Genetic analysis of loci associated with partial resistance to <Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis> in rapeseed (<Emphasis Type="Italic">Brassica napus</Emphasis> L.)

Sclerotinia stem rot is the most devastating disease of rapeseed (Brassica napus L.) in China. Quantitative trait loci (QTLs) involved in resistance to Sclerotinia sclerotiorum were detected in a rapeseed population of 128-F(2:3) families derived from a cross between the male sterility restorer line H5200 and a partial resistant line Ning RS-1. A total of 107 molecular markers including 72 RFLPs, 30 AFLPs, 3 SSRs and 2 RAPDs were employed to construct a genetic linkage map with 23 linkage groups covering 1,625.7 cM with an average space of 15.2 cM. Resistance was assessed empirically at two developmental stages: with a detached leaf inoculation at the seedling stage and in vivo stem inoculation at the mature plant stage. The observed resistance was scored for each plant as leaf resistance at the seedling stage (LRS) and stem resistance at the mature plant stage (SRM). A total of 13 loci were identified by one-way ANOVA and six QTLs were detected with MapMaker-QTL. We found that three of the six QTLs were associated with leaf resistance at the seedling stage and collectively accounted for 40.7% of the total phenotypic variation, each accounting for 23.2%, 16.6% and 13.6% respectively. Three QTLs were found corresponding to the disease resistance at the mature plant stage, explaining 49.0% of the phenotypic variation. Epistasis was observed for the resistance and the additive by additive interactions were the predominant type of epistasis. It was concluded that both single-locus QTLs and epistatic interactions played important roles in Sclerotinia resistance in rapeseed.     

Cytogenetic analysis of the susceptibility of the wheat line Hobbit sib (Dwarf A) to <Emphasis Type="Italic">Septoria tritici</Emphasis> blotch

Septoria tritici blotch, caused by Mycosphaerella graminicola (anamorph Septoria tritici), is one of the most important foliar diseases of wheat in much of the world. Susceptibility of host plants to septoria was investigated by cytogenetic analysis. A line of Hobbit sib (Dwarf A) in which translocated chromosome 5BS–7BS was nominally substituted by chromosome arms 5BS and 7BS from Bezostaya 1 had a much lower mean level of septoria than Hobbit sib itself. By the use of microsatellite markers, it was shown that the 5BS arm of this line had in fact been substituted by the homologous arm of Chinese Spring. Further investigation of substitution and nullitetrasomic lines demonstrated that chromosome arm 5BS of Hobbit sib possesses genes, which either promote susceptibility to septoria or suppress resistance. This chromosome arm has previously been shown to carry genes for resistance to yellow (stripe) rust and powdery mildew, implying a trade-off between resistances to these two diseases and to septoria in wheat breeding. Bezostaya 1 was found to have specific resistance to M. graminicola isolate IPO323, probably controlled by the gene Stb6 on chromosome arm 3AS, present in numerous wheat cultivars. It also had partial resistance to septoria distributed over several chromosomes, which may explain the value of this cultivar as a source of septoria resistance.     

Chromosomal location of a race-specific resistance gene to <Emphasis Type="Italic">Mycosphaerella graminicola</Emphasis> in the spring wheat ST6

