Fine genetic and physical mapping of the CRb gene conferring resistance to clubroot disease in Brassica rapa

The use of clubroot resistance (CR) genes is an effective and economical approach for controlling Plasmodiophora brassicae, the causal agent of clubroot disease in Chinese cabbage (Brassica rapa) and other Brassica crops. In a previous study, we identified and mapped the CRb locus on chromosome A03 of B. rapa in the doubled-haploid (DH) line ‘CR Shinki DH line’ of Chinese cabbage. In this study, CRb, a dominant gene conferring resistance to pathotype 4 of P. brassicae, was finely mapped in combination with bulked segregant analysis and bioinformatics analysis (BIA). Using 1,486 highly susceptible individuals and 2,896 individuals from two separate F₂ populations of ‘702-5’ (B. rapa ssp. chinensis) ×  ‘CR Shinki DH line,’ the CRb locus was narrowed to a region of approximately 0.14 cM between two flanking markers, TCR79 and TCR108. The sequences of seven newly developed markers linked to CRb were landed on bacterial artificial chromosome (BAC) of the reference B. rapa ‘Chiifu-401-42’ by BIA, and a physical map consisting of three BAC clones was constructed. The CRb locus was defined as an interval of approximately 83.5 kb on a BAC clone (KBrB085J21). The target interval contained one Toll-interleukin-1 receptor/nucleotide-binding site/leucine-rich repeat (TIR–NBS–LRR) gene, one NBS–LRR gene, and several putative regulatory genes in the B. rapa genome. The CRb gene was tightly linked to two other CR genes, CRa and CRb ^Kato . These results provide useful information for isolation of the CRb gene and tightly linked molecular markers for breeding CR in B. rapa.     

Mapping of isolate-specific QTLs for clubroot resistance in Chinese cabbage (<Emphasis Type="Italic">Brassica rapa</Emphasis> L. ssp. <Emphasis Type="Italic">pekinensis</Emphasis>)

A number of clubroot resistant (CR) Chinese cabbage cultivars have been developed in Japan using resistant genes from CR European fodder turnips (B. rapa ssp. rapifera). Clubroot resistance in European fodder turnips are known to be controlled by the combined action of several dominant resistance genes. We have developed three Chinese cabbage clubroot-resistant doubled haploid (DH) lines-T136-8, K10, and C9-which express resistance in different manners against two isolates of Plasmodiophora brassicae, M85 and K04. Depending on the isolates, we identified two CR loci, CRk and CRc. CRk was identified by quantitative trait loci (QTL) analysis of an F(2) population derived from a cross between K10 and Q5. This locus showed resistance to both isolates and is located close to Crr3 in linkage group R3. The other locus, CRc was identified by QTL analysis of an F(2) population derived from a cross between C9 and susceptible DH line, 6R. This locus was mapped to linkage group R2 and is independent from any published CR loci. We developed sequence-tagged site markers linked to this locus.     

Genotyping-by-sequencing map permits identification of clubroot resistance QTLs and revision of the reference genome assembly in cabbage (Brassica oleracea L.)

Clubroot is a devastating disease caused by Plasmodiophora brassicae and results in severe losses of yield and quality in Brassica crops. Many clubroot resistance genes and markers are available in Brassica rapa but less is known in Brassica oleracea. Here, we applied the genotyping-by-sequencing (GBS) technique to construct a high-resolution genetic map and identify clubroot resistance (CR) genes. A total of 43,821 SNPs were identified using GBS data for two parental lines, one resistant and one susceptible lines to clubroot, and 18,187 of them showed >5× coverage in the GBS data. Among those, 4,103 were credibly genotyped for all 78 F₂ individual plants. These markers were clustered into nine linkage groups spanning 879.9 cM with an average interval of 1.15 cM. Quantitative trait loci (QTLs) survey based on three rounds of clubroot resistance tests using F_{2 : 3} progenies revealed two and single major QTLs for Race 2 and Race 9 of P. brassicae, respectively. The QTLs show similar locations to the previously reported CR loci for Race 4 in B. oleracea but are in different positions from any of the CR loci found in B. rapa. We utilized two reference genome sequences in this study. The high-resolution genetic map developed herein allowed us to reposition 37 and 2 misanchored scaffolds in the 02–12 and TO1000DH genome sequences, respectively. Our data also support additional positioning of two unanchored 3.3 Mb scaffolds into the 02–12 genome sequence.     

