Fine mapping QMi-C11 a major QTL controlling root-knot nematodes resistance in Upland cotton

The identification and utilization of a high-level of host plant resistance is the most effective and economical approach to control root-knot nematode (Meloidogyne incognita). In an earlier study, we identified a major quantitative trait locus (QTL) for resistance to root-knot nematode in the M-120 RNR Upland cotton line (Gossypium hirsutum L.) of the Auburn 623 RNR source. The QTL is located in a 12.9-cM interval flanked by the two SSR markers CIR069 and CIR316 on the distal segment of chromosome 11. To construct a fine map around the target region, a bulked segregation analysis was performed using two DNA pools consisting of five individuals, with each being homozygous for the two parental alleles. From a survey of 1,152 AFLP primer combinations, 9 AFLP markers closely linked to the target region were identified. By screening an additional 1,221 F₂ individuals developed from the initial mapping population, the Mi-C11 locus was delimited to a 3.6-cM interval flanked by the SSR marker CIR069 and the AFLP marker E14M27-375. These results further elucidate the genetic fine structure of the Mi-C11 locus and provide the basis for map-based isolation of the nematode resistance gene in M-120 RNR.     

QTL mapping for resistance to root-knot nematodes in the M-120 RNR Upland cotton line (Gossypium hirsutum L.) of the Auburn 623 RNR source

Root-knot nematodes Meloidogyne incognita (Kofoid and White) can cause severe yield loss in cotton (Gossypium hirsutum L.). The objectives of this study were to determine the inheritance and genomic location of genes conferring root-knot nematode resistance in M-120 RNR, a highly resistant G. hirsutum line with the Auburn 623 RNR source of resistance. Utilizing two interspecific F₂ populations developed from the same M-120 RNR by Gossypium barbadense (cv. Pima S-6) cross, genome-wide scanning with RFLP markers revealed a marker on Chromosome 7 and two on Chromosome 11 showing significant association with the resistant phenotype. The association was confirmed using SSR markers with the detection of a minor and a major dominant QTL on Chromosome 7 and 11, respectively. Combined across the two populations, the major QTL on Chromosome 11 Mi-C11 had a LOD score of 19.21 (9.69 and 9.61 for Pop1 and Pop2, respectively) and accounted for 63.7% (52.6 and 65.56% for Pop1 and Pop2, respectively) of the total phenotypic variation. The minor QTL locus on Chromosome 7 Mi ₁ -C07 had a LOD score of 3.48 and accounted for 7.7% of the total phenotypic variation in the combined dataset but was detected in only one population. The allele from the M-120 RNR parent contributed to increased resistance in the Mi-C11 locus, but surprisingly, the Pima S-6 allele contributed to increased resistance in the Mi-C07 locus. The M-120 RNR allele in the Mi-C11 locus, derived from the Auburn 623 RNR, is likely to have originated from the Clevewilt 6 cultivar. Results from this study indicated that the SSR marker CIR316 may replace the laborious greenhouse screening in breeding programs to identify genotypes resistant to M. incognita.     

SSR markers closely associated with genes for resistance to root-knot nematode on chromosomes 11 and 14 of Upland cotton

Molecular markers closely linked to genes that confer a high level of resistance to root-knot nematode (RKN) [Meloidogyne incognita (Kofoid & White) Chitwood] in cotton (Gossypium hirsutum L.) germplasm derived from Auburn 623 RNR would greatly facilitate cotton breeding programs. Our objectives were to identify simple sequence repeat (SSR) markers linked to RKN resistance quantitative trait loci (QTL) and map these markers to specific chromosomes. We developed three recombinant inbred line (RIL) populations by single seed descent from the crosses of RKN-resistant parents M-240 RNR (M240), developed from the Auburn 623 RNR source, moderately resistant Clevewilt 6 (CLW6), one of the parents of Auburn 623 RNR, and susceptible parent Stoneville 213 (ST213). These crosses were CLW6 × ST213, M240 × CLW6, and M240 × ST213. RILs from these populations were grown under greenhouse conditions, inoculated with RKN eggs, scored for root gall index, eggs plant⁻¹, and eggs g⁻¹ root. Plants were also genotyped with SSR markers. Results indicated that a minimum of two major genes were involved in the RKN resistance of M240. One gene was localized to chromosome 11 and linked to the marker CIR 316-201. This CIR 316-201 allele was also present in CLW6 but not in Mexico Wild (MW) (PI593649), both of which are parents of Auburn 623 RNR. A second RKN resistance gene was localized to the short arm of chromosome 14 and was linked to the SSR markers BNL3545-118 and BNL3661-185. These two marker alleles were not present in CLW6 but were present in MW. Our data also suggest that the chromosome 11 resistance QTL primarily affects root galling while the QTL on chromosome 14 mediates reduced RKN egg production. The SSRs identified in this study should be useful to select plants with high levels of RKN resistance in segregating populations derived from Auburn 623 RNR.     

