European Turnip Research Articles

A chromosome-level reference genome facilitates the discovery of clubroot resistant gene Crr5 in Chinese cabbage

Abstract Brassica rapa includes a variety of important vegetable and oilseed crops, yet it is significantly challenged by clubroot disease. Notably, the majority of genotypes of B. rapa with published genomes exhibit high susceptibility to clubroot disease. The present study presents a high-quality chromosome-level sequence of the genome of the DH40 clubroot-resistant (CR) line, a doubled haploid line derived from the hybrid progeny of a European turnip (ECD01) and two lines of Chinese cabbage. The assembled genome spans 420.92 Mb, with a contig N50 size of 11.97 Mb. Comparative genomics studies revealed that the DH40 line is more closely related to the Chinese cabbage Chiifu than to the turnip ECD04. The DH40 genome provided direct reference and greatly facilitate the map-based cloning of the clubroot resistance gene Crr5, encoding a nucleotide-binding leucine-rich repeat (NLR) protein. Further functional analysis demonstrated that Crr5 confers clubroot resistance in both Chinese cabbage and transgenic Arabidopsis. It responds to inoculation with Plasmodiophora brassicae and is expressed in both roots and leaves. Subcellular localization shows that Crr5 is present in the nucleus. Notably, the TIR domain of Crr5 can autoactivate and trigger cell death. In addition, we developed two Crr5-specific KASP markers and showcased their successful application in breeding CR Chinese cabbage through marker-assisted selection. Overall, our research offers valuable resources for genetic and genomic studies in B. rapa and deepens our understanding of the molecular mechanisms underlying clubroot resistance against Plasmodiophora brassicae.

Multiomics analysis of a resistant European turnip ECD04 during clubroot infection reveals key hub genes underlying resistance mechanism

The clubroot disease has become a worldwide threat for crucifer crop production, due to its soil-borne nature and difficulty to eradicate completely from contaminated field. In this study we used an elite resistant European fodder turnip ECD04 and investigated its resistance mechanism using transcriptome, sRNA-seq, degradome and gene editing. A total of 1751 DEGs were identified from three time points after infection, among which 7 hub genes including XTH23 for cell wall assembly and two CPK28 genes in PTI pathways. On microRNA, we identified 17 DEMs and predicted 15 miRNA-target pairs (DEM-DEG). We validated two pairs (miR395-APS4 and miR160-ARF) by degradome sequencing. We investigated the miR395-APS4 pair by CRISPR-Cas9 mediated gene editing, the result showed that knocking-out APS4 could lead to elevated clubroot resistance in B. napus. In summary, the data acquired on transcriptional response and microRNA as well as target genes provide future direction especially gene candidates for genetic improvement of clubroot resistance on Brassica species.

Eco-Geographical and Botanical Patterns of Resistance to Lepidoptera Insects in Brassica rapa L.

In the context of the widespread expansion of damage by herbivorous pests of Brassica crops, taking into account the requirements for minimizing pesticide pollution of the environment, it is important to have fundamental knowledge of the geographical features of the distribution of pests and about the botanical confinement of plant resistance in order to develop a strategy for creating new Brassica cultivars with complex resistance to insects. The relevance of our work is related to the study of the variability in the degree of resistance of the extensive genetic diversity of Brassica rapa accessions to the main herbivorous pests of Brassica crops in contrasting ecological and geographical zones of the Russian Federation (Arctic, northwestern, and southern zones). We have studied the distribution and food preferences of Lepidoptera insects (diamondback moth Plutella xylostella and cabbage moth Mamestra brassicae) on a set of 100 accessions from the VIR B. rapa collection (Chinese cabbage, pakchoi, wutacai, zicaitai, mizuna, and leaf and root turnips) in the field in three zones of the Russian Federation. We have found that the diamondback moth and cabbage moth are largely harmful in three zones of the European part of the Russian Federation, although the degree of damage to plants by these insects varies by year of cultivation. On average, for the set studied during the two years of the experiment, the degree of plant damage by both pests in the Arctic zone was low and almost low, and in the northwestern and southern zones, it was medium. It was noted that diamondback moth damage was greater in the northwestern zone in both years and in the southern and Arctic zones in 2021, while in 2022, the degree of cabbage moth damage was slightly higher in the southern and Arctic zones. Under the conditions of field diamondback moth damage, the accessions of Chinese cabbage, wutacai, and mizuna turned out to be the most resistant (the damage score was 1.92-1.99), whereas the accessions of wutacai and pakchoi were the most resistant to the cabbage moth (the damage score was 1.62-1.78). A high variability in the degree of resistance of Brassica crops to Lepidoptera insects from complete resistance to susceptibility was revealed. We have identified sources of resistance to insects, including complex resistance in all study areas, among landraces and some modern cultivars of Chinese cabbage, pakchoi, wutacai, and mizuna from Japan and China, as well as European turnips. The highest susceptibility to pests in the studied set was noted in the accession of root turnip "Hinona" (k-1422, USA) (average damage score of 3.24-3.53 points). We were not able to establish the morphological features of resistant plants or the geographical confinement of the origin of resistance of B. rapa crop accessions.

