Genetic analysis of resistance to <Emphasis Type="Italic">soil-borne wheat mosaic virus</Emphasis> derived from <Emphasis Type="Italic">Aegilops tauschii</Emphasis>

Genetic Analysis of Resistance to Soil-Borne Wheat Mosaic Virus Derived from Aegilops tauschii. Euphytica. Soil-Borne Wheat Mosaic Virus (SBWMV), vectored by the soil inhabiting organism Polymyxa graminis, causes damage to wheat (Triticum aestivum) yields in most of the wheat growing regions of the world. In localized fields, the entire crop may be lost to the virus. Although many winter wheat cultivars contain resistance to SBWMV, the inheritance of resistance is poorly understood. A linkage analysis of a segregating recombinant inbred line population from the cross KS96WGRC40 × Wichita identified a gene of major effect conferring resistance to SBWMV in the germplasm KS96WGRC40. The SBWMV resistance gene within KS96WGRC40 was derived from accession TA2397 of Aegilops taushcii and is located on the long arm of chromosome 5D, flanked by microsatellite markers Xcfd10 and Xbarc144. The relationship of this locus with a previously identified QTL for SBWMV resistance and the Sbm1 gene conferring resistance to soil-borne cereal mosaic virus is not known, but suggests that a gene on 5DL conferring resistance to both viruses may be present in T. aestivum, as well as the D-genome donor Ae. tauschii.     

Allelic variation for the rust resistance gene <Emphasis Type="Italic">Lr34</Emphasis>/<Emphasis Type="Italic">Yr18</Emphasis> in Canadian wheat cultivars

The wheat (Triticum aestivum L.) gene Lr34/Yr18 conditions resistance to leaf rust, stripe rust, and stem rust, along with other diseases such as powdery mildew. This makes it one of the most important genes in wheat. In Canada, Lr34 has provided effective leaf rust resistance since it was first incorporated into the cultivar Glenlea, registered in 1972. Recently, molecular markers were discovered that are either closely linked to this locus, or contained within the gene. Canadian wheat cultivars released from 1900 to 2007, breeding lines and related parental lines, were tested for sequence based markers caSNP12, caIND11, caIND10, caSNP4, microsatellite markers wms1220, cam11, csLVMS1, swm10, csLV34, and insertion site based polymorphism marker caISBP1. Thirty different molecular marker haplotypes were found among the 375 lines tested; 5 haplotypes had the resistance allele for Lr34, and 25 haplotypes had a susceptibility allele at this locus. The numbers of lines in each haplotype group varied from 1 to 140. The largest group was represented by the leaf rust susceptible cultivar “Thatcher” and many lines derived from “Thatcher”. The 5 haplotypes that had the resistance allele for Lr34 were identical for the markers tested within the coding region of the gene but differed in the linked markers wms1220, caISBP1, cam11, and csLV34. The presence of the resistance or susceptibility allele at the Lr34 locus was tracked through the ancestries of the Canadian wheat classes, revealing that the resistance allele was present in many cultivars released since the 1970s, but not generally in the older cultivars.     

Dominant gene for common bean resistance to common bacterial blight caused by <Emphasis Type="Italic">Xanthomonas</Emphasis><Emphasis Type="Italic">axonopodis</Emphasis> pv. <Emphasis Type="Italic">phaseoli</Emphasis>

The common bacterial blight pathogen [Xanthomonas axonopodis pv. phaseoli (Xap)] is a limiting factor for common bean (Phaseolus vulgaris L.) production worldwide and resistance to the pathogen in most commercial cultivars is inadequate. Variability in virulence of the bacterial pathogen has been observed in strains isolated from Puerto Rico and Central America. A few common bean lines show a differential reaction when inoculated with different Xap strains, indicating the presence of pathogenic races. In order to study the inheritance of resistance to common bacterial blight in common bean, a breeding line that showed a differential foliar reaction to Xap strains was selected and was crossed with a susceptible parent. The inheritance of resistance to one of the selected Xap races was determined by analysis of segregation patterns in the F₁, F₂, F₃ and F₄ generations from the cross between the resistant parent PR0313-58 and the susceptible parent ‘Rosada Nativa’. The F₁, F₂ and F₃ generations were tested under greenhouse conditions. Resistant and susceptible F_3:4 sister lines were tested in the field. The statistical analysis of all generations followed the model for a dominant resistance gene. The resistant phenotype was found to co-segregate with the SCAR SAP6 marker, located on LG 10. These results fit the hypothesis that resistance is controlled by a single dominant gene. The symbol proposed for the resistance gene is Xap-1 and for the bacterial race, XapV1.     