Parasitic Variability of Meloidogyne incognita Populations on Susceptible and Resistant Cotton

Root gall induction and egg production by the four recognized host races and two cytological races of Meloidogyne incognita were compared on cotton Gossypium hirsutum cvs. Deltapine 16 (root-knot susceptible) and Auburn 634 (highly resistant). The 12 nematode populations included in the study were from various parts of the world. No population increases occurred on the highly resistant cultivar. After 45 days, populations of host races 1 and 2 induced slight root galling on both cuhivars with only limited reproduction. Host race 4 populations induced moderate root galling with higher reproduction on Deltapine 16 than that of race 1 or race 2 populations. Host race 3 populations induced severe root galling with population density increases of 7-30-fold. In a complementary study, 24 cotton cultivars or breeding lines were compared for suitability as hosts for a typical population of M. incognita race 3. The poorest hosts, ''Aubnru 623,'' ''Auburn 634,'' and ''McNair 220,'' yielded fewer eggs after 45 days than were added initially. The best hosts - ''M-8.'' ''DES 24-8,'' ''McNair 235,'' and ''Coker 20l'' - yielded > 5 times as many eggs as were added initially.     

Resistance gene analogue markers are mapped to homeologous chromosomes in cultivated tetraploid cotton

Degenerate primers designed from conserved motifs of known plant resistance gene products were used to amplify genomic DNA sequences from the root-knot nematode (Meloidogyne incognita) resistance genetic source, Upland cotton (Gossypium hirsutum) cultivar Auburn 634 RNR. A total of 165 clones were isolated, and sequence analysis revealed 57 of the clones to be novel nucleotide sequences, many containing the resistance (R)-protein nucleotide-binding site motif. A cluster analysis was performed with resistance gene analogue (RGA) nucleotide sequences isolated in this study, in addition to 99 cotton RGA nucleotide sequences already deposited in GenBank, to generate a phylogenetic tree of cotton R genes. The cotton RGA nucleotide sequences were arranged into 11 groups and 56 sub-groups, based on genetic distances. Multiple sequence alignments were performed on the RGA sequences of each sub-group, and either the consensus sequences or individual RGA sequences were used to design 61 RGA-sequence-tagged site primers. A recombinant inbred line (RIL) population of cultivated tetraploid cotton was genotyped using RGA-specific primers that amplified polymorphic fragments between the two RIL parents. Nine RGA markers were mapped to homeologous chromosomes 12 and 26, based on linkage to existing markers that are located on these chromosomes.     

Association mapping of stem rust race TTKSK resistance in US barley breeding germplasm

Key message

Loci conferring resistance to the highly virulent African stem rust race TTKSK were identified in advanced barley breeding germplasm and positioned to chromosomes 5H and 7H using an association mapping approach.

