Synthetic Hexaploid Wheat Lines Research Articles

Applicability of Aegilops tauschii drought tolerance traits to breeding of hexaploid wheat

Few genes are available to develop drought-tolerant bread wheat (Triticum aestivum L.) cultivars. One way to enhance bread wheat’s genetic diversity would be to take advantage of the diversity of wild species by creating synthetic hexaploid wheat (SW) with the genomic constitution of bread wheat. In this study, we compared the expression of traits encoded at different ploidy levels and evaluated the applicability of Aegilops tauschii drought-related traits using 33 Ae. tauschii accessions along with their corresponding SW lines under well-watered and drought conditions. We found wide variation in Ae. tauschii, and even wider variation in the SW lines. Some SW lines were more drought-tolerant than the standard cultivar Cham 6. Aegilops tauschii from some regions gave better performing SW lines. The traits of Ae. tauschii were not significantly correlated with their corresponding SW lines, indicating that the traits expressed in wild diploid relatives of wheat may not predict the traits that will be expressed in SW lines derived from them. We suggest that, regardless of the adaptability and performance of the Ae. tauschii under drought, production of SW could probably result in genotypes with enhanced trait expression due to gene interactions, and that the traits of the synthetic should be evaluated in hexaploid level.

Distinct genetic regulation of flowering time and grain-filling period based on empirical study of D-genome diversity in synthetic hexaploid wheat lines

Aegilops tauschii Coss., the D-genome progenitor of hexaploid wheat, has a wide species range in central Eurasia. Large natural variation in Ae. tauschii offers potential for improving modern varieties of common wheat. Maturation time, an important agronomic trait for wheat breeding, is tied to flowering time and grain-filling period. Here, we established 82 wheat synthetic lines derived through endoreduplication, forming triploid gametes, in interspecific hybrids obtained by crossing a tetraploid wheat cultivar, Langdon, with 69 Ae. tauschii accessions. Empirical study of the hexaploid synthetics showed abundant variation in heading, flowering, and maturation time and the grain-filling period throughout the two growth seasons examined. The wide variation observed in heading time in Ae. tauschii was maintained in the hexaploid background of the synthetic wheat lines. Significantly positive correlations were observed among heading, flowering, and maturation time in the hexaploid synthetics. On the other hand, no significant correlations were found between grain-filling period and the other traits examined. Some comparisons between two selected synthetic wheat lines having similar flowering time stably exhibited differences in their grain-filling periods. These observations suggest that two major genetic pathways independently determine wheat maturation time; one controls heading and flowering time and the other regulates grain-filling period.

Identification and molecular mapping of a resistance gene to powdery mildew from the synthetic wheat line M53

Powdery mildew disease caused by Blumeria graminis f. sp. tritici (Bgt) is an economically important disease in wheat worldwide. The identification of germplasms resistant to the disease can not only facilitate the breeding of resistant cultivars, but can also broaden the diversity of resistance genes. The Mexican M53 is a synthetic hexaploid wheat line developed at the International Maize and Wheat Improvement Center (CIMMYT) from the cross between Triticum durum and Aegilops tauschii249. Infection of M53 with 15 different pathogen races revealed that the resistance in M53 was race-dependent and effective against the majority of the tested Bgt races, including the race 15 predominant in the Beijing wheat growing area. Inoculation of the parents of M53 with the race 15 demonstrated that M53 and Ae. tauschii249 were resistant, whereas T. durum was susceptible. The inoculation of three segregating F(2) populations developed from the crosses between M53 and three susceptible Chinese wheat cultivars with the race 15 showed that the resistant gene in M53 segregated in a single dominant manner. Amplified fragment length polymorphism (AFLP) and simple sequence repeat (SSR) markers were used to map the gene in a segregating F(2) population consisting of 213 lines developed from the cross Wan7107 × M53. Two closely linked AFLP markers, Apm109 and Apm161, were identified to flank the gene with genetic distances of 1.0cM and 3.0cM, respectively. The recognized gene was assigned to the long arm of chromosome 5D as determined by three linked SSR markers, Xwmc289b, Xgwm583, and Xgwm292, and by the physical mapping of Apm109 using Chinese Spring nullisomic-tetrasomic and ditelosomic stocks. The resistance gene identified in M53, temporarily designated as Pm-M53, could be used in local wheat-breeding programs to improve powdery mildew resistance.