Interploidy Introgression Shaped Adaptation during the Origin and Domestication History of Brassica napus.

Polyploidy is recurrent across the tree of life and known as an evolutionary driving force in plant diversification and crop domestication. How polyploid plants adapt to various habitats has been a fundamental question that remained largely unanswered. Brassica napus is a major crop cultivated worldwide, resulting from allopolyploidy between unknown accessions of diploid B. rapa and B. oleracea. Here, we used whole-genome resequencing data of accessions representing the majority of morphotypes and ecotypes from the species B. rapa, B. oleracea, and B. napus to investigate the role of polyploidy during domestication. To do so, we first reconstructed the phylogenetic history of B. napus, which supported the hypothesis that the emergence of B. napus derived from the hybridization of European turnip of B. rapa and wild B. oleracea. These analyses also showed that morphotypes of swede and Siberian kale (used as vegetable and fodder) were domesticated before rapeseed (oil crop). We next observed that frequent interploidy introgressions from sympatric diploids were prominent throughout the domestication history of B. napus. Introgressed genomic regions were shown to increase the overall genetic diversity and tend to be localized in regions of high recombination. We detected numerous candidate adaptive introgressed regions and found evidence that some of the genes in these regions contributed to phenotypic diversification and adaptation of different morphotypes. Overall, our results shed light on the origin and domestication of B. napus and demonstrate interploidy introgression as an important mechanism that fuels rapid diversification in polyploid species.

Draft Genome Assembly and Transcriptome Dataset for European Turnip (Brassica rapa L. ssp. rapifera), ECD4 Carrying Clubroot Resistance.

Draft Genome Assembly and Transcriptome Dataset for European Turnip (Brassica rapa L. ssp. rapifera), ECD4 Carrying Clubroot Resistance.

QTL Mapping and Candidate Gene Identification of Swollen Root Formation in Turnip.

The swollen root is an important agronomic trait and is a determinant of yield for turnips, which are cultivated as both vegetables and fodder. However, the genetic mechanism of swollen root formation is poorly understood. In this study, we analyzed the F2 and BC1P2 populations derived from a cross between “10601” (European turnip with swollen root, Brassica rapa ssp. rapifera, AA, 2n = 2× = 20) and “10603” (Chinese cabbage with normal root, Brassica rapa ssp. pekinensis, AA, 2n = 2× = 20), and suggested that the swollen root is a quantitative trait. Two major quantitative trait loci (QTLs), FR1.1 (Fleshy root 1.1) and FR7.1 (Fleshy root 7.1), were identified by QTL-seq analysis and further confirmed by QTL mapping in F2 and BC1P2 populations. The QTL FR1.1 with a likelihood of odd (LOD) of 7.01 explained 17.2% of the total phenotypic variations for root diameter and the QTL FR7.1 explained 23.0% (LOD = 9.38) and 31.0% (LOD = 13.27) of the total phenotypic variations in root diameter and root weight, respectively. After a recombinant screening, the major QTL FR7.1 was further narrowed down to a 220 kb region containing 47 putative genes. A candidate gene, Bra003652, which is a homolog of AT1G78240 that plays an essential role in cell adhesion and disorganized tumor-like formation in Arabidopsis thaliana, was identified in this region. In addition, expression and parental allele analysis supported that Bra003652 was a possible candidate gene of QTL FR7.1 for swollen root formation in turnip. Our research may provide new insight into the molecular mechanism of swollen root formation in root crops.