Abstract

African races of the stem rust pathogen (Puccinia graminis f. sp. tritici) are a serious threat to barley production worldwide because of their wide virulence. To discover and characterize resistance to African stem rust race TTKSK in US barley breeding germplasm, over 3,000 lines/cultivars were assessed for resistance at the seedling stage in the greenhouse and also the adult plant stage in the field in Kenya. Only 12 (0.3 %) and 64 (2.1 %) lines exhibited a resistance level comparable to the resistant control at the seedling and adult plant stage, respectively. To map quantitative trait loci (QTL) for resistance to race TTKSK, an association mapping approach was conducted, utilizing 3,072 single nucleotide polymorphism (SNP) markers. At the seedling stage, two neighboring SNP markers (0.8 cM apart) on chromosome 7H (11_21491 and 12_30528) were found significantly associated with resistance. The most significant one found was 12_30528; thus, the resistance QTL was named Rpg-qtl-7H-12_30528. At the adult plant stage, two SNP markers on chromosome 5H (11_11355 and 12_31427) were found significantly associated with resistance. This resistance QTL was named Rpg-qtl-5H-11_11355 for the most significant marker identified. Adult plant resistance is of paramount importance for stem rust. The marker associated with Rpg-qtl-5H-11_11355 for adult plant resistance explained only a small portion of the phenotypic variation (0.02); however, this QTL reduced disease severity up to 55.0 % under low disease pressure and up to 21.1 % under heavy disease pressure. SNP marker 11_11355 will be valuable for marker-assisted selection of adult plant stem rust resistance in barley breeding.     

Comparative QTL analysis of root lesion nematode resistance in barley

Key message

This study demonstrates for the first time that resistance to different root lesion nematodes ( P. neglectus and P. penetrans ) is controlled by a common QTL. A major resistance QTL ( Rlnnp6H ) has been mapped to chromosome 6H using two independent barley populations.

Abstract

Root lesion nematodes (Pratylenchus spp.) are important pests in cereal production worldwide. We selected two doubled haploid populations of barley (Igri × Franka and Uschi × HHOR 3073) and infected them with Pratylenchus penetrans and Pratylenchus neglectus. Nematode multiplication rates were measured 7 or 10 weeks after infection. In both populations, continuous phenotypic variations for nematode multiplication rates were detected indicating a quantitative inheritance of resistance. In the Igri × Franka population, four P. penetrans resistance QTLs were mapped with 857 molecular markers on four linkage groups (2H, 5H, 6H and 7H). In the Uschi × HHOR 3073 population, eleven resistance QTLs (P. penetrans and P. neglectus) were mapped with 646 molecular markers on linkage groups 1H, 3H, 4H, 5H, 6H and 7H. A major resistance QTL named Rlnnp6H (LOD score 6.42–11.19) with a large phenotypic effect (27.5–36.6 %) for both pests was mapped in both populations to chromosome 6H. Another resistance QTL for both pests was mapped on linkage group 5H (Igri × Franka population). These data provide first evidence for common resistance mechanisms against different root lesion nematode species. The molecular markers are a powerful tool for the selection of resistant barley lines among segregating populations because resistance tests are time consuming and laborious.     

Inheritance of Resistance to Meloidoygne incognita in Primitive Cotton Accessions from Mexico

Few sources of resistance to root-knot nematodes (Meloidogyne incognita) in upland cotton (Gossypium hirsutum) have been utilized to develop resistant cultivars, making this resistance vulnerable to virulence in the pathogen population. The objectives of this study were to determine the inheritance of resistance in five primitive accessions of G. hirsutum (TX1174, TX1440, TX2076, TX2079, and TX2107) and to determine allelic relations with the genes for resistance in the genotypes Clevewilt-6 (CW) and Wild Mexico Jack Jones (WMJJ). A half-diallel experimental design was used to create 28 populations from crosses among these seven sources of resistance and the susceptible cultivar DeltaPine 90 (DP90). Resistance to M. incognita was measured as eggs per g roots in the parents, F(1) and F(2) generations of each cross. The resistance in CW and WMJJ was inherited as recessive traits, as reported previously for CW, whereas the resistance in the TX accessions was inherited as a dominant trait. Chi square analysis of segregation of resistance in the F(2) was used to estimate the numbers of genes that conditioned resistance. Resistance in CW and WMJJ appeared to be a multigenic trait whereas the resistance in the TX accessions best fit either a one or two gene model. The TX accessions were screened with nine SSR markers linked to resistance loci in other cotton genotypes. The TX accessions lacked the allele amplified by SSR marker CR316 and linked to resistance in CW and other resistant genotypes derived from this source. Four of five TX genotypes lacked the amplification products from the marker BNL1231 that is also associated with the resistant allele on Chromosome 11 in WMJJ, CW, NemX, M120 RNR and Auburn 634 RNR. However, all five TX genotypes produced the same amplification products from three SSR markers linked to the resistant allele on Chromosome 14 in M120 RNR and M240 RNR. The TX accessions have unique resistance genes that are likely to be useful in efforts to develop resistant cotton cultivars with increased durability.     