QTL analysis for thousand-grain weight under terminal drought stress in bread wheat (<Emphasis Type="Italic">Triticum</Emphasis><Emphasis Type="Italic">aestivum</Emphasis> L.)

Grain yield under post-anthesis drought stress is one of the most complex traits, which is inherited quantitatively. The present study was conducted to identify genes determining post-anthesis drought stress tolerance in bread wheat through Quantitative Trait Loci (QTLs) analysis. Two cultivated bread wheat accessions were selected as parental lines. Population phenotyping was carried out on 133 F_2:3 families. Two field experiments and two experiments in the greenhouse were conducted at IPK-Gatersleben, Germany with control and post-anthesis stress conditions in each experiment. Thousand-grain weight was recorded as the main wheat yield component, which is reduced by post-anthesis drought stress. Chemical desiccation was applied in three experiments as simulator of post-anthesis drought stress whereas water stress was applied in one greenhouse experiment. Analysis of variance showed significant differences among the F_2:3 families. The molecular genetic linkage map including 293 marker loci associated to 19 wheat chromosomes was applied for QTL analysis. The present study revealed four and six QTLs for thousand-grain weight under control and stress conditions, respectively. Only one QTL on chromosome 4BL was common for both conditions. Five QTLs on chromosomes 1AL, 4AL, 7AS, and 7DS were found to be specific to the stress condition. Both parents contributed alleles for drought tolerance. Taking the known reciprocal translocation of chromosomes 4AL/7BS into account, the importance of the short arms of homoeologous group 7 is confirmed for drought stress.     

Genetics of resistance to <Emphasis Type="Italic">Cucumber mosaic virus</Emphasis> in <Emphasis Type="Italic">Cucumis sativus</Emphasis> var. <Emphasis Type="Italic">hardwickii</Emphasis> R. Alef

The genetics of resistance to Cucumber mosaic virus (CMV) in Cucumis sativus var. hardwickii R. Alef, the wild progenitor of cultivated cucumber was assessed by challenge inoculation and by natural infection of CMV. Among the 31 genotypes of C. sativus var. hardwickii collected from 21 locations in India the lowest mean percent disease intensity (PDI) was recorded in IC-277048 (6.33%) while the highest PDI was observed in IC-331631 (75.33%). All the four cultivated varieties (DC-1, DC-2, CHC-1 and CHC-2) showed very high PDI and susceptible disease reaction. Based on mean PDI, 8 genotypes were categorized as resistant, 13 as moderately resistant, 9 as moderately susceptible and one as susceptible. A chi-square test of frequency distribution based on mean PDI in F₂ progenies of six resistant × susceptible crosses revealed monogenic recessive Mendelian ratio 1(R):3(S) to be the best fit. This monogenic recessive model was further confirmed by 1(R):1(S) ratio as the best fit for back cross with resistant parent and no fit for either 3:1 or 1:1 in the back cross with the susceptible parent. The results revealed that CMV resistance in C. sativus var. hardwickii was controlled by a single recessive gene. Considering the cross compatibility between C. sativus var. hardwickii and cultivated cucumber, the resistance trait can be easily transferred to cultivated species through simple backcross breeding.     

Search in <Emphasis Type="Italic">Solanum</Emphasis> (section <Emphasis Type="Italic">Lycopersicon</Emphasis>) germplasm for sources of broad-spectrum resistance to four <Emphasis Type="Italic">Tospovirus</Emphasis> species