Association of Clubroot Resistance Locus PbBa8.1 With a Linkage Drag of High Erucic Acid Content in the Seed of the European Turnip.

Clubroot caused by Plasmodiophora brassicae is a severe threat to the production of Brassica napus, worldwide. The cultivation of resistant varieties is the most efficient and environmentally friendly way to limit disease spread. We developed a highly resistant B. napus line, ZHE226, containing the resistance locus PbBa8.1. However, ZHE226 seeds contain high erucic acid content, which limits its cultivation owing to its low edible oil quality. A segregation population of BC3F2 was developed by crossing ECD04, a resistant European turnip donor, with Huangshuang5, an elite variety with no erucic acid in its seeds, as a recurrent plant. Fine mapping using the bulk segregation analysis sequencing (BSA-Seq) approach detected PbBa8.1 within a 2.9 MB region on chromosome A08. Interestingly, the previously reported resistance gene Crr1a was found in the same region. Genetic analysis revealed that the CAP-134 marker for Crr1a was closely linked with clubroot resistance (CR). Thus, PbBa8.1 and Crr1a might be allelic for CR. Moreover, comparative and genetic analysis showed that high erucic acid in the seeds of ZHE226 was due to linkage drag of fatty acid elongase 1 (FAE1) in the ECD04 line, which was located in the interval of PbBa8.1 with a physical and genetic distance of 729 Kb and 1.86 cm, respectively. Finally, a clubroot-resistant line with a low erucic acid content was successfully developed through gene-specific molecular marker assistant selection from BC4F4. These results will accelerate CR breeding programs in B. napus.

Whole-genome resequencing reveals Brassica napus origin and genetic loci involved in its improvement

Brassica napus (2n = 4x = 38, AACC) is an important allopolyploid crop derived from interspecific crosses between Brassica rapa (2n = 2x = 20, AA) and Brassica oleracea (2n = 2x = 18, CC). However, no truly wild B. napus populations are known; its origin and improvement processes remain unclear. Here, we resequence 588 B. napus accessions. We uncover that the A subgenome may evolve from the ancestor of European turnip and the C subgenome may evolve from the common ancestor of kohlrabi, cauliflower, broccoli, and Chinese kale. Additionally, winter oilseed may be the original form of B. napus. Subgenome-specific selection of defense-response genes has contributed to environmental adaptation after formation of the species, whereas asymmetrical subgenomic selection has led to ecotype change. By integrating genome-wide association studies, selection signals, and transcriptome analyses, we identify genes associated with improved stress tolerance, oil content, seed quality, and ecotype improvement. They are candidates for further functional characterization and genetic improvement of B. napus.

BrRLP48, Encoding a Receptor-Like Protein, Involved in Downy Mildew Resistance in Brassica rapa.

Downy mildew, caused by Hyaloperonospora parasitica, is a major disease of Brassica rapa that causes large economic losses in many B. rapa-growing regions of the world. The genotype used in this study was based on a double haploid population derived from a cross between the Chinese cabbage line BY and a European turnip line MM, susceptible and resistant to downy mildew, respectively. We initially located a locus Br-DM04 for downy mildew resistance in a region about 2.7 Mb on chromosome A04, which accounts for 22.3% of the phenotypic variation. Using a large F2 mapping population (1156 individuals) we further mapped Br-DM04 within a 160 kb region, containing 17 genes encoding proteins. Based on sequence annotations for these genes, four candidate genes related to disease resistance, BrLRR1, BrLRR2, BrRLP47, and BrRLP48 were identified. Overexpression of both BrRLP47 and BrRLP48 using a transient expression system significantly enhanced the downy mildew resistance of the susceptible line BY. But only the leaves infiltrated with RNAi construct of BrRLP48 could significantly reduce the disease resistance in resistant line MM. Furthermore, promoter sequence analysis showed that one salicylic acid (SA) and two jasmonic acid-responsive transcript elements were found in BrRLP48 from the resistant line, but not in the susceptible one. Real-time PCR analysis showed that the expression level of BrRLP48 was significantly induced by inoculation with downy mildew or SA treatment in the resistant line MM. Based on these findings, we concluded that BrRLP48 was involved in disease resistant response and the disease-inducible expression of BrRLP48 contributed to the downy mildew resistance. These findings led to a new understanding of the mechanisms of resistance and lay the foundation for marker-assisted selection to improve downy mildew resistance in Brassica rapa.