Genetic Variation of Seed Dormancy in Synthetic Hexaploid Wheat‐Derived Populations

Aegilops tauschii, the D genome donor of wheat (Triticum aestivum L.), has strong seed dormancy and genes controlling the trait may be used to improve resistance of cultivars to preharvest sprouting (PHS). Thus, this research project was initiated to identify dormancy genes from Ae. tauschii‐derived synthetic hexaploid wheat (SHW). Four permanent segregating populations, developed by crossing four SHW lines with a common wheat line, were grown under field conditions to evaluate dormancy with threshed seeds (kernels) and/or intact seeds on the spike from 2006 to 2008. Significant genotypic and year effects on both kernel and on‐spike seed dormancy were detected in each population, with heritability estimates of about 0.4 (0.32–0.48) across the years. Seeds on the spikes were much more dormant than kernels, suggesting the presence of germination inhibitors in the covering tissues. Significant correlations between kernel and on‐spike seed germination percentages were detected in all populations; the estimated correlation coefficient (r) was greater for genotypic (0.80) than for phenotypic (0.63) or environmental (0.51) components. Thus, available SHW lines could be an alternative source of dormancy genes for enhancing resistance of wheat cultivars to PHS.

Evaluation and characterization of high-molecular weight 1D glutenin subunits from Aegilops tauschii in synthetic hexaploid wheats

The high-molecular weight (HMW) glutenin subunits of bread wheat are major determinants of end-use quality. The objective of this study was to determine the 1Dx and 1Dy subunits present in 43 synthetic hexaploid wheat (SHW) lines derived by crossing durum ‘Langdon’ to 43 Aegilops tauschii accessions. Protein samples were initially electrophoresed multiple times on SDS-PAGE gels to arrange subunits into similar groups and then were electrophoresed on urea/SDS-PAGE gels. Initial results with SDS-PAGE gels indicated that there were six 1Dx and six 1Dy subunits in these SHW lines. However, results of the urea/SDS-PAGE indicated that some of the subunit groups could be further differentiated into additional subunits. A total of eleven 1Dx and eight 1Dy subunits including the newly designated subunits 1Dx2 t-1, 1Dx2 t-2, 1Dx2 t-3, 1Dx1.5 t-1, 1Dx2.1 t-1, 1Dy10 t-1, and 1Dy12 t-1 were identified, and they composed 17 1Dx and 1Dy combinations in the SHW lines. Eight of the combinations included at least one novel subunit and hence they were novel Glu-D1 alleles. Our results indicated that urea/SDS-PAGE can be very useful in identifying new HMW glutenin subunits. Quality testing of the SHW lines will determine if any of the alleles are useful in improving wheat-baking quality.

Synthetic hexaploid wheat and its utilization for wheat genetic improvement in China

Synthetic hexaploid wheat (Triticum turgidumxAegilops tauschii) was created to explore for novel genes from T. turgidum and Ae. tauschii that can be used for common wheat improvement. In the present paper, research advances on the utilization of synthetic hexaploid wheat for wheat genetic improvement in China are reviewed. Over 200 synthetic hexaploid wheat (SHW) accessions from the International Maize and Wheat Improvement Centre (CIMMYT) were introduced into China since 1995. Four cultivars derived from these, Chuanmai 38, Chuanmai 42, Chuanmai 43 and Chuanmai 47, have been released in China. Of these, Chuanmai 42, with large kernels and resistance to stripe rust, had the highest average yield (>6 t/ha) among all cultivars over two years in Sichuan provincial yield trials, outyielding the commercial check cultivar Chuanmai 107 by 22.7%. Meanwhile, by either artificial chromosome doubling via colchicine treatment or spontaneous chromosome doubling via a union of unreduced gametes (2n) from T. turgidum-Ae. tauschii hybrids, new SHW lines were produced in China. Mitotic-like meiosis might be the cytological mechanism of spontaneous chromosome doubling. SHW lines with genes for spontaneous chromosome doubling may be useful for producing new SHW-alien amphidiploids and double haploid in wheat genetic improvement.