Genetic mapping of clubroot resistance genes in oilseed rape

Clubroot caused by the obligate biotrophic protist Plasmodiophora brassicae is a major disease of Brassica species. Clubroot resistances introduced from B. oleracea var. ‘Böhmerwaldkohl’ and resistance from B. rapa ECD-04 were genetically mapped in oilseed rape (B. napus L.). A doubled haploid (DH) population of rape seed was developed by crossing a resistant DH-line derived from a resynthesized B. napus with the susceptible cultivar ‘Express’. The DH population was tested in the greenhouse against seven P. brassicae isolates showing low and high virulence toward B. oleracea or/and B. rapa. DH-lines with highest or lowest disease scores were used in a bulked segregant analysis (BSA), and 43 polymorphic AFLPs were identified. A genetic map of the whole genome was constructed using 338 AFLP and 156 anchored SSR markers. Nineteen QTL were detected on chromosomes N02, N03, N08, N13, N15, N16 and N19 giving resistance to seven different isolates. Race-specific effects were observed for all QTL, none of the QTL conferred resistance to all isolates. The phenotypic variance explained by the respective QTL ranged between 10.3 and 67.5%. All QTL could be assigned to both ancestral genomes of B. napus. In contrast to previous reports, a clear differentiation into major QTL from B. rapa and minor QTL from B. oleracea could not be found. Composite interval mapping confirmed the linkage relationships determined by BSA, thus demonstrating that markers for oligogenic traits can be selected by merely testing the distributional extremes of a segregating population.     

QTL mapping of clubroot resistance in radish (Raphanus sativus L.)

A QTL analysis for clubroot resistance (CR) of radish was performed using an F₂ population derived from a crossing of a CR Japanese radish and a clubroot-susceptible (CS) Chinese radish. F₃ plants obtained by selfing of F₂ plants were used for the CR tests. The potted seedlings were inoculated and the symptom was evaluated 6 weeks thereafter. The mean disease indexes of the F₃ plants were used for the phenotype of the F₂. The results of two CR tests were analyzed for the presence of QTL. A linkage map was constructed using AFLP and SSR markers; it spanned 554 cM and contained 18 linkage groups. A CR locus was observed in the top region of linkage group 1 in two tests. Therefore, the present results suggest that a large part of radish CR is controlled by a single gene or closely linked genes in this radish population, although minor effects of other genomic areas cannot be ruled out. The CR locus was named Crs1. Markers linked to Crs1 showed sequence homology to the genomic region of the top of chromosome 3 of Arabidopsis, as in the case of Crr3, a CR locus in Brassica rapa. These markers should be useful for breeding CR cultivars of radish. As Japanese radishes are known to be highly resistant or immune to clubroot, these markers may also be useful in the introgression of this CR gene to Brassica crops.     

Genetics of Clubroot Resistance in <Emphasis Type="Italic">Brassica</Emphasis> Species

Clubroot disease, caused by the obligate plant pathogen Plasmodiophora brassicae Wor., is one of the most economically important diseases affecting Brassica crops in the world. The genetic basis of clubroot resistance (CR) has been well studied in three economically important Brassica species: B. rapa, B. oleracea, and B. napus. In B. rapa, mainly in Chinese cabbage, one minor and seven major CR genes introduced from European fodder turnips have been identified. Mapping of these CR genes localized Crr1 on R8, Crr2 on R1, CRc on R2, and Crr4 on R6 linkage groups of Chinese cabbage. Genes Crr3, CRa, CRb, and CRk mapped to R3, but at two separate loci, CRa and CRb are independent of Crr3 and CRk, which are closely linked. Further analysis suggested that Crr1, Crr2, and CRb have similar origins in the ancestral genome as in chromosome 4 of Arabidopsis thaliana. Genetic analysis of clubroot resistance genes in B. oleracea suggests that they are quantitative traits. Twenty-two quantitative trait loci (QTLs) were mapped in different linkage groups of B. oleracea. In B. napus, genetic analysis of clubroot resistance was found to be governed by one or two dominant genes, whereas resistance conferred by two recessive genes is reported. The quantitative analysis approach, however, proved that they are polygenic. In total, at least 16 QTLs have been detected on eight chromosomes of B. napus, N02, N03, N08, N09, N13, N15, N16, and N19. The chromosomal location of the other six QTLs is not clear. Cloning of any of these QTLs or resistance loci was not, however, possible until recently. Progress in genomics, particularly the techniques of comparative mapping and genome sequencing, supplements cloning and allows improved characterization of CR genes. Further development of DNA markers linked to CR genes will in turn hasten the breeding of clubroot-resistant Brassica cultivars.     