The genus Tospovirus was considered as monotypic with Tomato spotted wilt virus (TSWV) being the only assigned species. However, extensive studies with worldwide isolates revealed that this genus comprises a number of species with distinct virulence profiles. The Neotropical South America is one center of Tospovirus diversity with many endemic species. Groundnut ringspot virus (GRSV), TSWV, Tomato chlorotic spot virus (TCSV), and Chrysanthemum stem necrosis virus (CSNV) are the predominant tomato-infecting species in Brazil. Sources of resistance were found in Solanum (section Lycopersicon) mainly against TSWV isolates from distinct continents, but there is an overall lack of information about resistance to other viral species. One-hundred and five Solanum (section Lycopersicon: Solanaceae) accessions were initially evaluated for their reaction against a GRSV isolate by analysis of symptom expression and systemic virus accumulation using DAS-ELISA. A subgroup comprising the most resistant accessions was re-evaluated in a second assay with TSWV, TCSV, and GRSV isolates and in a third assay with a CSNV isolate. Seven S. peruvianum accessions displayed a broad-spectrum resistance to all viral species with all plants being free of symptoms and systemic infection. Sources of resistance were also found in tomato cultivars with the Sw-5 gene and also in accessions of S. pimpinellifolium, S. chilense, S. arcanum, S. habrochaites, S. corneliomuelleri, and S. lycopersicum. The introgression/incorporation of these genetic factors into cultivated tomato varieties might allow the development of genetic materials with broad-spectrum resistance, as well as with improved levels of phenotypic expression.     

QTL mapping for developmental behavior of plant height in wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

A recombinant inbred line (RIL) population with 305 lines derived from a cross of Hanxuan 10 × Lumai 14 was used to identify the dynamic quantitative trait loci (QTL) for plant height (PH) in wheat (Triticum aestivum L.). Plant heights of RILs were measured at five stages in three environments. Total of seven genomic regions covering PH QTL clusters on different chromosomes identified from a DH population derived from the same cross as the RIL were used as the candidate QTLs and extensively analyzed. Five additive QTLs and eight pairs of epistatic QTLs significantly affecting plant height development were detected by unconditional QTL mapping method. Six additive QTLs and four pairs of epistatic QTLs were identified using conditional mapping approach. Among them, three additive QTLs (QPh.cgb-1B.3, QPh.cgb-4D.1, QPh.cgb-5B.2) and three pairs of epistatic QTLs (QPh.cgb-1B.1–QPh.cgb-1B.3, QPh.cgb-2A.1–QPh.cgb-2D.1, QPh.cgb-2D.1–QPh.cgb-5B.2) were common QTLs detected by both methods. Three QTLs (QPh.cgb-4D.1, QPh.cgb-5B.3, QPh.cgb-5B.4) were expressed under both drought and well-water conditions. The present data are useful for wheat genetic manipulations through molecular marker-assisted selection (MAS), and provides new insights into understanding the genetic mechanism and regulation network underlying the development of plant height in crops. Our result in this study indicated that combining unconditional and conditional mapping methods could make it possible to reveal not only the stable/conserved QTLs for the developmental traits such as plant height but also the dynamic expression feature of the QTLs.     

Wild <Emphasis Type="Italic">Coffea arabica</Emphasis> resistant to <Emphasis Type="Italic">Meloidogyne paranaensis</Emphasis> and genetic parameters for resistance

Arabica coffee production is based on highly productive cultivars; however, these cultivars are susceptible to infestation by several biotic agents, including root-knot nematodes. Collections of wild Coffea arabica germplasm represent an important source of genetic variability for resistant cultivar development. In this study, 1046 plants derived from 71 wild coffee trees were evaluated with respect to Meloidogyne paranaensis resistance. In addition to information on plants reactions, we also evaluated the genetic parameters related to resistance. Progenies from the five most promising plants were also evaluated regarding resistance to M. incognita and M. exigua. The yield potential of selected plants was estimated through analysis of data for fruits harvested in 4 different years. Forty-seven plants were considered resistant based on reproduction factor values. The estimated heritability was high for all analyzed variables leading to substantial selection gain, mainly at the progeny mean level. On the basis of heritabilities and genetic correlations, we conclude that selection could be performed based on values of the gall and egg mass index. However, higher genetic gain could be obtained based on nematode count variables. A second experiment confirmed the reactions of the selected five plants to M. paranaensis, and multiple resistance was detected in three of them. The resistant accessions also have yield potential.     