Combinations of Independent Dominant Loci Conferring Clubroot Resistance in All Four Turnip Accessions (Brassica rapa) From the European Clubroot Differential Set

Clubroot disease is devastating to Brassica crop production when susceptible cultivars are planted in infected fields. European turnips are the most resistant sources and their resistance genes have been introduced into other crops such oilseed rape (Brassica napus L.), Chinese cabbage and other Brassica vegetables. The European clubroot differential (ECD) set contains four turnip accessions (ECD1–4). These ECD turnips exhibited high levels of resistance to clubroot when they were tested under controlled environmental conditions with Canadian field isolates. Gene mapping of the clubroot resistance genes in ECD1–4 were performed and three independent dominant resistance loci were identified. Two resistance loci were mapped on chromosome A03 and the third on chromosome A08. Each ECD turnip accession contained two of these three resistance loci. Some resistance loci were homozygous in ECD accessions while others showed heterozygosity based on the segregation of clubroot resistance in 20 BC1 families derived from ECD1 to 4. Molecular markers were developed linked to each clubroot resistance loci for the resistance gene introgression in different germplasm.

Evaluation of Clubroot Resistance in Chinese Cabbage and Its Inheritance in the European Turnip Line ‘IT033820’, a New Genetic Resource

Clubroot caused by the protist Plasmodiophora Brassicae is one of the most destructive diseases of Brassica crops. Developing Chinese cabbage cultivars with durable clubroot resistance (CR) is an important goal of breeding programs, which will require new genetic resources to be identified and introduced. In this study, we evaluated resistance to P. Brassicae race 4 using 26 Chinese cabbage (B. rapa ssp. pekinensis.) cultivars compared to the clubroot-susceptible Chinese cabbage inbred line ‘BP079’ and the clubroot-resistant European turnip (B. rapa ssp. rapifera) inbred line ‘IT033820’. No symptoms of clubroot disease were found in ‘IT033820’ infected with P. Brassicae race 4, whereas the Chinese cabbage cultivars exhibited disease symptoms to various degrees. The Chinese cabbage cultivars that were reported to be clubroot-susceptible were susceptible to P. Brassicae race 4; however, seven of the 20 cultivars reported to be clubroot-resistant were susceptible to this race of P. Brassicae to varying degrees. Resting spores of P. Brassicae were abundant within the infected root tissues of ‘BP079’, as revealed by light microscopy and scanning electron microscopy (SEM), but they were not detected in root tissues of ‘IT033820’. Although resting spores were not detected by light microscopy in root tissues of the clubroot-resistant Chinese cabbage cultivar ‘Kigokoro 75’, a few spores were observed by SEM. The F1 hybrids from a cross between ‘IT033820’ and ‘BP079’ showed no disease symptoms, and all BC1P1 progenies from a cross between the F1 hybrid and ‘IT033820’ exhibited a resistance phenotype. In the BC1P2 population from a cross between the F1 hybrid and ‘BP079’, this trait segregated at a ratio of 3(R):1(S) (χ2 = 1.333, p = 0.248) at a 5% significance level. Inoculated BC1P2 plants were either highly resistant or highly susceptible to the pathogen, indicating that the CR to race 4 of P. Brassicae carried by ‘IT033820’ is dominant. In the F2 population, this trait segregated at a ratio of 15(R):1(S) (χ2 = 0.152, p = 0.696) at a 5% significance level, suggesting that CR in ‘IT033820’ is mainly controlled by two dominant genes. Therefore, ‘IT033820’ represents a promising genetic resource for developing durable CR breeding lines in Chinese cabbage.