Detection and analysis of QTLs based on RAPD markers for polygenic resistance to Plasmodiophora brassicae Woron in Brassica oleracea L.

Resistance to Plasmodiophora brassicae Woron, the causal fungus of clubroot, was examined in an F₂ population of a cross between a clubroot-resistant kale (Brassica oleracea L. var. acephala) and a susceptible cauliflower (Brassica oleracea L. var. botrytis). QTL detection was performed with RAPD markers. Two resistance notations, carried out at different times after inoculation, were used. Three markers were associated with these two notations and three were specifically linked to only one notation. QTL analysis suggests the existence of at least two genetic mechanisms implicated in the resistance phenomenon.     

Identification of Novel QTLs for Isolate-Specific Partial Resistance to Plasmodiophora brassicae in Brassica rapa

Plasmodiophora brassicae, the causal agent of clubroot disease of the Brassica crops, is widespread in the world. Quantitative trait loci (QTLs) for partial resistance to 4 different isolates of P. brassicae (Pb2, Pb4, Pb7, and Pb10) were investigated using a BC₁F₁ population from a cross between two subspecies of Brassica rapa, i.e. Chinese cabbage inbred line C59-1 as a susceptible recurrent parent and turnip inbred line ECD04 as a resistant donor parent. The BC₁F₂ families were assessed for resistance under controlled conditions. A linkage map constructed with simple sequence repeats (SSR), unigene-derived microsatellite (UGMS) markers, and specific markers linked to published clubroot resistance (CR) genes of B. rapa was used to perform QTL mapping. A total of 6 QTLs residing in 5 CR QTL regions of the B. rapa chromosomes A01, A03, and A08 were identified to account for 12.2 to 35.2% of the phenotypic variance. Two QTL regions were found to be novel except for 3 QTLs in the respective regions of previously identified Crr1, Crr2, and Crr3. QTL mapping results indicated that 1 QTL region was common for partial resistance to the 2 isolates of Pb2 and Pb7, whereas the others were specific for each isolate. Additionally, synteny analysis between B. rapa and Arabidopsis thaliana revealed that all CR QTL regions were aligned to a single conserved crucifer blocks (U, F, and R) on 3 Arabidopsis chromosomes where 2 CR QTLs were detected in A. thaliana. These results suggest that some common ancestral genomic regions were involved in the evolution of CR genes in B. rapa.     

Variation in abundance of predicted resistance genes in the Brassica oleracea pangenome

Brassica oleracea is an important agricultural species encompassing many vegetable crops including cabbage, cauliflower, broccoli and kale; however, it can be susceptible to a variety of fungal diseases such as clubroot, blackleg, leaf spot and downy mildew. Resistance to these diseases is meditated by specific disease resistance genes analogs (RGAs) which are differently distributed across B. oleracea lines. The sequenced reference cultivar does not contain all B. oleracea genes due to gene presence/absence variation between individuals, which makes it necessary to search for RGA candidates in the B. oleracea pangenome. Here we present a comparative analysis of RGA candidates in the pangenome of B. oleracea. We show that the presence of RGA candidates differs between lines and suggests that in B. oleracea, SNPs and presence/absence variation drive RGA diversity using separate mechanisms. We identified 59 RGA candidates linked to Sclerotinia, clubroot, and Fusarium wilt resistance QTL, and these findings have implications for crop breeding in B. oleracea, which may also be applicable in other crops species.     