Asparagine synthetase genes (<Emphasis Type="Italic">AsnS1</Emphasis> and <Emphasis Type="Italic">AsnS2</Emphasis>) in durum wheat: structural analysis and expression under nitrogen stress

Wheat is one of the most widely grown cereal crops based on the amount of calories it provides in the human diet. Durum wheat (Triticum turgidum ssp. durum) is largely used for production of pasta and other products. In order to use genetic knowledge to improve the understanding of N-use efficiency, we carried out, for the first time in durum wheat, the isolation and the characterization of four members of the asparagine synthetase (AsnS) gene family. Phylogenetic inference clustered the Ttu-AsnS1 (1.1 and 1.2) and Ttu-AsnS2 (2.1 and 2.2) genes in AsnS gene class I, which is present in monocots and dicots. Class I genes underwent a subsequent duplication leading to the formation of two subgroups. Plants of Svevo cultivar were grown under N-stress conditions and expression of the four AsnS genes was investigated at three developmental stages (seedling, booting, and late milk development), crucial for N absorption, assimilation and remobilization. AsnS1 genes were down-regulated in N-stressed roots, stems and leaves during seedling growth and booting, but seemed to play a role in N remobilization in flag leaves during grain filling. AsnS2 genes were scarcely expressed in roots, stems, and leaves. In N-stressed spikes there was no differential expression in any of the genes. The genes were mapped in silico using a durum wheat SNP map, assigning Ttu-AsnS1 genes to chromosome 5 and Ttu-AsnS2 to chromosome 3. These findings provide a better understanding of the role of ASN genes in response to N stress in durum wheat.     

Allelic composition and associated quality traits of the <Emphasis Type="Italic">Glu-1</Emphasis> and <Emphasis Type="Italic">Glu-3</Emphasis> loci in selected modern Ethiopian durum wheat varieties

Gluten protein determines the processing quality of both durum wheat and bread wheat. The glutenin subunits compositions and associated quality traits of 20 Ethiopian durum wheat varieties were systematically analyzed using SDS-PAGE and Payne numbers. A total of 16 glutenin patterns were identified. At the Glu-A1 locus, all varieties scored the null allele. The predominant glutenin alleles at the Glu-B1 locus were Glu-B1b (7+8) and Glu-B1e (20). In Glu-3, the most abundant glutenin subunits were Glu-A3a and Glu-B3c. Based on the Payne scores, the varieties Yerer, Ginchi, Candate, and Foka were identified to have allelic composition suitable for pasta making. The cluster analysis using agglomerative hierarchical clustering (AHC) method classified the varieties into four similarity classes. Based on the findings of this experiment, suggestions were made for allelic composition improvement through introgression of superior alleles from known Glu-1 and Glu-3 sources.     

The use of <Emphasis Type="Italic">Vp1</Emphasis> in real time RT-PCR to select for pre-harvest sprouting tolerance in triticale

In spite of the availability of laboratory and field tests there is still a major problem to select pre-harvest sprouting (PHS) tolerant triticale varieties in a reliable, field-independent way. One approach to minimize the influence of environmental conditions and physio-morphological traits on PHS detection is using molecular genetic tools. The ‘viviparous’ Vp1 gene has been repeatedly described to play an important role in dormancy in wheat. A quantitative RT-PCR assay based on the expression of the Vp1 gene has been developed. Specific primers were designed for detecting Vp1 in both wheat and triticale. The expression levels of Vp1 were normalized using reference genes and relatively quantified with the comparative C_t-method. However, the first results indicate that the achieved Vp1 expression levels at 50 days post anthesis are not useful to select for PHS tolerance, both in wheat and triticale. This negative outcome so far is possibly due to the existence of several splicing events or to the late assaying moment in the kernel development, when Vp1 expression is found to be low.     