Genetic divergence of turnip (Brassica rapa L. em. Metzg. subsp. rapa) inferred from simple sequence repeats in chloroplast and nuclear genomes and morphology

Turnip (Brassica rapa L. em. Metzg. subsp. rapa), which is considered to be a primitive type of cultivated B. rapa, is cultivated worldwide as vegetable and fodder. To elucidate the phylogenetic relationships of Eurasian turnips, we examined their morphology and analyzed 6 cpSSR and 18 nuSSR loci in 87 accessions. Examination of seed coat mucilage and leaf hairs revealed existence of geographic distinctions. Two haplogroups were categorized among 12 haplotypes identified by the analysis of cpSSRs and two clusters were detected based on nuSSRs. These haplogroups and clusters were different between eastern and western Eurasia. Although morphological differences were detected between eastern and western Japan, no clear differences of haplotypes and clusters were found in Japanese turnips. Accessions from continental Asia showed various haplogroups and clusters and higher levels of genetic diversity than those from other regions. These results, in addition to previous studies suggest that central Asia is the sole geographic origin of turnips that Asian turnips did not originate as descendants of European turnips, and that almost all Japanese turnips were derived from central Asia.

Morphology, carbohydrate composition and vernalization response in a genetically diverse collection of Asian and European turnips (Brassica rapa subsp. rapa).

Brassica rapa displays enormous morphological diversity, with leafy vegetables, turnips and oil crops. Turnips (Brassica rapa subsp. rapa) represent one of the morphotypes, which form tubers and can be used to study the genetics underlying storage organ formation. In the present study we investigated several characteristics of an extensive turnip collection comprising 56 accessions from both Asia (mainly Japanese origin) and Europe. Population structure was calculated using data from 280 evenly distributed SNP markers over 56 turnip accessions. We studied the anatomy of turnip tubers and measured carbohydrate composition of the mature turnip tubers of a subset of the collection. The variation in 16 leaf traits, 12 tuber traits and flowering time was evaluated in five independent experiments for the entire collection. The effect of vernalization on flowering and tuber formation was also investigated. SNP marker profiling basically divided the turnip accessions into two subpopulations, with admixture, generally corresponding with geographical origin (Europe or Asia). The enlarged turnip tuber consists of both hypocotyl and root tissue, but the proportion of the two tissues differs between accessions. The ratio of sucrose to fructose and glucose differed among accessions, while generally starch content was low. The evaluated traits segregated in both subpopulations, with leaf shape, tuber colour and number of shoots per tuber explaining most variation between the two subpopulations. Vernalization resulted in reduced flowering time and smaller tubers for the Asian turnips whereas the European turnips were less affected by vernalization.

Fine Mapping a Clubroot Resistance Locus in Chinese Cabbage

There are various clubroot pathogen (Plasmodiophora brassicae) resistance genes within Brassica species with european turnip (B. rapa ssp. rapifera) being identified as potentially the best source of resistance for the development of clubroot-resistant cultivars in chinese cabbage (B. rapa ssp. pekinensis). To use clubroot resistance genes effectively, it is necessary to map these genes so that molecular markers inside or closely linked to these resistance genes can be developed. Using molecular marker-assisted selection, the clubroot resistance genes can be effectively transferred from cultivar to cultivar and from species to species. In this report, one clubroot resistance locus was mapped on linkage group A3 using five segregating populations developed from five chinese cabbage cultivars, suggesting that all the five cultivars shared the same clubroot resistance locus. Furthermore, one of these five chinese cabbage cultivars was used to develop a large segregating population to fine-map this clubroot resistance locus to a 187-kilobp chromosomal region. Molecular markers that are closely linked to the mapped clubroot resistance locus have been developed that can be used for marker-assisted selection in chinese cabbage and canola/rapeseed (B. rapa and B. napus) breeding programs.

An Improved Brassica rapa Genetic Linkage Map and Locus-specific Variations in a Doubled Haploid Population

In this study, we describe the construction of an improved Chinese cabbage genetic linkage map by integrating simple sequence repeats (SSRs) and insertion/deletion polymorphisms (InDels) into a previously published map of a doubled haploid (DH) population. The population was derived from a cross between the Chinese cabbage line BY (Brassica rapa ssp. pekinensis) and a European turnip line MM (Brassica rapa L. ssp. rapifera). A total of 629 markers were aligned to ten linkage groups, with a total map length of 1,173.8 cM, and an average distance between markers of 1.87 cm. Of the 126 SSRs and 133 InDels mapped, 46 and 34 were novel, respectively. A comparison of the linkage map with the B. rapa genome showed that more than 93 % of the markers, including 112 SSRs and 129 InDels, could be anchored unambiguously to a specific location on one of the ten chromosomes. In most cases, the order of markers on the linkage map and physical map was similar; however, the majority of linkage groups contained a number of markers whose positions were either transposed or had moved slightly forwards or backwards. During microspore culture, it was observed that 11 SSRs and one InDel showed either variation in size, or the appearance of new marker bands in the DH lines. As a first step to addressing this SSR/InDel marker instability, six SSR and one InDel loci were sequenced, which revealed that the size variation was due mainly to changes in repeat-motif number or to the insertion/deletion of new fragments of DNA.