Mapping of days to flower and seed yield in spring oilseed <Emphasis Type="Italic">Brassica napus</Emphasis> carrying genome content introgressed from <Emphasis Type="Italic">Brassica oleracea</Emphasis>

Earliness of flowering and maturity and high seed yield are important objectives of breeding spring Brassica napus canola. Previously, we have introgressed earliness of flowering from Brassica oleracea into spring B. napus canola through interspecific crossing between these two species. In this paper, we report quantitative trait locus (QTL) mapping of days to flower and seed yield by use of publicly available markers and markers designed based on flowering time genes and a doubled haploid population, derived from crossing of the spring canola parent and an early flowering line developed from a B. napus × B. oleracea cross, tested in nine field trials for over 5 years. Five genomic regions associated with days to flower were identified on C1, C2, C3, and C6 of which the single QTL of C1 was detected in all trials; in all cases, the allele introgressed from B. oleracea reduced the number of days to flower. BLASTn search in the Brassica genomes located the physical position of the QTL markers and identified putative flowering time genes in these regions. In the case of seed yield, ten QTL from eight linkage groups were detected; however, none could be consistently detected in all trials. The QTL region of C1 associated with days to flower did not show significant association with seed yield in more than 80% of the field trials; however, in a single trial, the allele introgressed from B. oleracea exerted a negative effect on seed yield. Thus, the genomic regions and molecular markers identified in this research could potentially be used in breeding for the development of early flowering B. napus canola cultivars without affecting seed yield in a majority of the environments.     

Identification of genomic regions involved in resistance against Sclerotinia sclerotiorum from wild Brassica oleracea

The lack of resistant source has greatly restrained resistance breeding of rapeseed (Brassica napus, AACC) against Sclerotinia sclerotiorum which causes severe yield losses in rapeseed production all over the world. Recently, several wild Brassica oleracea accessions (CC) with high level of resistance have been identified (Mei et al. in Euphytica 177:393–400, 2011), bringing a new hope to improve Sclerotinia resistance of rapeseed. To map quantitative trait loci (QTL) for Sclerotinia resistance from wild B. oleracea, an F2 population consisting of 149 genotypes, with several clones of each genotypes, was developed from one F1 individual derived from the cross between a resistant accession of wild B. oleracea (B. incana) and a susceptible accession of cultivated B. oleracea var. alboglabra. The F2 population was evaluated for Sclerotinia reaction in 2009 and 2010 under controlled condition. Significant differences among genotypes and high heritability for leaf and stem reaction indicated that genetic components accounted for a large portion of the phenotypic variance. A total of 12 QTL for leaf resistance and six QTL for stem resistance were identified in 2 years, each explaining 2.2–28.4 % of the phenotypic variation. The combined effect of alleles from wild B. oleracea reduced the relative susceptibility by 22.5 % in leaves and 15 % in stems on average over 2 years. A 12.8-cM genetic region on chromosome C09 of B. oleracea consisting of two major QTL intervals for both leaf and stem resistance was assigned into a 2.7-Mb genomic region on chromosome A09 of B. rapa, harboring about 30 putative resistance-related genes. Significant negative corrections were found between flowering time and relative susceptibility of leaf and stem. The association of flowering time with Sclerotinia resistance is discussed.     

Identification and mapping in spring wheat of genetic factors controlling stem rust resistance and the study of their epistatic interactions across multiple environments