Variation in Dehydration Tolerance, ABA Sensitivity and Related Gene Expression Patterns in D-Genome Progenitor and Synthetic Hexaploid Wheat Lines

The wild wheat Aegilops tauschii Coss. has extensive natural variation available for breeding of common wheat. Drought stress tolerance is closely related to abscisic acid (ABA) sensitivity. In this study, 17 synthetic hexaploid wheat lines, produced by crossing the tetraploid wheat cultivar Langdon with 17 accessions of Ae. tauschii, were used for comparative analysis of natural variation in drought tolerance and ABA sensitivity. Ae. tauschii showed wide natural variation, with weak association between the traits. Drought-sensitive accessions of Ae. tauschii exhibited significantly less ABA sensitivity. D-genome variations observed at the diploid genome level were not necessarily reflected in synthetic wheats. However, synthetic wheats derived from the parental Ae. tauschii accessions with high drought tolerance were significantly more tolerant to drought stress than those from drought-sensitive accessions. Moreover, synthetic wheats with high drought tolerance showed significantly higher ABA sensitivity than drought-sensitive synthetic lines. In the hexaploid genetic background, therefore, weak association of ABA sensitivity with drought tolerance was observed. To study differences in gene expression patterns between stress-tolerant and -sensitive lines, levels of two Cor/Lea and three transcription factor gene transcripts were compared. The more tolerant accession of Ae. tauschii tended to accumulate more abundant transcripts of the examined genes than the sensitive accession under stress conditions. The expression patterns in the synthetic wheats seemed to be additive for parental lines exposed to drought and ABA treatments. However, the transcript levels of transcription factor genes in the synthetic wheats did not necessarily correspond to the postulated levels based on expression in parental lines. Allopolyploidization altered the expression levels of the stress-responsive genes in synthetic wheats.

Resistance to Russian Wheat Aphid Biotype 2 in CIMMYT Synthetic Hexaploid Wheat Lines

The Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), devastates wheat and barley production on all continents except Australia. Although D. noxia-resistant cultivars exist, virulent D. noxia populations exist in Asia, North America, and South America that have the ability to overcome resistance. In this study, synthetic hexaploid wheat genotypes created at the International Maize and Wheat Improvement Center (CIMMYT) were evaluated and characterized for resistance to North American biotype 2 (RWA2). Fewer RWA2 were produced on several genotypes, and D. noxia-induced leaf rolling and chlorosis were reduced on these genotypes as well. Aphid numbers were positively correlated with chlorosis and with leaf rolling. However, some genotypes were highly resistant to leaf rolling and chlorosis while supporting large RWA2 populations. There were negative correlations between leaf chlorosis and leaf dry weight (r = -0.267, df = 106, P = 0.006) and between aphid numbers and leaf dry weight (r = -0.297, df = 105, P = 0.002). These results indicate that chlorosis and aphid number individually explain at least 27% of the changes observed in leaf dry weights. Interestingly, there was no correlation between leaf rolling and leaf dry weight. The RWA2-resistant lines identified, which are also resistant to D. noxia populations in Mexico and to greenbug (Schizaphis graminum Rondani) biotype G, are strong candidates for use in improving the genetic diversity in bread wheat for resistance to different biotypes of both S. graminum and D. noxia.

Identification of novel QTLs for seedling and adult plant leaf rust resistance in a wheat doubled haploid population

Pyramiding of genes that confer partial resistance is a method for developing wheat (Triticum aestivum L.) cultivars with durable resistance to leaf rust caused by Puccinia triticina. In this research, a doubled haploid population derived from the cross between the synthetic hexaploid wheat (SHW) (xAegilotriticum spp.) line TA4152-60 and the North Dakota breeding line ND495 was used for identifying genes conferring partial resistance to leaf rust in both the adult plant and seedling stages. Five QTLs located on chromosome arms 3AL, 3BL, 4DL, 5BL and 6BL were associated with adult plant resistance with the latter four representing novel leaf rust resistance QTLs. Resistance effects of the 4DL QTL were contributed by ND495 and the effects of the other QTLs were contributed by the SHW line. The QTL on chromosome arm 3AL had large effects and also conferred seedling resistance to leaf rust races MJBJ, TDBG and MFPS. The other major QTL, which was on chromosome arm 3BL, conferred seedling resistance to race MFPS and was involved in a significant interaction with a locus on chromosome arm 5DS. The QTLs and the associated molecular markers identified in this research can be used to develop wheat cultivars with potentially durable leaf rust resistance.