Septoria tritici blotch, caused by Mycosphaerella graminicola, is a serious foliar disease of wheat worldwide. Qualitative, race-specific resistance sources have been identified and utilized for resistant cultivar development. However, septoria tritici blotch resistant varieties have succumbed to changes in virulence of M. graminicola on at least three continents. The use of resistance gene pyramids may slow or prevent the breakdown of resistance. A clear understanding of the genetics of resistance and the identification of linked PCR-based markers will facilitate the recovery of wheat lines carrying multiple septoria tritici blotch resistance genes. The resistance gene in ST6 to isolate MG2 of M. graminicola was mapped with microsatellite markers in two populations, ST6/Erik and ST6/Katepwa. Bulk segregant analysis identified a marker on chromosome 4AL putatively linked to the resistance gene. A large linkage group was identified in each population using additional microsatellite markers mapping to chromosome 4AL. The resistance gene in ST6 mapped to the distal end of chromosome 4AL in each mapping population and was designated Stb7. Three of the microsatellite loci, Xwmc313, Xwmc219 and Xgwm160, mapped within 3.5 cM of Stb7; however, none flanked Stb7. Xwmc313 was the closest and mapped 0.3 and 0.5 cM from Stb7 in the crosses ST6/Katepwa and ST6/Erik, respectively. WMC313 will be very useful for marker-assisted selection of Stb7 in Canadian breeding programs because the ST6 allele of Xwmc313 was not identified in any of the Canadian common wheat cultivars tested.Communicated by P. Langridge     

QTL mapping for some grain traits in bread wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

Grain traits are important agronomic attributes with the market value as well as milling yield of bread wheat. In the present study, quantitative trait loci (QTL) regulating grain traits in wheat were identified. Data for grain area size (GAS), grain width (GWid), factor form density (FFD), grain length-width ratio (GLWR), thousand grain weight (TGW), grain perimeter length (GPL) and grain length (GL) were recorded on a recombinant inbred line derived from the cross of NW1014?×?HUW468 at Meerut and Varanasi locations. A linkage map of 55 simple sequence repeat markers for 8 wheat chromosomes was used for QTL analysis by Composite interval mapping. Eighteen QTLs distributed on 8 chromosomes were identified for seven grain traits. Of these, five QTLs for GLWR were found on chromosomes 1A, 6A, 2B, and 7B, three QTLs for GPL were located on chromosomes 4A, 5A and 7B and three QTLs for GAS were mapped on 5D and 7D. Two QTLs were identified on chromosomes 4A and 5A for GL and two QTLs for GWid were identified on chromosomes 7D and 6A. Similarly, two QTLs for FFD were found on chromosomes 1A and 5D. A solitary QTL for TGW was identified on chromosome 2B. For several traits, QTLs were also co-localized on chromosomes 2B, 4A, 5A, 6A, 5D, 7B and 7D. The QTLs detected in the present study may be validated for specific crosses and then used for marker-assisted selection to improve grain quality in bread wheat.     

QTL mapping of flag leaf-related traits in wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

Key message

QTL controlling flag leaf length, flag leaf width, flag leaf area and flag leaf angle were mapped in wheat.

Abstract

This study aimed to advance our understanding of the genetic mechanisms underlying morphological traits of the flag leaves of wheat (Triticum aestivum L.). A recombinant inbred line (RIL) population derived from ND3331 and the Tibetan semi-wild wheat Zang1817 was used to identify quantitative trait loci (QTLs) controlling flag leaf length (FLL), flag leaf width (FLW), flag leaf area (FLA), and flag leaf angle (FLANG). Using an available simple sequence repeat genetic linkage map, 23 putative QTLs for FLL, FLW, FLA, and FLANG were detected on chromosomes 1B, 2B, 3A, 3D, 4B, 5A, 6B, 7B, and 7D. Individual QTL explained 4.3–68.52% of the phenotypic variance in different environments. Four QTLs for FLL, two for FLW, four for FLA, and five for FLANG were detected in at least two environments. Positive alleles of 17 QTLs for flag leaf-related traits originated from ND3331 and 6 originated from Zang1817. QTLs with pleiotropic effects or multiple linked QTL were also identified on chromosomes 1B, 4B, and 5A; these are potential target regions for fine-mapping and marker-assisted selection in wheat breeding programs.

     

Exploring the quantitative resistance to <Emphasis Type="Italic">Pseudomonas syringae</Emphasis> pv. <Emphasis Type="Italic">phaseolicola</Emphasis> in common bean (<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> L.)