QTL mapping and analysis of epistatic interactions for grain yield and yield-related traits in <Emphasis Type="Italic">Triticum turgidum</Emphasis> L. var. <Emphasis Type="Italic">durum</Emphasis>

A quantitative trait loci (QTL) analysis of grain yield and yield-related traits was performed on 93 durum wheat recombinant inbred lines derived from the cross UC1113 × Kofa. The mapping population and parental lines were analyzed considering 19 traits assessed in different Argentine environments, namely grain yield, heading date, flowering time, plant height, biomass per plant, and spikelet number per ear, among others. A total of 224 QTL with logarithm of odds ratio (LOD) ≥ 3 and 47 additional QTL with LOD > 2.0 were detected. These QTL were clustered in 35 regions with overlapping QTL, and 12 genomic regions were associated with only one phenotypic trait. The regions with the highest number of multi-trait and stable QTL were 3BS.1, 3BS.2, 2BS.1, 1BL.1, 3AL.1, 1AS, and 4AL.3. The effects of epistatic QTL and QTL × environment interactions were also analyzed. QTL putatively located at major gene loci (Rht, Vrn, Eps, and Ppd) as well as additional major/minor QTL involved in the complex genetic basis of yield-related traits expressed in Argentine environments were identified. Interestingly, the 3AL.1 region was found to increase yield without altering grain quality or crop phenology.     

Diploid and tetraploid progenitors of wheat are valuable sources of resistance to the root lesion nematode <Emphasis Type="Italic">Pratylenchus</Emphasis><Emphasis Type="Italic">thornei</Emphasis>

The root lesion nematode Pratylenchus thornei is widely distributed in Australian wheat (Triticum aestivum) producing regions and can reduce yield by more than 50%, costing the industry AU$50 M/year. Genetic resistance is the most effective form of management but no commercial cultivars are resistant (R) and the best parental lines are only moderately R. The wild relatives of wheat have evolved in P. thornei-infested soil for millennia and may have superior levels of resistance that can be transferred to commercial wheats. To evaluate this hypothesis, a collection of 251 accessions of wheat and related species was tested for resistance to P. thornei under controlled conditions in glasshouse pot experiments over two consecutive years. Diploid accessions were more R than tetraploid accessions which proved more R than hexaploid accessions. Of the diploid accessions, 11 (52%) Aegilops speltoides (S-[B]-genome), 10 (43%) Triticum monococcum (A ^m -genome) and 5 (24%) Triticum urartu (A ^u -genome) accessions were R. One tetraploid accession (Triticum dicoccoides) was R. This establishes for the first time that P. thornei resistance is located on the A-genome and confirms resistance on the B-genome. Since previous research has shown that the moderate levels of P. thornei resistance in hexaploid wheat are dose-dependent, additive and located on the B and D-genomes, it would seem efficient to target A-genome resistance for introduction to hexaploid lines through direct crossing, using durum wheat as a bridging species and/or through the development of amphiploids. This would allow resistances from each genome to be combined to generate a higher level of resistance than is currently available in hexaploid wheat.     

Resistance to <Emphasis Type="Italic">Cucumber green mottle mosaic virus</Emphasis> in <Emphasis Type="Italic">Cucumis sativus</Emphasis>

Cucumber green mottle mosaic virus (CGMMV) is a severe threat for cucumber production worldwide. At present, there are no cultivars available in the market which show an effective resistance or tolerance to CGMMV infection, only wild Cucumis species were reported as resistant. Germplasm accessions of Cucumis sativus, as well as C. anguria and C. metuliferus, were mechanically infected with the European and Asian strains of CGMMV and screened for resistance, by scoring symptom severity, and conventional RT-PCR. The viral loads of both CGMMV strains were determined in a selected number of genotypes using quantitative RT-PCR. Severe symptoms were found following inoculation in C. metuliferus and in 44 C. sativus accessions, including C. sativus var. hardwickii. Ten C. sativus accessions, including C. sativus var. sikkimensis, showed intermediate symptoms and only 2 C. sativus accessions showed mild symptoms. C. anguria was resistant to both strains of CGMMV because no symptoms were expressed and the virus was not detected in systemic leaves. High amounts of virus were found in plants showing severe symptoms, whereas low viral amounts found in those with mild symptoms. In addition, the viral amounts detected in plants which showed intermediate symptoms at 23 and 33 dpi, were significantly higher in plants inoculated with the Asian CGMMV strain than those with the European strain. This difference was statistically significant. Also, the amounts of virus detected over time in plants did not change significantly. Finally, the two newly identified partially resistant C. sativus accessions may well be candidates for breeding programs and reduce the losses produced by CGMMV with resistant commercial cultivars.     