Quantitative variation of Revolver transposon-like genes in synthetic wheat and their structural relationship with the LARD element

Revolver is a multi-gene family dispersed through Triticeae genomes like transposons. Revolver is similar to class II transposable elements and shows considerable quantitative variation in wheat and its relatives. The highest copy number of Revolver is found in Secale cereale (RR) and the lowest in hexaploid wheat Triticum aestivum (AABBDD). In this study, Revolver copy numbers were determined in synthetic hexaploid wheat lines from crosses between Aegilops tauschii (DD) and T. turgidum tetraploid wheat species (T. dicoccoides, T. dicoccum, T. carthlicum and T. durum, AABB). Eight out of 18 lines showed significantly lower copies than the sum of their parents and seven lines were equal to the sum, suggesting that polyploidy caused loss of Revolver. Members of the Revolver family also showed structural variation in the 5′ region, especially in length. Revolver did not share any similarity with autonomous transposable elements. However, the long terminal repeats (LTRs) of the non-autonomous large retrotransposon derivative (LARD) in barley, showed 60% homology to both 5′ and 3′ ends of some variants of Revolver. LARD LTRs lack the Revolver region from the first exon to the middle of the first intron resulting in non-coding sequences. Evolutionary relationships between Revolver and LARD are discussed.

Identification of novel tan spot resistance loci beyond the known host-selective toxin insensitivity genes in wheat

Tan spot, caused by Pyrenophora tritici-repentis, is a destructive foliar disease of wheat causing significant yield reduction in major wheat growing areas throughout the world. The objective of this study was to identify quantitative trait loci (QTL) conferring resistance to tan spot in the synthetic hexaploid wheat (SHW) line TA4152-60. A doubled haploid (DH) mapping population derived from TA4152-60 x ND495 was inoculated with conidia produced by isolates of each of four virulent races of P. tritici-repentis found in North America. QTL analysis revealed a total of five genomic regions significantly associated with tan spot resistance, all of which were contributed by the SHW line. Among them, two novel QTLs located on chromosome arms 2AS and 5BL conferred resistance to all isolates tested. Another novel QTL on chromosome arm 5AL conferred resistance to isolates of races 1, 2 and 5, and a QTL specific to a race 3 isolate was detected on chromosome arm 4AL. None of these QTLs corresponded to known host selective toxin (HST) insensitivity loci, but a second QTL on chromosome arm 5BL conferred resistance to the Ptr ToxA producing isolates of races 1 and 2 and corresponded to the Tsn1 (Ptr ToxA sensitivity) locus. This indicates that the wheat-P. tritici-repentis pathosystem is much more complex than previously thought and that selecting for toxin insensitivity alone will not necessarily lead to tan spot resistance. The markers associated with the QTLs identified in this work will be useful for deploying the SHW line as a tan spot resistance source in wheat breeding.

Stem Rust, Tan Spot, Stagonospora nodorum Blotch, and Hessian Fly Resistance in Langdon Durum–Aegilops tauschii Synthetic Hexaploid Wheat Lines

Diseases and pests of wheat (Triticum aestivum L.) cause serious yield and quality losses to wheat grown worldwide. In the current study we tested synthetic hexaploid wheat (SHW) lines developed from various lines of Aegilops tauschii Cosson crossed with the tetraploid durum wheat (T. turgidum L.) cultivar Langdon. These SHW lines were tested along with their durum wheat and A. tauschii parents for resistance to stem rust (caused by Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. and E. Henn.), tan spot [caused by Pyrenophora tritici‐repentis (Died.) Drechs.], and Stagonospora nodorum blotch [SNB; caused by Phaeosphaeria nodorum (E. Mull.) Hedjar] as well as for resistance to Hessian fly [Mayetiola destructor (Say) (Diptera: Cecidomyiidae)]. Langdon durum and all SHW lines were resistant to all races tested for stem rust. Although the durum parent Langdon was susceptible to tan spot, SNB, and Hessian fly, resistance was identified in several synthetic lines for each disease or pest indicating that the A. tauschii lines used in constructing the SHW lines are potentially useful sources of resistance. These resistant SHW lines are presently being used to characterize new sources of the resistance and introgress the resistance into bread wheat.