Identification and QTL mapping of whitefly resistance components in Solanum galapagense

Solanum galapagense is closely related to the cultivated tomato and can show a very good resistance towards whitefly. A segregating population resulting from a cross between the cultivated tomato and a whitefly resistant S. galapagense was created and used for mapping whitefly resistance and related traits, which made it possible to study the genetic basis of the resistance. Quantitative trait loci (QTL) for adult survival co-localized with type IV trichome characteristics (presence, density, gland longevity and gland size). A major QTL (Wf-1) was found for adult survival and trichome characters on Chromosome 2. This QTL explained 54.1 % of the variation in adult survival and 81.5 % of the occurrence of type IV trichomes. A minor QTL (Wf-2) for adult survival and trichome characters was identified on Chromosome 9. The major QTL was confirmed in F3 populations. Comprehensive metabolomics, based on GCMS profiling, revealed that 16 metabolites segregating in the F2 mapping population were associated with Wf-1 and/or Wf-2. Analysis of the 10 most resistant and susceptible F2 genotypes by LCMS showed that several acyl sugars were present in significantly higher concentration in the whitefly resistant genotypes, suggesting a role for these components in the resistance as well. Our results show that whitefly resistance in S. galapagense seems to inherit relatively simple compared to whitefly resistance from other sources and this offers great prospects for resistance breeding as well as elucidating the underlying molecular mechanism(s) of the resistance.     

Progression of Root-knot Nematode Symptoms and Infection on Resistant and Susceptible Cottons

Progressive development in cotton root morphology of resistant A623 and susceptible M-8 cotton (Gossypium hirsutum L.) lines following infection by the root-knot nematode Meloidogyne incognita was studied in glass front boxes. Symptom development and radicle growth were observed; degree of galling, gall and egg mass diameter, and number of eggs per egg mass were recorded; and root segments were examined histologically. Small cracks caused by M. incognita appeared in the root epidermis and cortex soon after the cotyledons expanded on day 4. The cracks were longer and wider and extended through the cortex when the first true leaf became visible at day 8. Galls had formed on taproots by this time. When exposed to M. incognita, A623 had faster radicle growth (22%), fewer and smaller cracks in the root epidermis and cortex, fewer and smaller root galls, one-twelfth as many egg masses, and one-fourth as many eggs per egg mass as M-8. Root cracking, galling, and giant cell formation are major effects of M. incognita that may predispose cotton roots to pathogens resulting in synergistic interactions and diseases.     

Mapping resistance to the bird cherry-oat aphid and the greenbug in wheat using sequence-based genotyping

Key message

Identification of novel resistance QTL against wheat aphids. First QTL-resistance report for R. padi in wheat and chromosome 2DL for S. graminum . These sources have potential use in wheat breeding.