Pseudomonas syringae pv. phaseolicola is an important disease that causes halo blight in common bean. The genetic mechanisms underlying quantitative halo blight resistance are poorly understood in this species, as most disease studies have focused on qualitative resistance. The present work examines the genetic basis of quantitative resistance to the nine halo blight races in different organs (primary and trifoliate leaf, stem and pod) of an Andean recombinant inbred line (RIL) progeny. Using a multi-environment quantitative trait locus (QTL) mapping approach, 76 and 101 main-effect and epistatic QTLs were identified, respectively. Most of the epistatic interactions detected were due to loci without detectable QTL additive main effects. Main and epistatic QTLs detected were mainly consistent across the environment conditions. The homologous genomic regions corresponding to 26 of the 76 main-effect detected QTLs were positive for the presence of resistance-associated gene cluster encoding nucleotide-binding and leucine-rich repeat (NL) proteins and known defence genes. Main-effect QTLs for resistance to races 3, 4 and 5 in leaf, stem and pod were located on chromosome 2 within a 3.01-Mb region, where a cluster of nine NL genes was detected. The NL gene Phvul.002G323300 is located in this region, which can be considered an important putative candidate gene for the non-organ-specific QTL identified here. The present research provides essential information not only for the better understanding of the plant-pathogen interaction but also for the application of genomic assisted breeding for halo blight resistance in common bean.     

Mapping of quantitative trait loci for resistance to <Emphasis Type="Italic">Mycosphaerella pinodes</Emphasis> in <Emphasis Type="Italic">Pisum sativum</Emphasis> subsp. <Emphasis Type="Italic">syriacum</Emphasis>

Aschochyta blight, caused by Mycosphaerella pinodes, is one of the most economically serious pea pathogens, particularly in winter sowings. The wild Pisum sativum subsp. syriacum accession P665 shows good levels of resistance to this pathogen. Knowledge of the genetic factors controlling resistance to M. pinodes in this wild accession would facilitate gene transfer to pea cultivars; however, previous studies mapping resistance to M. pinodes in pea have never included this wild species. The objective of this study was to identify quantitative trait loci (QTL) controlling resistance to M. pinodes in P. sativum subsp. syriacum and to compare these with QTLs previously described for the same trait in P. sativum. A population formed by 111 F_6:7 recombinant inbred lines derived from a cross between accession P665 and a susceptible pea cultivar (Messire) was analysed using morphological, isozyme, RAPD, STS and EST markers. The map developed covered 1214 cM and contained 246 markers distributed in nine linkage groups, of which seven could be assigned to pea chromosomes. Six QTLs associated with resistance to M. pinodes were detected in linkage groups II, III, IV and V, which collectively explained between 31 and 75% of the phenotypic variation depending of the trait. While QTLs MpIII.1 and MpIII.2 were detected both for seedlings and field resistance, MpV.1 and MpII.1 were specific for growth chamber conditions and MpIII.3 and MpIV.1 for field resistance. Quantitative trait loci MpIII.1, MpII.1, MpIII.2 and MpIII.3 may coincide with other QTLs associated with resistance to M. pinodes previously described in P. sativum. Four QTLs associated with earliness of flowering were also identified. While dfIII.2 and dfVI.1, may correspond with other genes and QTLs controlling earliness in P. sativum, dfIII.1 and dfII.1 may be specific to P. sativum subsp. syriacum. Flowering date and growth habit were strongly associated with resistance to M. pinodes in the field evaluations. The relation observed between earliness, growth habit and resistance to M. pinodes is discussed.     

