during the mapping in the progenitors confirmed the presence of all TuYV resistance QTLs from B. rapa and B. oleracea. This is the first report of TuYV resistance mapped in the Brassica C genome and of an allotetraploid AACC line possessing dual resistance to TuYV originating from both of its progenitors. The introgression into OSR can now be accelerated, utilising marker-assisted selection, and this may reduce selection pressure for TuYV isolates that are able to overcome existing sources of resistance to TuYV.
Highlights
IntroductionB. napus is primarily grown as an oilseed crop but swede, kale, and fodder morphotypes exist
Brassica napus [oilseed rape (OSR), genome: AACC, 2n = 38] is an allotetraploid species that arose from recent (∼7,500 years ago; Chalhoub et al, 2014) and limited interspecific hybridisations between Brassica rapa and Brassica oleracea (Song and Osborn, 1992; Chalhoub et al, 2014)
A further 619 had a segregation ratio that deviated significantly from the expected 1:1 for a backcross population. Removal of these markers resulted in 2,479 useful markers; of which, 351 represented the overall recombination within SEA17016 and were used to make the minimal genetic map
Summary
B. napus is primarily grown as an oilseed crop but swede, kale, and fodder morphotypes exist. It is the third most important oil crop after palm (Elaeis guineensis) and soybean (Glycine max) worldwide (USDA, 2020) and is a major source of vegetable oils, biodiesels, industrial lubricants, and oilseed meals. The largest producers of OSR are Canada, the EU, and China (USDA, 2020). Beet western yellows virus (BWYV) is a major contributor to the shortfall in the OSR’s yield potential and has been listed in the top 10 pests and diseases of the crop in a recent survey of 10 countries (Zheng et al, 2020) Turnip yellows virus (TuYV) syn. beet western yellows virus (BWYV) is a major contributor to the shortfall in the OSR’s yield potential and has been listed in the top 10 pests and diseases of the crop in a recent survey of 10 countries (Zheng et al, 2020)
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