Abstract

The aphids Rhopalosiphum padi and Schizaphis graminum are important pests of common wheat (Triticum aestivum L.). Characterization of the genetic bases of resistance sources is crucial to facilitate the development of resistant wheat cultivars to these insects. We examined 140 recombinant inbred lines (RILs) from the cross of Seri M82 wheat (susceptible) with the synthetic hexaploid wheat CWI76364 (resistant). RILs were phenotyped for R. padi antibiosis and tolerance traits. Phenotyping of S. graminum resistance was based on leaf chlorosis in a greenhouse screening and the number of S. graminum/tiller in the field. RILs were also scored for pubescence. Using a sequence-based genotyping method, we located genomic regions associated with these resistance traits. A quantitative trait locus (QTL) for R. padi antibiosis (QRp.slu.4BL) that explained 10.2 % of phenotypic variation was found in chromosome 4BL and located 14.6 cM apart from the pubescence locus. We found no association between plant pubescence and the resistance traits. We found two QTLs for R. padi tolerance (QRp.slu.5AL and QRp.slu.5BL) in chromosomes 5AL and 5BL, with an epistatic interaction between a locus in chromosome 3AL (EnQRp.slu.5AL) and QRp.slu.5AL. These genomic regions explained about 35 % of the phenotypic variation. We re-mapped a previously reported gene for S. graminum resistance (putatively Gba) in 7DL and found a novel QTL associated with the number of aphids/tiller (QGb.slu-2DL) in chromosome 2DL. This is the first report on the genetic mapping of R. padi resistance in wheat and the first report where chromosome 2DL is shown to be associated with S. graminum resistance.     

Mapping of quantitative adult plant field resistance to leaf rust and stripe rust in two European winter wheat populations reveals co-location of three QTL conferring resistance to both rust pathogens

Key message

We detected several, most likely novel QTL for adult plant resistance to rusts. Notably three QTL improved resistance to leaf rust and stripe rust simultaneously indicating broad spectrum resistance QTL.

Abstract

The rusts of wheat (Puccinia spp.) are destructive fungal wheat diseases. The deployment of resistant cultivars plays a central role in integrated rust disease management. Durability of resistance would be preferred, but is difficult to analyse. The Austrian winter wheat cultivar Capo was released in the 1989 and grown on a large acreage during more than two decades and maintained a good level of quantitative leaf rust and stripe rust resistance. Two bi-parental mapping populations: Capo × Arina and Capo × Furore were tested in multiple environments for severity of leaf rust and stripe rust at the adult plant stage in replicated field experiments. Quantitative trait loci associated with leaf rust and stripe rust severity were mapped using DArT and SSR markers. Five QTL were detected in multiple environments associated with resistance to leaf rust designated as QLr.ifa-2AL, QLr.ifa-2BL, QLr.ifa-2BS, QLr.ifa-3BS, and QLr.ifa-5BL, and five for resistance to stripe rust QYr.ifa-2AL, QYr.ifa-2BL, QYr.ifa-3AS, QYr.ifa-3BS, and QYr.ifa-5A. For all QTL apart from two (QYr.ifa-3AS, QLr.ifa-5BL) Capo contributed the resistance improving allele. The leaf rust and stripe rust resistance QTL on 2AL, 2BL and 3BS mapped to the same chromosome positions, indicating either closely linked genes or pleiotropic gene action. These three multiple disease resistance QTL (QLr.ifa-2AL/QYr.ifa-2AL, QLr.ifa.2BL/QYr.ifa-2BL, QLr.ifa-3BS/QYr.ifa.3BS) potentially contribute novel resistance sources for stripe rust and leaf rust. The long-lasting resistance of Capo apparently rests upon a combination of several genes. The described germplasm, QTL and markers are applicable for simultaneous resistance improvement against leaf rust and stripe rust.     

Modification of Resistance Expression of Phaseolus vulgaris to Meloidogyne incognita by Elevated Soil Temperatures

The effect of temperature on the reaction of susceptible (Canario Divex) and resistant (A 211) bean pure lines to Meloidogyne incognita was studied with soil temperature tanks housed in a growth chamber at 22 or 24 C. Soil temperature remained constant at 16, 22, 24, 26, 30, or 32 C in several trials. Bean line A 211 was resistant at 16 and 22 C but was susceptible at 24 C and above. Resistance to root-knot nematode reproduction was affected by a lower temperature (24 C) than was resistance to root galling (26 C) in A 211. Incubation of A 211 at 30 C for 3 and 16 days after inoculation with M. incognita resulted in a significant increase in nematode reproduction and root galling, respectively. The resistant reactions of A 211 to nematode reproduction and root galling were retained when inoculated plants were incubated at 21 C for a minimum of 16 and 23 days, respectively, prior to high temperature treatment.     