Avirulence in the wheat septoria tritici leaf blotch fungus Mycosphaerella graminicola is controlled by a single locus

Segregation of avirulence in Mycosphaerella graminicola, a heterothallic ascomycete that causes wheat septoria tritici leaf blotch, was studied in F1, BC1, and F2 populations by inoculation assays on five wheat cultivars in the seedling stage and by amplified fragment length polymorphism and random amplified polymorphic DNA analyses. F1 was generated by crossing isolates IPO323 (avirulent) and IPO94269 (virulent). All F1, BC1, and F2 progeny isolates were virulent on the susceptible check cultivar Taichung 29 and were avirulent on the resistant check cultivar Kavkav-K4500. Avirulence segregation was observed in F1 and in several BC1 and F2 generations on the differential cultivars Shafir, Kavkaz, and Veranopolis at a 1:1 ratio. Avirulence for the three differential cultivars always cosegregated. We conclude that avirulence in isolate IPO323 is controlled by a single, seemingly complex locus.     

Identification and validation of genomic regions that affect shoot fly resistance in sorghum [<Emphasis Type="Italic">Sorghum bicolor</Emphasis> (L.) Moench]

Shoot fly is one of the most important pests affecting the sorghum production. The identification of quantitative trait loci (QTL) affecting shoot fly resistance enables to understand the underlying genetic mechanisms and genetic basis of complex interactions among the component traits. The aim of the present study was to detect QTL for shoot fly resistance and the associated traits using a population of 210 RILs of the cross 27B (susceptible) × IS2122 (resistant). RIL population was phenotyped in eight environments for shoot fly resistance (deadheart percentage), and in three environments for the component traits, such as glossiness, seedling vigor and trichome density. Linkage map was constructed with 149 marker loci comprising 127 genomic-microsatellite, 21 genic-microsatellite and one morphological marker. QTL analysis was performed by using MQM approach. 25 QTL (five each for leaf glossiness and seedling vigor, 10 for deadhearts, two for adaxial trichome density and three for abaxial trichome density) were detected in individual and across environments. The LOD and R ² (%) values of QTL ranged from 2.44 to 24.1 and 4.3 to 44.1%, respectively. For most of the QTLs, the resistant parent, IS2122 contributed alleles for resistance; while at two QTL regions, the susceptible parent 27B also contributed for resistance traits. Three genomic regions affected multiple traits, suggesting the phenomenon of pleiotrophy or tight linkage. Stable QTL were identified for the traits across different environments, and genetic backgrounds by comparing the QTL in the study with previously reported QTL in sorghum. For majority of the QTLs, possible candidate genes were identified. The QTLs identified will enable marker assisted breeding for shoot fly resistance in sorghum.     

Identification of quantitative trait loci for resistance to <Emphasis Type="Italic">Xanthomonas campestris</Emphasis> pv. <Emphasis Type="Italic">campestris</Emphasis> in <Emphasis Type="Italic">Brassica rapa</Emphasis>

Resistance to six known races of black rot in crucifers caused by Xanthomonas campestris pv. campestris (Pammel) Dowson is absent or very rare in Brassica oleracea (C genome). However, race specific and broad-spectrum resistance (to type strains of all six races) does appear to occur frequently in other brassica genomes including B. rapa (A genome). Here, we report the genetics of broad spectrum resistance in the B. rapa Chinese cabbage accession B162, using QTL analysis of resistance to races 1 and 4 of the pathogen. A B. rapa linkage map comprising ten linkage groups (A01–A10) with a total map distance of 664 cM was produced, based on 223 AFLP bands and 23 microsatellites from a F₂ population of 114 plants derived from a cross between the B. rapa susceptible inbred line R-o-18 and B162. Interaction phenotypes of 125 F₂ plants were assessed using two criteria: the percentage of inoculation sites in which symptoms developed, and the severity of symptoms per plant. Resistance to both races was correlated and a cluster of highly significant QTL that explained 24–64% of the phenotypic variance was located on A06. Two additional QTLs for resistance to race 4 were found on A02 and A09. Markers closely linked to these QTL could assist in the transference of the resistance into different B. rapa cultivars or into B. oleracea.     