Molecular characterization of the CRa gene conferring clubroot resistance in Brassica rapa

Clubroot disease is one of the major diseases affecting Brassicaceae crops, and a number of these crops grown commercially, such as Chinese cabbage (Brassica rapa L. ssp. pekinensis), are known to be highly susceptible to clubroot disease. To provide protection from this disease, plant breeders have introduced genes for resistance to clubroot from the European turnip into susceptible lines. The CRa gene confers specific resistance to the clubroot pathogen Plasmodiophora brassicae isolate M85. Fine mapping of the CRa locus using synteny to the Arabidopsis thaliana genome and partial genome sequences of B. rapa revealed a candidate gene encoding a TIR-NBS-LRR protein. Several structural differences in this candidate gene were found between susceptible and resistant lines, and CRa expression was observed only in the resistant line. Four mutant lines lacking clubroot resistance were obtained by the UV irradiation of pollen from a resistant line, and all of these mutant lines carried independent mutations in the candidate TIR-NBS-LRR gene. This genetic and molecular evidence strongly suggests that the identified gene is CRa. This is the first report on the molecular characterization of a clubroot Resistance gene in Brassicaceae and of the disease resistance gene in B. rapa.

Mapping quantitative trait loci (QTL) for clubroot resistance in Brassica rapa L.

SummaryThe genes related to clubroot resistance (CR) from a new genetic resource of European turnip (Brassica rapa ssp. rapifera) were identified, and their chromosomal locations were determined using quantitative trait loci (QTL) analysis. One hundred and ninety F2 plants derived from a cross between a clubroot-resistant inbred turnip line (IT033820) and a susceptible inbred Chinese cabbage (B. rapa ssp. pekinensis) line (BP079) were used to construct a genetic linkage map. The map contained 477 markers, including 401 amplified fragment length polymorphism (AFLP) and 56 simple sequence repeat (SSR) markers, in ten linkage groups (LGs). It covered a total length of 1,149.1 cM, with an average interval of 2.53 cM between markers. The majority of individual F2 plants were found to be highly resistant to clubroot disease based on phenotypic evaluations following inoculation with race 4 of Plasmodiophora brassicae. Using composite interval mapping analysis, two major QTLs (Pb-Br3 and Pb-Br8) and one minor QTL associated with resistance to race 4 of P. brassicae were detected. Pb-Br3 and the minor QTL were located on LG3, while Pb-Br8 was on LG8. Most of the markers identified by bulk segregant analysis (BSA) were located in the region of the two major QTLs. Along with the markers developed in the present study, the Pb-Br3 region included the previously published markers HC352b-SCAR and TCR08, for the loci CRa and CRb, respectively. Five markers, including OPW06_297 identified by BSA, were located in the Pb-Br8 region, spanning 5.9 cM. The QTL Pb-Br8 differed from previously reported CR genes. The two major QTLs explained 85.4% of the total phenotypic variation. The QTLs Pb-Br3 and Pb-Br8 were marked by two convenient co-dominant sequence characterised amplified region (SCAR) and cleaved amplified polymorphic sequence (CAPS) markers, CRW06_297 and CRU12_SspI, respectively. These markers will assist in future marker-assisted selection to accelerate the breeding of clubroot-resistant Brassica crops.