Introgression of homeologous quantitative trait loci (QTLs) for resistance to the root-knot nematode [Meloidogyne arenaria (Neal) Chitwood] in an advanced backcross-QTL population of peanut (Arachis hypogaea L.)

Resistance to root-knot nematodes [Meloidogyne arenaria (Neal) Chitwood] is needed for cultivation of peanut in major peanut-growing areas, but significant resistance is lacking in the cultivated species (Arachis hypogaea L.). Markers to two closely-linked genes introgressed from wild relatives of peanut have been identified previously, but phenotypic evidence for the presence of additional genes in wild species and introgression lines has eluded quantitative trait locus (QTL) identification. Here, to improve sensitivity to small-effect QTLs, an advanced backcross population from a cross between a Florunner component line and the synthetic amphidiploid TxAG-6 [Arachis batizocoi × (A. cardenasii × A. diogoi)]^4× was screened for response to root-knot nematode infection. Composite interval mapping results suggested a total of seven QTLs plus three putative QTLs. These included the known major resistance gene plus a second QTL on LG1, and a potentially homeologous B-genome QTL on LG11. Additional potential homeologs were identified on linkage group (LG) 8 and LG18, plus a QTL on LG9.2 and putative QTLs on LG9.1 and 19. A QTL on LG15 had no inferred resistance-associated homeolog. Contrary to expectation, two introgressed QTLs were associated with susceptibility, and QTLs at some homeologous loci were found to confer opposite phenotypic responses. Long-term functional conservation accompanied by rapid generation of functionally divergent alleles may be a singular feature of NBS-LRR resistance gene clusters, contributing to the richness of resistance alleles available in wild relatives of crops. The significance for peanut evolution and breeding is discussed.     

Fine mapping and identification of candidate genes controlling the resistance to southern root-knot nematode in PI 96354

Meloidogyne incognita (Kofoid and White) Chitwood (Mi) is the most economically damaging species of the root-knot nematode to soybean and other crops in the southern USA. PI 96354 was identified to carry a high level of resistance to galling and Mi egg production. Two Quantitative Trait Locus (QTLs) were found to condition the resistance in PI 96354 including a major QTL and a minor QTL on chromosome 10 and chromosome 18, respectively. To fine map the major QTL on chromosome 10, F_5:6 recombinant inbred lines from the cross between PI 96354 and susceptible genotype Bossier were genotyped with Simple Sequence Repeats (SSR) markers to identify recombinational events. Analysis of lines carrying key recombination events placed the Mi-resistant allele on chromosome 10 to a 235-kb region of the ‘Williams 82’ genome sequence with 30 annotated genes. Candidate gene analysis identified four genes with cell wall modification function that have several mutations in promoter, exon, 5′, and 3′UTR regions. qPCR analysis showed significant difference in expression levels of these four genes in Bossier compared to PI 96354 in the presence of Mi. Thirty Mi-resistant soybean lines were found to have same SNPs in these 4 candidate genes as PI 96354 while 12 Mi-susceptible lines possess the ‘Bossier’ genotype. The mutant SNPs were used to develop KASP assays to detect the resistant allele on chromosome 10. The four candidate genes identified in this study can be used in further studies to investigate the role of cell wall modification genes in conferring Mi resistance in PI 96354.     

QTL analysis for transgressive resistance to root-knot nematode in interspecific cotton (Gossypium spp.) progeny derived from susceptible parents