Mapping of quantitative trait loci for partial resistance to<Emphasis Type="Italic"> Mycosphaerella pinodes</Emphasis> in pea (<Emphasis Type="Italic">Pisum sativum</Emphasis> L.), at the seedling and adult plant stages

The inheritance of resistance to Ascochyta blight, an economically important foliar disease of field pea (Pisum sativum L.) worldwide, was investigated. Breeding resistant pea varieties to this disease, caused by Mycosphaerella pinodes, is difficult due to the availability of only partial resistance. We mapped and characterized quantitative trait loci (QTLs) for resistance to M. pinodes in pea. A population of 135 recombinant inbred lines (RILs), derived from the cross between DP (partially resistant) and JI296 (susceptible), was genotyped with morphological, RAPD, SSR and STS markers. A genetic map was elaborated, comprising 206 markers distributed over eight linkage groups and covering 1,061 cM. The RILs were assessed under growth chamber and field conditions at the seedling and adult plant stages, respectively. Six QTLs were detected at the seedling stage, which together explained up to 74% of the variance. Ten QTLs were identified at the adult plant stage in the field, and together these explained 56.6–67.1% of the variance, depending on the resistance criteria and the organ considered. Four QTLs were detected under both growth chamber and field conditions, suggesting they were not plant-stage dependent. Three QTLs for flowering date and three QTLs for plant height were also identified in the RIL population, some of which co-located with QTLs for resistance. The relationship between QTLs for resistance to M. pinodes, plant height and flowering date is discussed.Communicated by H.C. Becker     

Genome-wide association mapping of spot blotch resistance to three different pathotypes of <Emphasis Type="Italic">Cochliobolus sativus</Emphasis> in the USDA barley core collection

Spot blotch, caused by Cochliobolus sativus, is an economically important disease of barley. To identify genetic loci conferring resistance to three different pathotypes of C. sativus, a worldwide barley core collection (BCC) consisting of 1480 accessions from the USDA National Small Grains Collection were genotyped with the barley 9k Illumina Infinium iSELECT assay and phenotyped at the seedling stage with three C. sativus isolates ND85F (pathotype 1), ND90Pr (pathotype 2), and ND4008 (pathotype 7). Association mapping analysis was performed with the Whole_Panel containing 1480 barley accessions, as well as Two-rowed_Panel and Six-rowed_Panel consisting of 621 two-rowed and 857 six-rowed barley accessions, respectively. For resistance to isolate ND4008, one quantitative trait locus (QTL, QRcs-6H-P7) was detected in all three panels. Three other QTL (QRcs-1H-P7, QRcs-2H-P7, and QRcs-3H-P7) were detected in Whole_Panel, Six-rowed_Panel, and Two-rowed_Panel, respectively. For resistance to isolate ND90Pr, one QTL (QRcs-1H-P2) was identified in the Whole_Panel and the Two-rowed_Panel, and the other QTL (QRcs-6H-P2) was only identified in the Six-rowed_Panel. For resistance to isolate ND85F, three QTL (QRcs-1H-P1, QRcs-3H-P1, QRcs-7H-2-P1) were detected in all three panels, and one QTL (QRcs-7H-1-P1) was only detected in the Two-rowed_Panel. Among the ten QTL detected, four (QRcs-1H-P1, QRcs-3H-P1, QRcs-7H-2-P1, and QRcs-1H-P2) were mapped to chromosome regions containing previously identified QTL for spot blotch resistance, while six (QRcs-1H-P7, QRcs-2H-P7, QRcs-3H-P7, QRcs-6H-P7, QRcs-6H-P2, and QRcs-7H-1-P1) were novel. The SNP markers associated with the QTL identified in this study will be useful for breeding barley cultivars with resistance to multiple pathotypes of C. sativus.     

The genetic basis of flowering time and photoperiod sensitivity in rapeseed <Emphasis Type="Italic">Brassica napus</Emphasis> L.