Stem rust (Puccinia graminis f. sp. tritici) is responsible for major production losses in hexaploid wheat (Triticum aestivum L.) around the world. The spread of stem rust race Ug99 and variants is a threat to worldwide wheat production and efforts are ongoing to identify and incorporate resistance. The objectives of this research were to identify quantitative trait loci (QTL) and to study their epistatic interactions for stem rust resistance in a population derived from the Canadian wheat cultivars AC Cadillac and Carberry. A doubled haploid (DH) population was developed and genotyped with DArT^® and SSR markers. The parents and DH lines were phenotyped for stem rust severity and infection response to Ug99 and variant races in 2009, 2010 and 2011 in field rust nurseries near Njoro, Kenya, and to North American races in 2011 and 2012 near Swift Current, SK, Canada. Seedling infection type to race TTKSK was assessed in a bio-containment facility in 2009 and 2012 near Morden, MB. Eight QTL for stem rust resistance and three QTL for pseudo-black chaff on nine wheat chromosomes were identified. The phenotypic variance (PV) explained by the stem rust resistance QTL ranged from 2.4 to 48.8 %. AC Cadillac contributed stem rust resistance QTL on chromosomes 2B, 3B, 5B, 6D, 7B and 7D. Carberry contributed resistance QTL on 4B and 5A. Epistatic interactions were observed between loci on 4B and 5B, 4B and 7B, 6D and 3B, 6D and 5B, and 6D and 7B. The stem rust resistance locus on 6D interacted synergistically with 5B to improve the disease resistance through both crossover and non-crossover interactions depending on the environment. Results from this study will assist in planning breeding for stem rust resistance by maximizing QTL main effects and epistatic interactions.     

Fine mapping of the Rrs1 resistance locus against scald in two large populations derived from Spanish barley landraces

Key message

In two Spanish barley landraces with outstanding resistance to scald, the Rrs1 _Rh4 locus was fine mapped including all known markers used in previous studies and closely linked markers were developed.

Abstract

Scald, caused by Rhynchosporium commune, is one of the most prevalent barley diseases worldwide. A search for new resistance sources revealed that Spanish landrace-derived lines SBCC145 and SBCC154 showed outstanding resistance to scald. They were crossed to susceptible cultivar Beatrix to create large DH-mapping populations of 522 and 416 DH lines that were scored for disease resistance in the greenhouse using two R. commune isolates. To ascertain the pattern of resistance, parents and reference barley lines with known scald resistance were phenotyped with a panel of differential R. commune isolates. Subpopulations were genotyped with the Illumina GoldenGate 1,536 SNP Assay and a large QTL in the centromeric region of chromosome 3H, known to harbour several scald resistance genes and/or alleles, was found in both populations. Five SNP markers closest to the QTL were converted into CAPS markers. These CAPS markers, together with informative SSR markers used in other scald studies, confirmed the presence of the Rrs1 locus. The panel of differential scald isolates indicated that the allele carried by both donors was Rrs1 _Rh4 . The genetic distance between Rrs1 and its flanking markers was 1.2 cM (11_0010) proximally and 0.9 cM (11_0823) distally, which corresponds to a distance of just below 9 Mbp. The number and nature of scald resistance genes on chromosome 3H are discussed. The effective Rrs1 allele found and the closely linked markers developed are already useful tools for molecular breeding programs and provide a good step towards the identification of candidate genes.     

Microsatellite analysis of wheat chromosome 2D allows the reconstruction of chromosomal inheritance in pedigrees of breeding programmes

Fusarium head blight (FHB, scab) is a fungal disease of wheat and other small cereals that is found in both temperate and semi-tropical regions. FHB causes severe yield and quality losses, but the most-serious concern is the possible mycotoxin contamination of cereal food and feed. Breeding for FHB resistance by conventional selection is feasible, but tedious and expensive. This study was conducted to identify and map DNA markers associated with FHB resistance genes in wheat. A population of 364 F₁-derived doubled-haploid (DH) lines from the cross ’CM-82036’ (resistant)/’Remus’ (susceptible) was evaluated for Type II resistance (spread within the spike) during 2 years under field conditions. Marker analysis was performed on 239 randomly chosen DH lines. Different marker types were applied, with an emphasis on AFLP and SSR markers. Analysis of variance, as well as simple and composite interval mapping, were applied. Three genomic regions were found significantly associated with FHB resistance. The most-prominent effect was detected on the short arm of chromosome 3B, explaining up to 60% of the phenotypic variance for Type II FHB resistance. A further QTL was located on chromosome 5A and a third one on 1B. The QTL regions on 3B and 5A were tagged with flanking SSR markers, the 1B QTL was found associated with the high-molecular-weight glutenin locus. These results indicate that FHB resistance is under control of a few major QTLs operating together with unknown numbers of minor genes. Marker-assisted selection for these major QTLs involved in FHB resistance appears feasible and should accelerate the development of resistant and agronomically improved wheat cultivars. Received: 25 January 2001 / Accepted: 18 February 2001     