Whole genome mapping in a wheat doubled haploid population using SSRs and TRAPs and the identification of QTL for agronomic traits

Genetic maps are useful for detecting quantitative trait loci (QTL) associated with quantitative traits and for marker-assisted selection (MAS) in breeding. In this research, we used the wheat × maize method to develop a doubled haploid (DH) population derived from the synthetic hexaploid wheat (SHW) line TA4152-60 and the North Dakota hard red spring wheat line ND495. The population consisted of 213 lines, of which a subset of 120 lines was randomly selected and used to construct linkage maps of all 21 chromosomes and for QTL detection. The whole genome maps consisted of 632 markers including 410 SSRs, 218 TRAPs, 1 RFLP, and 3 phenotypic markers, and spanned 3,811.5 cM with an average density of one marker per 6.03 cM. Telomere sequence-based TRAPs allowed us to define the ends of seven linkage groups. Analysis revealed major QTLs associated with the traits of days to heading on chromosomes 5A and 5B, plant height on chromosomes 4D and 5A, and spike characteristics on chromosomes 3D, 4A, 4D, 5A and 5B. The DH population and genetic map will be a useful tool for the identification of disease resistance QTL and agronomically important loci, and will aid in the identification and development of markers for MAS.

Genetic variation for quality traits in synthetic wheat germplasm

Synthetic hexaploid wheats offer breeders ready access to potentially novel genetic variation associated with the D genome of Aegilops tauschii. In order to assess the application of this germplasm to wheat quality improvement, collections of primary and derived synthetic hexaploid wheat lines were surveyed for traits that determine colour and colour stability in Asian noodles and the frequency of a genetic defect know as late maturity α-amylase (LMA). The range of variation was then compared with bread wheat and durum wheat cultivar collections. Primary synthetics contained substantial genetic variation for quality traits associated with colour and colour stability of Asian noodles including near-zero extremes for polyphenol oxidase and lipoxygenase. These extremes represent a significant advantage compared with current bread wheat cultivars and are similar to the best durum wheats. While alternative strategies for reducing polyphenol oxidase and lipoxygenase are available, the synthetics nevertheless provide a useful resource for wheat breeders attempting to develop improved wheat cultivars for the Asian noodle market. Unfortunately, however, most primary synthetics were prone to late maturity α-amylase and mature grain contained unacceptably high levels of α-amylase. Elimination of this genetic defect, or selection within breeding populations for low or non-LMA, is both time consuming and labour intensive and presents a significant obstacle to exploitation of variation for other traits. As proof of concept, near-zero polyphenol oxidase (PPO) lines, free from LMA, were recovered from backcross populations involving a high LMA primary synthetic.

Molecular mapping of a dominant non-glaucousness gene from synthetic hexaploid wheat (Triticum aestivum L.)

The leaf and stem surfaces of many land plants are covered with a cuticular wax layer that confers a glaucous appearance or white bloom. Synthetic hexaploid wheat Line 3672 was non-glaucous, and common wheat Line 9753 was glaucous. The cuticular wax was characterized using Scanning Electron Microscopy (SEM). A hybrid using 9753 as female parent and 3672 as male parent was made and 108 F2 plants and their F3 progenies were used to map the non-glaucouness gene. Non-glaucousness in Line 3672 was controlled by a single dominant gene, temporarily designated Iw3672. Five SSR markers mapped on chromosome 2DS were linked to Iw3672. Additionally, two EST-derived markers and a SNP marker were developed and were also linked to Iw3672. The order of the eight markers and Iw3672 was Xte6 730 /Xbarc124 520 —Iw3672 —Xwe6 2100/2150 —Xcau96 287 —Xcfd51 180/200/230 —Xwe7—Xgdm5 190 —Xgdm35 246/250 , with the genetic distance for each interval being 0.9 cM, 1.4 cM, 0.9 cM, 0.9 cM, 1.9 cM, 7.2 cM and 2.5 cM, respectively. We concluded that Iw3672 is physically mapped on the distal region of wheat chromosome 2DS.