The objective of this study was to dissect the genetic control of days to flowering (DTF) and photoperiod sensitivity (PS) into the various components including the main-effect quantitative trait loci (QTLs), epistatic QTLs and QTL-by-environment interactions (QEs). Doubled haploid (DH) lines were produced from an F₁ between two spring Brassica napus cultivars Hyola 401 and Q2. DTF of the DH lines and parents were investigated in two locations, one location with a short and the other with a long photoperiod regime over two years. PS was calculated by the delay in DTF under long day as compared to that under short day. A genetic linkage map was constructed that comprised 248 marker loci including SSR, SRAP, and AFLP markers. Further QTL analysis resolved the genetic components of flowering time and PS into the main-effect QTLs, epistatic QTLs, and QEs. A total of 7 main-effect QTLs and 11 digenic interactions involving 21 loci located on 13 out of the 19 linkage groups were detected for the two traits. Three main-effect QTLs and four pairs of epistatic QTLs were involved in QEs conferring DTF. One QTL on linkage group (LG) 18 was revealed to simultaneously affect DTF and PS and explain for the highest percentage of the phenotypic variation. The implications of the results for B. napus breeding have been discussed. The text was submitted by the authors in English.     

Mapping of yield-related QTLs in pepper in an interspecific cross of <Emphasis Type="Italic">Capsicum annuum</Emphasis> and <Emphasis Type="Italic">C. frutescens</Emphasis>

An advanced backcross QTL study was performed in pepper using a cross between the cultivated species Capsicum annuum cv. Maor and the wild C. frutescens BG 2816 accession. A genetic map from this cross was constructed, based on 248 BC(2) plants and 92 restriction fragment length polymorphism (RFLP) markers distributed throughout the genome. Ten yield-related traits were analyzed in the BC(2) and BC(2)S(1) generations, and a total of 58 quantitative trait loci (QTLs) were detected; the number of QTLs per trait ranged from two to ten. Most of the QTLs were found in 11 clusters, in which similar QTL positions were identified for multiple traits. Unlike the high percentage of favorable QTL alleles discovered in wild species of tomato and rice, only a few such QTL alleles were detected in BG 2816. For six QTLs (10%), alleles with effects opposite to those expected from the phenotype were detected in the wild species. The use of common RFLP markers in the pepper and tomato maps enabled possible orthologous QTLs in the two species to be determined. The degree of putative QTL orthology for the two main fruit morphology traits-weight and shape-varied considerably. While all eight QTLs identified for fruit weight in this study could be orthologous to tomato fruit weight QTLs, only one out of six fruit shape QTLs found in this study could be orthologous to tomato fruit shape QTLs.     

QTL analysis of resistance to Fusarium head blight in Swiss winter wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

Fusarium head blight (FHB) of wheat is a widespread and destructive disease which occurs in humid and semi-humid areas. FHB epidemics can cause serious yield and quality losses under favorable climatic conditions, but the major concern is the contamination of grains with mycotoxins. Resistance to FHB is quantitatively inherited and greatly influenced by the environment. Its evaluation is costly and time-consuming. The genetic basis of FHB resistance has mainly been studied in spring wheat. The objective of this study was to map quantitative trait loci (QTLs) for resistance to FHB in a population of 240 recombinant inbred lines (RILs) derived from a cross between the two Swiss winter wheat cultivars Arina (resistant) and Forno (susceptible). The RILs were genotyped with microsatellite and RFLP markers. The resulting genetic map comprises 380 loci and spans 3,086 cM. The 240 RILs were evaluated for resistance to FHB in six field trials over 3 years. Composite interval mapping (CIM) analyses carried out on FHB AUDPC (i.e. mean values across six environments) revealed eight QTLs which altogether explained 47% of the phenotypic variance. The three main QTLs were mapped on the long arms of chromosomes 6D (R²=22%), 5B (R²=14%) and 4A (R²=10%). The QTL detected on 5B originated from the susceptible parent Forno. Other QTLs with smaller effects on FHB resistance were detected on chromosomes 2AL, 3AL, 3BL, 3DS and 5AL.Communicated by H.C. Becker     

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.     