<Emphasis Type="Italic">Agrobacterium</Emphasis>-mediated genetic transformation of pigeon pea [<Emphasis Type="Italic">Cajanus cajan</Emphasis> (L.) Millsp.] for resistance to legume pod borer <Emphasis Type="Italic">Helicoverpa armigera</Emphasis>

Agrobacterium-mediated genetic transformation was performed using embryonic axes explants of pigeon pea. Both legume pod borer resistant gene (cry1Ac) and plant selectable marker neomycine phosphor transferase (nptII) genes under the constitutive expression of the cauliflower mosaic virus 35S promoter (CaMV35S) assembled in pPZP211 binary vector were used for the experiments. An optimum average of 44.61% successfully hardened dot blot Southern hybridization positive plants were obtained on co-cultivation media supplemented with 200 μM acetosyringone without L-cysteine. The increased transformation efficiency from a baseline of 11.53% without acetosyringone to 44.61% with acetosyringone was further declined with the addition of different concentrations of L-cysteine to co-cultivation media. Transgenic shoots were selected on 50 and 75 mg L⁻¹ kanamycin. Rooting efficiency was 100% on half-strength Murashige and Skoog medium with 20 g L⁻¹ sucrose and 0.5 mg L⁻¹ indole butyric acid in the absence of kanamycin. Furthermore, 100% seed setting was found among all the transgenic events. The plants obtained were subjected to multi- and nochoice tests to determine the behavioral responses and mortality through Helicoverpa armigera bioassays on the leaf and relate their relationship with the expression of cry1Ac protein which was found to be less in leaf as compared to the floral buds, anther, pod, and seed.     

Application of multi-phase experiments in plant pathology to identify genetic resistance to <Emphasis Type="Italic">Diaporthe toxica</Emphasis> in <Emphasis Type="Italic">Lupinus albus</Emphasis>

Phenotyping assays in plant pathology using detached plant parts are multi-phase experimental processes. Such assays involve growing plants in field or controlled-environment trials (Phase 1) and then subjecting a sample removed from each plant to disease assessment, usually under laboratory conditions (Phase 2). Each phase may be subject to non-genetic sources of variation. To be able to separate these sources of variation in both phases from genetic sources of variation requires a multi-phase experiment with an appropriate experimental design and statistical analysis. To achieve this, a separate randomization is required for each phase, with additional replication in Phase 2. In this article, Phomopsis leaf and pod blight (caused by Diaporthe toxica) of Lupinus albus was used as a case study to apply a multi-phase experimental approach to identify genetic resistance to this pathogen, and demonstrate the principles of sound experimental design and analysis in detached plant part assays. In seven experiments, 250 breeding lines, cultivars, landraces, and recombinant in-bred lines from a mapping population of L. albus were screened using detached, inoculated leaves, and/or pods. The experimental, non-genetic variance in Phase 2 varied in magnitude compared to the Phase 1 experimental, non-genetic variance. The reliability of prediction for resistance to Phomopsis pod blight was high (mean of 0.70 in seven experiments), while reliability of prediction for leaf assays was lower (mean 0.35–0.51 depending on the scoring method used).     

Generation means analysis of <Emphasis Type="Italic">wheat streak mosaic</Emphasis> virus resistance in winter wheat

Wheat streak mosaic virus (WSMV; Family: potyviridae; Genus: Tritimovirus) is a major threat to winter wheat (Triticum aestivum L. em. Thell) production worldwide, yet little is known about the genetic control of resistance. Our objective was to determine the mode of inheritance and type of gene action of WSMV resistance in two winter wheat crosses involving a resistant line, OK65C93-8, and two susceptible cultivars, Tandem and Vista. For each cross, parents, F₁, F₂, and backcross plants were inoculated and evaluated for WSMV resistance in two replicated greenhouse experiments. Generation means analysis indicated that additive, dominance, and epistatic effects were all involved in the inheritance of WSMV resistance. Broad-sense heritability estimates for visual symptom rating and ELISA values were high for both crosses (0.84–0.91). Narrow-sense heritability estimates were low in the Tandem/OK65C93-8 cross (0.43–0.45) and moderate in the Vista/OK65C93-8 cross (0.71–0.74). Due to the presence of greater non-additive gene effects combined with low narrow-sense heritability in the Tandem/OK65C93-8 cross, selecting for WSMV resistance in this cross would be complex if using conventional methods. On the other hand, the significant contribution of additive gene effects combined with moderate narrow-sense heritability in the Vista/OK65C93-8 cross suggested that it could be exploited to select for WSMV resistance. Progress from selection for WSMV resistance in early generations of winter wheat may vary among populations as indicated in this study. Therefore, evaluating genetic control of parental combinations may be warranted prior to selecting for WSMV resistance from this source.     