Identification of quantitative trait loci conferring resistance to tan spot in a biparental population derived from two Nebraska hard red winter wheat cultivars

Tan spot, caused by Pyrenophora tritici-repentis (Ptr), is a destructive foliar disease in all types of cultivated wheat worldwide. Genetics of tan spot resistance in wheat is complex, involving insensitivity to fungal-produced necrotrophic effectors (NEs), major resistance genes, and quantitative trait loci (QTL) conferring race-nonspecific and race-specific resistance. The Nebraska hard red winter wheat (HRWW) cultivar ‘Wesley’ is insensitive to Ptr ToxA and highly resistant to multiple Ptr races, but the genetics of resistance in this cultivar is unknown. In this study, we used a recombinant inbred line (RIL) population derived from a cross between Wesley and another Nebraska cultivar ‘Harry’ (Ptr ToxA sensitive and highly susceptible) to identify QTL associated with reaction to tan spot caused by multiple races/isolates. Sensitivity to Ptr ToxA conferred by the Tsn1 gene was mapped to chromosome 5B as expected. The Tsn1 locus was a major susceptibility QTL for the race 1 and race 2 isolates, but not for the race 2 isolate with the ToxA gene deleted. A second major susceptibility QTL was identified for all the Ptr ToxC-producing isolates and located to the distal end of the chromosome 1A, which likely corresponds to the Tsc1 locus. Three additional QTL with minor effects were identified on chromosomes 7A, 7B, and 7D. This work indicates that both Ptr ToxA-Tsn1 and Ptr ToxC-Tsc1 interactions are important for tan spot development in winter wheat, and Wesley is highly resistant largely due to the absence of the two tan spot sensitivity genes.     

The genetic architecture of seedling resistance to Septoria tritici blotch in the winter wheat doubled-haploid population Solitär × Mazurka

Breeding for resistance to Septoria tritici blotch (STB), caused by Mycosphaerella graminicola (anamorph: Septoria tritici), is an essential component in controlling this important foliar disease of wheat. Inheritance of seedling resistance to seven worldwide pathogen isolates has been studied in a doubled-haploid (DH) population derived from a cross between the field resistant cultivar Solitär and the susceptible cultivar Mazurka. Multiple quantitative trait locus (QTL) mapping revealed major and minor genetic effects on resistance as well as several epistatic relationships in the seedling stage. Solitär conferred resistance to isolate IPO323, governed by Stb6 on chromosome 3A, as well as to IPO99015, IPO92034, Hu1 and Hu2 controlled by a QTL on chromosome arm 1BS, possibly corresponding to Stb11 and minor QTL on chromosomes 1B, 3D, 6B and 7D. Resistance of Mazurka to IPO90015 and BBA22 was caused by a QTL located in a region on 4AL which harbours Stb7 or Stb12. QTL specific to pycnidial coverage on 3B and specific to necrosis on 1A could be discovered for isolate IPO92034. Pairwise epistatic interactions were reliably detected with five isolates. Although their contributions to the total variance are generally low, the genotypic effect of the QTL by QTL interaction of 4AL (Stb7 or Stb12) and 3AS (Stb6) made up almost 15% of disease expression. Altogether, the results suggest a complex inheritance of resistance to STB in the seedling stage in terms of isolate-specificity and resistance mechanisms, which have implications for marker-assisted breeding in an attempt to pyramid STB resistance genes.     