Use of CACTA mobile genetic elements for revision of phylogenetic relationships in Brassica rapa L. species

To supplement our previous studies of genetic diversity in Brassica rapa L. by SSR analysis, for the first time, we used Class II transposable elements to clarify the phylogenetic relationships. The core collection of 96 accessions stored at the Vavilov Institute of Plant Industry is used in the study. Experiments with 12 S-SAP primer pairs gave rise to 123 polymorphic markers. Using TE markers, we divide B. rapa crops into two major clusters: East Asian vegetables and Indo-European-Asian oilseed and turnip. The first cluster is divided into the subclusters of pak-choi and bok choi according to botanical classification and the results of SSR analysis. The earlier classification is improved by subclassification of pak-choi into groups, including identification of the separate group of headed cabbages, revision of the positions of the Hiroshimana and Mizuna forms, and subcategorization of oilseed by geographical origin. However, more accurate positions of accessions in the system of the species are obtained by construction of a SAHN dendrogram with 149 SSR and 123 S-SAP markers. The diagram shows that only a few accessions of European turnip are dispersed among the accessions of the other cluster but some accessions form a separate group in the Nepal-Indian subcluster. They are assumed to be the oldest forms closely related to the first domesticated Central-Asian B. rapa form.

Linkage Map Construction and Quantitative Trait Loci Analysis for Bolting Based on a Double Haploid Population of Brassica rapa

AbstractEarly bolting of Chinese cabbage (Brassica rapa L.) during spring cultivation often has detrimental effects on the yield and quality of the harvested products. Breeding late bolting varieties is a major objective of Chinese cabbage breeding programs. In order to analyze the genetic basis of bolting traits, a genetic map of B. rapa was constructed based on amplified fragment‐length polymorphism (AFLP), sequence‐related amplified polymorphism (SRAP), simple sequence repeat (SSR), random amplification of polymorphic DNA (RAPD), and isozyme markers. Marker analysis was carried out on 81 double haploid (DH) lines obtained by microspore culture from F1 progeny of two homozygous parents: B. rapa L. ssp. pekinensis (BY) (an extra‐early bolting Chinese cabbage line) and B. rapa L. ssp. rapifera (MM) (an extra‐late bolting European turnip line). A total of 326 markers including 130 AFLPs, 123 SRAPs, 16 SSRs, 43 RAPDs and 14 isozymes were used to construct a linkage map with 10 linkage groups covering 882 cM with an average distance of 2.71 cM between loci. The bolting trait of each DH line was evaluated by the bolting index under controlled conditions. Quantitative trait loci (QTL) analysis was conducted using multiple QTL model mapping with MapQTL5.0 software. Eight QTLs controlling bolting resistance were identified. These QTLs, accounting for 14.1% to 25.2% of the phenotypic variation with positive additive effects, were distributed into three linkage groups. These results provide useful information for molecular marker‐assisted selection of late bolting traits in Chinese cabbage breeding programs.(Handling editor: Yong‐Biao Xue)

Genetic Analysis of Clubroot Resistance in Brassica Crops

Clubroot disease is caused by an obligate parasite, Plasmodiophora brassicae, and is one of the most serious diseases of Brassica crops worldwide. The pathogen is a eukaryote, and is a member of Plasmodiophorales. Recent phylogenic studies did not reveal its close relationship to fungi, but suggest a classification in Protozoa or Protoctista. The infection is thought to occur through two phases. The pathogen induces massive clubs on root, and prevents host growth. A number of clubroot resistance (CR) Chinese cabbage (Brassica rapa) cultivars have been bred using European turnips as sources of CR genes. Field populations of the pathogen have a wide variation of virulence, which causes the infection of CR Chinese cabbage cultivars. The CR gene of B. rapa is extensively studied with the aid of molecular markers. At least four independent loci are identified and mapped. They are all derived from European turnips. Crr1, and Crr2 are derived from Siloga and mapped in linkage groups R8 and R6, respectively. Crr3 from Milan White, and CRb from Gelria R are both mapped in R3. The molecular markers linked to Crr1, Crr2 and CRb showed homology to the central part of the long arm of chromosome 4 of Arabidopsis thaliana. These findings suggest that these CR genes are derived from the same region of the ancestral genome. In contrast, Crr3 is thought to have different origin, since its linkage markers showed homology to chromosome 3 of A. thaliana. Although another CR gene CRa is reported, the relationship between above mentioned four CR loci and CRa is not yet clarified. Genetic studies on CR genes in B. oleracea revealed a polygenic nature of the trait in this species and also suggest the presence of one CR gene having large effect. However, correspondence among the CR genes reported in these studies is not clarified because of the lack of information on base sequence of the linkage markers in this species. Strategies for breeding of more resistant Chinese cabbage cultivars to overcome the variation on virulence of field populations of the pathogen are presented.