Genetic characterization and molecular mapping of Hessian fly resistance genes derived from Aegilops tauschii in synthetic wheat

Two synthetic hexaploid wheat lines (xAegilotriticum spp., 2n = 6x = 42, genomes AABBDD), SW8 and SW34, developed from the crosses of the durum wheat cultivar Langdon (Triticum turgidum L. var. durum, 2n = 4x = 28, genomes AABB) with two Aegilops tauschii Cosson accessions (2n = 2x = 14, genome DD), were determined to carry Hessian fly [Mayetiola destructor (Say)] resistance genes derived from the Ae. tauschii parents. SW8 was resistant to the Hessian fly biotype Great Plains (GP) and strain vH13 (virulent to H13). SW34 was resistant to biotype GP, but susceptible to strain vH13. Allelism tests indicated that resistance genes in SW8 and SW34 may be allelic to H26 and H13 or correspond to paralogs at both loci, respectively. H26 and H13 were localized to chromosome 4D and 6D, respectively, in previous studies. Molecular mapping in the present study, however, assigned the H26 locus to chromosome 3D rather than 4D. On the other hand, mapping of the resistance gene in SW34 verified the previous assignment of the H13 locus to chromosome 6D. Linkage analysis and physical mapping positioned the H26 locus to the chromosomal deletion bin 3DL3-0.81-1.00. A linkage map for each of these two resistance genes was constructed using simple sequence repeat (SSR) and target region amplification polymorphism (TRAP) markers.

Molecular mapping of hybrid necrosis genes Ne1 and Ne2 in hexaploid wheat using microsatellite markers

Hybrid necrosis is the gradual premature death of leaves or plants in certain F1 hybrids of wheat (Triticum aestivum L.), and it is caused by the interaction of two dominant complementary genes Ne1 and Ne2 located on chromosome arms 5BL and 2BS, respectively. To date, molecular markers linked to these genes have not been identified and linkage relationships of the two genes with other important genes in wheat have not been established. We observed that the F1 hybrids from the crosses between the bread wheat variety 'Alsen' and four synthetic hexaploid wheat (SHW) lines (TA4152-19, TA4152-37, TA4152-44, and TA4152-60) developed at the International Maize and Wheat Improvement Center (CIMMYT) exhibited hybrid necrosis. This study was conducted to determine the genotypes of TA4152-60 and Alsen at the Ne1 and Ne2 loci, and to map the genes using microsatellite markers in backcross populations. Genetic analysis indicated that Alsen has the genotype ne1ne1Ne2Ne2 whereas the SHW lines have Ne1Ne1ne2ne2. The microsatellite marker Xbarc74 was linked to Ne1 at a genetic distance of 2.0 cM on chromosome arm 5BL, and Xbarc55 was 3.2 cM from Ne2 on 2BS. Comparison of the genetic maps with the chromosome deletion-based physical maps indicated that Ne1 lies in the proximal half of 5BL, whereas Ne2 is in the distal half of 2BS. Genetic linkage analysis showed that Ne1 was about 35 cM proximal to Tsn1, a locus conferring sensitivity to the host selective toxin Ptr ToxA produced by the tan spot fungus. The closely linked microsatellite markers identified in this study can be used to genotype parental lines for Ne1 and Ne2 or to eliminate the two hybrid necrosis genes using marker-assisted selection.

Wheat‐Alien Species Derivatives: A Novel Source of Resistance to Fusarium Head Blight in Wheat

Fusarium head blight (FHB), caused mainly by Fusarium graminearum Schwabe, is a destructive disease of wheat (Triticum spp.) in humid growth conditions throughout the world. Genetic resistance of the host plant is considered the most effective and sustainable method of defense against FHB; however, only limited sources of resistance are available in wheat. Relatives of wheat have proven to be an invaluable gene pool for wheat improvement. The objective of this study was to explore relatives of wheat for FHB resistance. We evaluated 293 lines derived from the crosses of wheat with its relatives for resistance to spread of FHB infection over two greenhouse seasons. of these 293 derivatives, 66 were susceptible, 153 appeared moderately resistant, and 74 lines exhibited a level of resistance comparable with T. aestivum L. ‘Sumai 3’, the most widely used source of resistance to FHB. Alien species involved in development of these derivatives include T. tauschii (Coss.) Schmal., Roegneria kamoji C. Koch, R. ciliaris (Trin.) Nevski, Leymus racemosus Lam., Thinopyrum ponticum (Podp.) Barkworth & D.R. Dewey, Th. elongatum (Host) D.R. Dewey, Th. junceum (L.) Love, Th. intermedium (Host) Barkworth & D.R. Dewey, Dasypyrum villosa L., Secale cereale L., and oat (Avena sativa L.). The wheat‐alien species derivatives identified as resistant to FHB include wheat‐alien species amphiploids, synthetic hexaploid wheat lines, and wheat‐alien species substitution and translocation lines. These derivatives could serve as novel sources to enhance resistance of wheat to FHB.