The southern root-knot nematode (RKN, Meloidogyne incognita) is a major soil-inhabiting plant parasite that causes significant yield losses in cotton (Gossypium spp.). Progeny from crosses between cotton genotypes susceptible to RKN produced segregants in subsequent populations which were highly resistant to this parasite. A recombinant inbred line (RIL) population of 138 lines developed from a cross between Upland cotton TM-1 (G. hirsutum L.) and Pima 3-79 (G. barbadense L.), both susceptible to RKN, was used to identify quantitative trait loci (QTLs) determining responses to RKN in greenhouse infection assays with simple sequence repeat (SSR) markers. Compared to both parents, 53.6% and 52.1% of RILs showed less (P<0.05) root-galling index (GI) and had lower (P<0.05) nematode egg production (eggs per gram root, EGR). Highly resistant lines (transgressive segregants) were identified in this RIL population for GI and/or EGR in two greenhouse experiments. QTLs were identified using the single-marker analysis nonparametric mapping Kruskal-Wallis test. Four major QTLs located on chromosomes 3, 4, 11, and 17 were identified to account for 8.0 to 12.3% of the phenotypic variance (R(2)) in root-galling. Two major QTLs accounting for 9.7% and 10.6% of EGR variance were identified on chromosomes 14 and 23 (P<0.005), respectively. In addition, 19 putative QTLs (P<0.05) accounted for 4.5-7.7% of phenotypic variance (R(2)) in GI, and 15 QTLs accounted for 4.2-7.3% of phenotypic variance in EGR. In lines with alleles positive for resistance contributed by both parents in combinations of two to four QTLs, dramatic reductions of >50% in both GI and EGR were observed. The transgressive segregants with epistatic effects derived from susceptible parents indicate that high levels of nematode resistance in cotton may be attained by pyramiding positive alleles using a QTL mapping approach.     

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.     

Response of Resistant and Susceptible Bell Pepper (Capsicum annuum) to a Southern California Meloidogyne incognita Population from a Commercial Bell Pepper Field

To determine the presence and level of root-knot nematode (Meloidogyne spp.) infestation in Southern California bell pepper (Capsicum annuum) fields, soil and root samples were collected in April and May 2012 and analyzed for the presence of root-knot nematodes. The earlier samples were virtually free of root-knot nematodes, but the later samples all contained, sometimes very high numbers, of root-knot nematodes. Nematodes were all identified as M. incognita. A nematode population from one of these fields was multiplied in a greenhouse and used as inoculum for two repeated pot experiments with three susceptible and two resistant bell pepper varieties. Fruit yields of the resistant peppers were not affected by the nematodes, whereas yields of two of the three susceptible pepper cultivars decreased as a result of nematode inoculation. Nematode-induced root galling and nematode multiplication was low but different between the two resistant cultivars. Root galling and nematode reproduction was much higher on the three susceptible cultivars. One of these susceptible cultivars exhibited tolerance, as yields were not affected by the nematodes, but nematode multiplication was high. It is concluded that M. incognita is common in Southern California bell pepper production, and that resistant cultivars may provide a useful tool in a nonchemical management strategy.     

Phenotypic Expression of rkn1-Mediated Meloidogyne incognita Resistance in Gossypium hirsutum Populations

The root-knot nematode Meloidogyne incognita is a damaging pest of cotton (Gossypium hirsutum) worldwide. A major gene (rkn1) conferring resistance to M. incognita was previously identified on linkage group A03 in G. hirsutum cv. Acala NemX. To determine the patterns of segregation and phenotypic expression of rkn1, F₁, F₂, F_2:3, BC₁F₁ and F_2:7 recombinant inbred lines (RIL) from intraspecific crosses between Acala NemX and a closely related susceptible cultivar Acala SJ-2 were inoculated in greenhouse tests with M. incognita race 3. The resistance phenotype was determined by the extent of nematode-induced root galling and nematode egg production on roots. Suppression of root galling and egg production was highly correlated among individuals in all tests. Root galling and egg production on heterozygous plants did not differ from the susceptible parent phenotype 125 d or more after inoculation, but were slightly suppressed with shorter screening (60 d), indicating that rkn1 behaved as a recessive gene or an incompletely recessive gene, depending on the screening condition. In the RIL, rkn1 segregated in an expected 1 resistant: 1 susceptible ratio for a major resistance gene. However, within the resistant class, 21 out of 34 RIL were more resistant than the resistant parent Acala NemX, indicating transgressive segregation. These results suggest that rkn1-based resistance in G. hirsutum can be enhanced in progenies of crosses with susceptible genotypes. Allelism tests and molecular genetic analysis are needed to determine the relationship of rkn1 to other M. incognita resistance sources in cotton.