Mapping the <Emphasis Type="Italic">compactum</Emphasis> locus in wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) and its relationship to other spike morphology genes of the Triticeae

The spikes of club wheat are significantly more compact than spikes of common wheat due to the action of the dominant allele of the compactum (C) locus. Little is known about the location of C on chromosome 2D and the relationship between C and to other spike-compacting genes. Thus, a study was undertaken to place C on linkage maps and a chromosome deletion bin, and to assess its relatedness to the spike compacting genes zeocriton (Zeo) from barley and soft glume (Sog) from T. monococcum. Genetic mapping was based on recombinant inbred lines (RILs) from a cross between the cultivars Coda (club) and Brundage (common) and F₂ progeny from a cross between the club wheat Corrigin and a chromosome 2D substitution line [Chinese Spring (Ae. tauschii 2D)]. The C locus was flanked by Xwmc144 and Xwmc18 in the RIL population and it was completely linked to Xcfd116, Xgwm358 and Xcfd17 in the F₂ population. C could not be unambiguously placed to a chromosome bin because markers that were completely linked to C or flanked this locus were localized to chromosome bins on either side of the centromere (C-2DS1 and C-2DL3). Since C has been cytogenetically mapped to the long arm of chromosome 2D, we suspect C is located in bin C-2DL3. Comparative mapping suggested that C and Sog were present in homoeologous regions on chromosomes 2D and 2A^m, respectively. On the other hand, C and Zeo, on chromosome 2H, did not appear to be orthologous.     

Identification of molecular markers linked to adult plant leaf rust resistance gene <Emphasis Type="Italic">Lr48</Emphasis> in wheat and detection of <Emphasis Type="Italic">Lr48</Emphasis> in the Thatcher near-isogenic line with gene <Emphasis Type="Italic">Lr25</Emphasis>

The recessive adult plant resistance (APR) gene Lr48 in wheat was tagged with flanking random amplified polymorphic DNA (RAPD) markers. Markers S336₇₇₅ in coupling and S3₄₅₀ in repulsion with Lr48 were identified in wheat line CSP44. Tests of these markers on available Thatcher near-isogenic lines (NILs) detected the likely presence of Lr48 in TcLr25. A test of allelism of APR involving the cross TcLr25 × CSP44 indicated that Lr48 was present in both lines. A separate experiment on inheritance of resistance in an F₂ population of TcLr25 × Agra Local confirmed the presence of a dominant seedling resistance gene (Lr25) and a recessive APR gene (Lr48) in TcLr25. This study demonstrated the value of molecular markers in identifying the presence of masked genes in genetic stocks where direct phenotyping failed to detect their presence.     

Epistasis and heritability of resistance to <Emphasis Type="Italic">Phytophthora nicotianae</Emphasis> in pepper (<Emphasis Type="Italic">Capsicum annuum</Emphasis> L.)

Gene effects of resistance to two isolates of Phytophthora nicotianae in two crosses of pepper were investigated using separate generation means analysis. Additive-dominance models were inadequate in all cases. Digenic parameter models were adequate in three cases and the probability of goodness of fit of models was negatively correlated with the aggressiveness of the pathogen. None of these models explained variation among generation means in the combined cross Beldi × CM334 with P. nicotianae isolate Pn_2. Additive × additive, dominance × dominance and dominance × additive effects were significant in most cases. Additive and dominance effects (of negative sign) contribute more to resistance than to susceptibility. Additive variance was greater than environmental and dominance variance and ranged from 0.038 to 0.224. Narrow-sense heritabilities were dependent upon the cross and inoculate and ranged from 86 to 92%. The results of this study indicate that selection with more aggressive isolates of the pathogen will be useful for enhancing resistance in pepper.