Identification of quantitative trait loci for resistance to two species of root-lesion nematode (Pratylenchus thornei and P. neglectus) in wheat

Pratylenchus thornei and P. neglectus are two species of root-lesion nematode that cause substantial yield losses in wheat. No commercially available wheat variety has resistance to both species. A doubled-haploid population developed from a cross between the synthetic hexaploid wheat line CPI133872 and the bread wheat Janz was used to locate and tag quantitative trait loci (QTLs) associated with resistance to both P. thornei and P. neglectus. Wheat plants were inoculated with both species of nematode in independent replicated glasshouse trials repeated over 2 years. Known locations of wheat microsatellite markers were used to construct a framework map. After an initial single-marker analysis to detect marker-trait linkages, chromosome regions associated with putative QTLs were targetted with microsatellite markers to increase map density in the chromosome regions of interest. In total, 148 wheat microsatellite markers and 21 amplified fragment length polymorphism markers were mapped. The codominant microsatellite marker Xbarc183 on the distal end of chromosome 6DS was allelic for resistance to both P. thornei and P. neglectus. The QTL were designated QRlnt.lrc-6D.1 and QRlnn.lrc-6D.1, for the 2 traits, respectively. The allele inherited from CPI133872 explained 22.0–24.2% of the phenotypic variation for P. thornei resistance, and the allele inherited from Janz accounted for 11.3–14.0% of the phenotypic variation for P. neglectus resistance. Composite interval mapping identified markers that flank a second major QTL on chromosome 6DL (QRlnt.lrc-6D.2) that explained 8.3–13.4% of the phenotypic variation for P. thornei resistance. An additional major QTL associated with P. neglectus resistance was detected on chromosome 4DS (QRlnn.lrc-4D.1) and explained a further 10.3–15.4% of the phenotypic variation. The identification and tagging of nematode resistance genes with molecular markers will allow appropriate allele combinations to be selected, which will aid the successful breeding of wheat with dual nematode resistance.

Microsatellite markers associated with two Aegilops tauschii-derived greenbug resistance loci in wheat

A new source of greenbug (Schizaphis graminum Rondani) resistance derived from Aegilops tauschii (Coss.) Schmal was identified in W7984, a synthetic hexaploid wheat line and one parent of the International Triticeae Mapping Initiative (ITMI) mapping population. Segregation analysis of responses to greenbug feeding in a set of recombinant inbred lines (RILs) identified a single, dominant gene governing the greenbug resistance in W7984, which was placed in chromosome arm 7DL by linkage analysis with molecular markers in the ITMI population. Allelism tests based on the segregation of responses to greenbug feeding in F2 and testcross plants revealed that the greenbug resistance in W7984 and Largo, another synthetic line carrying the greenbug resistance gene Gb3, was controlled by different but linked loci. Using the ITMI reference map and a target mapping strategy, we have constructed a microsatellite map of Gb3 in a mapping population of 130 F7 RILs from Largo x TAM 107 and identified one marker (Xwmc634) co-segregating with Gb3 and four markers (Xbarc76, Xgwm037, Xgwm428 and Xwmc824) closely linked with Gb3. Deletion mapping of selected microsatellite markers flanking the Gb3 locus placed this resistance gene into the distal 18% region of 7DL. Comparative mapping in the ITMI and Largo x TAM 107 populations using the same set of microsatellite markers provided further evidence that greenbug resistance in W7984 and Largo is conditioned by two different loci. We suggest that the greenbug resistance gene in W7984 be designated Gb7. The microsatellite map of Gb3 constructed from this study should be a valuable tool for marker-assisted selection of Gb3-conferred greenbug resistance in wheat breeding.