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.     

Genome-wide identification of QTL conferring high-temperature adult-plant (HTAP) resistance to stripe rust (<Emphasis Type="Italic">Puccinia striiformis</Emphasis> f. sp<Emphasis Type="Italic">. tritici</Emphasis>) in wheat

High-temperature adult-plant (HTAP) resistance to stripe rust (caused by Puccinia striiformis f. sp. tritici) is a durable type of resistance in wheat (Triticum aestivum L.). This study identified quantitative trait loci (QTL) conferring HTAP resistance to stripe rust in a population consisting of 169 F_8:10 recombinant inbred lines (RILs) derived from a cross between a susceptible cultivar Rio Blanco and a resistant germplasm IDO444. HTAP resistance was evaluated for both disease severity and infection type under natural infection over two years at two locations. The genetic linkage maps had an average density of 6.7 cM per marker across the genome and were constructed using 484 markers including 96 wheat microsatellite (SSR), 632 Diversity Arrays Technology (DArT) polymorphisms, two sequence-tagged-site (STS) from semi-dwarf genes Rht1 and Rht2, and two markers for low molecular-weight glutenin gene subunits. QTL analysis detected a total of eight QTL significantly associated with HTAP resistance to stripe rust with two on chromosome 2B, two on 3B and one on each of 1A, 4A, 4B and 5B. QTL on chromosomes 2B and 4A were the major loci derived from IDO444 and explained up to 47 and 42% of the phenotypic variation for disease severity and infection type, respectively. The remaining five QTL accounted for 7–10% of the trait variation. Of these minor QTL, the resistant alleles at the two QTL QYrrb.ui-3B.1 and QYrrb.ui-4B derived from Rio Blanco and reduced infection type only, while the resistant alleles at the other three QTL, QYrid.ui-1A, QYrid.ui-3B.2 and QYrid.ui-5B, all derived from IDO444 and reduced either infection type or disease severity. Markers linked to 2B and 4A QTL should be useful for selection of HTAP resistance to stripe rust.     

Identification of candidate genes of QTLs for seed weight in <Emphasis Type="Italic">Brassica napus</Emphasis> through comparative mapping among <Emphasis Type="Italic">Arabidopsis</Emphasis> and <Emphasis Type="Italic">Brassica</Emphasis>species

Background

Map-based cloning of quantitative trait loci (QTLs) in polyploidy crop species remains a challenge due to the complexity of their genome structures. QTLs for seed weight in B. napus have been identified, but information on candidate genes for identified QTLs of this important trait is still rare.

Results

In this study, a whole genome genetic linkage map for B. napus was constructed using simple sequence repeat (SSR) markers that covered a genetic distance of 2,126.4 cM with an average distance of 5.36 cM between markers. A procedure was developed to establish colinearity of SSR loci on B. napus with its two progenitor diploid species B. rapa and B. oleracea through extensive bioinformatics analysis. With the aid of B. rapa and B. oleracea genome sequences, the 421 homologous colinear loci deduced from the SSR loci of B. napus were shown to correspond to 398 homologous loci in Arabidopsis thaliana. Through comparative mapping of Arabidopsis and the three Brassica species, 227 homologous genes for seed size/weight were mapped on the B. napus genetic map, establishing the genetic bases for the important agronomic trait in this amphidiploid species. Furthermore, 12 candidate genes underlying 8 QTLs for seed weight were identified, and a gene-specific marker for BnAP2 was developed through molecular cloning using the seed weight/size gene distribution map in B. napus.

Conclusions

Our study showed that it is feasible to identify candidate genes of QTLs using a SSR-based B. napus genetic map through comparative mapping among Arabidopsis and B. napus and its two progenitor species B. rapa and B. oleracea. Identification of candidate genes for seed weight in amphidiploid B. napus will accelerate the process of isolating the mapped QTLs for this important trait, and this approach may be useful for QTL identification of other traits of agronomic significance.