Construction of a genetic map based on high-throughput SNP genotyping and genetic mapping of a TuMV resistance locus in Brassica rapa

Brassica rapa is a member of the Brassicaceae family and includes vegetables and oil crops that are cultivated worldwide. The introduction of durable resistance against turnip mosaic virus (TuMV) into agronomically important cultivars has been a significant challenge for genetic and horticultural breeding studies of B. rapa. Based on our previous genome-wide analysis of DNA polymorphisms between the TuMV-resistant doubled haploid (DH) line VC40 and the TuMV-susceptible DH line SR5, we constructed a core genetic map of the VCS-13M DH population, which is composed of 83 individuals derived from microspore cultures of a F₁ cross between VC40 and SR5, by analyzing the segregation of 314 sequence-characterized genetic markers. The genetic markers correspond to 221 SNPs and 31 InDels of genes as well as 62 SSRs, covering 1,115.9 cM with an average distance of 3.6 cM between the adjacent marker loci. The alignment and orientation of the constructed map showed good agreement with the draft genome sequence of Chiifu, thus providing an efficient strategy to map genic sequences. Using the genetic map, a novel dominant TuMV resistance locus (TuMV-R) in the VCS-13M DH population was identified as a 0.34 Mb region in the short arm of chromosome A6 in which four CC–NBS–LRR resistance genes and two pathogenesis-related-1 genes reside. The genetic map developed in this study can play an important role in the genetic study of TuMV resistance and the molecular breeding of B. rapa.     

Genetic mapping of a fusarium wilt resistance gene in Brassica oleracea

Fusarium wilt caused by Fusarium oxysporum f. sp. conglutinans is one of the most important diseases of Brassica crops, resulting in severe reductions in yield and quality. To characterize the inheritance pattern of fusarium resistance, a cross between a susceptible broccoli and a resistant cabbage was subjected to segregation analysis. Results indicated that resistance was controlled by a single dominant allele. This gene was named Foc-Bo1 and mapped to linkage group seven (O7) by both the segregation test and quantitative trait locus (QTL) analysis. The QTL on O7 was detected with a logarithm of odds score (LOD) of 19.5, which was above the threshold value with genome-wide 1% significance level (2.01). A minor QTL was also detected on O4 with a LOD score of 2.06. Inoculation tests indicated that stable expression of fusarium resistance at high temperatures required Foc-Bo1 homozygosity. The association between Foc-Bo1 and the closest simple sequence repeat marker (KBrS003O1N10) was analyzed in three F3 populations. Based on these studies, KBrS003O1N10 represents an effective marker-assisted selection (MAS) tool for breeding fusarium wilt resistance into Brassica oleracea crops. To our knowledge, this is the first paper to map the fusarium-resistance gene in Brassica species and to validate the effectiveness of MAS in improving fusarium resistance in these important plants.     

QTL for resistance to root lesion nematode (Pratylenchus thornei) from a synthetic hexaploid wheat source

Key message

A whole genome average interval mapping approach identified eight QTL associated with P. thornei resistance in a DH population from a cross between the synthetic-derived wheat Sokoll and cultivar Krichauff.

Abstract

Pratylenchus thornei are migratory nematodes that feed and reproduce within the wheat root cortex, causing cell death (lesions) resulting in severe yield reductions globally. Genotypic selection using molecular markers closely linked to Pratylenchus resistance genes will accelerate the development of new resistant cultivars by reducing the need for laborious and expensive resistance phenotyping. A doubled haploid wheat population (150 lines) from a cross between the synthetic-derived cultivar Sokoll (P. thornei resistant) and cultivar Krichauff (P. thornei moderately susceptible) was used to identify quantitative trait loci (QTL) associated with P. thornei resistance. The resistance identified in the glasshouse was validated in a field trial. A genetic map was constructed using Diversity Array Technology and the QTL regions identified were further targeted with simple sequence repeat (SSR) and single-nucleotide polymorphism (SNP) markers. Six significant and two suggestive P. thornei resistance QTL were detected using a whole genome average interval mapping approach. Three QTL were identified on chromosome 2B, two on chromosome 6D, and a single QTL on each of chromosomes 2A, 2D and 5D. The QTL on chromosomes 2BS and 6DS mapped to locations previously identified to be associated with Pratylenchus resistance. Together, the QTL on 2B (QRlnt.sk-2B.1–2B.3) and 6D (QRlnt.sk-6D.1 and 6D.2) explained 30 and 48 % of the genotypic variation, respectively. Flanking PCR-based markers based on SSRs and SNPs were developed for the major QTL on 2B and 6D and provide a cost-effective high-throughput tool for marker-assisted breeding of wheat with improved P. thornei resistance.