Inheritance of resistance to Fusarium graminearum in wheat

To study the inheritance of resistance in wheat to Fusarium graminearum, six resistant cultivars from China were crossed to two susceptible cultivars. The parents and their progenies were evaluated in the greenhouse for resistance to the spread of scab within a spike. A central floret was inoculated by injecting a droplet of inoculum at the time of anthesis. Inoculated plants were kept in a moist chamber for three subsequent nights. The proportion of scabbed spikelets was recorded six-times from 3-days to 21-days after inoculation, and the area under the disease progress curve (AUDPC) was calculated from these proportions. One to three genes, depending on the cultivar, conditioned resistance to scab as reflected by the AUDPC. A simple additive-dominance effect model fitted the segregation data for 8 of the 11 crosses. Dominance and epistatic effects were significant in a few crosses. These effects increased resistance in some crosses but decreased resistance in others. However, relative to additive effects, dominant and epistatic effects accounted for only a small portion of the genetic effects in the populations evaluated. The importance of additive effects means that it should be possible to accumulate different genes to enhance resistance to scab in wheat. Received: 15 December 1998 / Accepted: 17 June 1999     

The genetics of rust resistance in sugar cane seedling populations

The inheritance of rust resistance was studied in sugar cane seedling populations using a factorial mating design over 1 summer and 2 winter seasons. Frequency distributions for rust infection pooled over 2 winter seasons for resistant x resistant parents were highly skewed with the majority of progenies grouped towards the resistant classes, whereas crosses between susceptible x highly susceptible parents tended to be skewed with the majority of progenies grouped towards the susceptible ones. Both categories of crosses produced transgressive segregants at either extremes. Distribution of infection within progeny of the selfed resistant parent ’R 570’ and distribution in the majority of crosses tended to support the hypothesis of a major gene with a dominant effect for resistance. However, the action of other minor genes acting in a quantitative way is also suggested. The female (F) and male (M) variance components were very important, and F×M interaction indicated the existence of non-additive genetic effects. F×S, M×S and F×M×S interaction mean squares were generally low or insignificant. Broad-sense heritability for the individual season ratings and for the combined ratings was high (0.75–0.90), whereas narrow-sense heritability was generally moderate (0.40–0.52) with the additive genetic effects accounting for 44–68% of the total genetic variation. The implications of these findings in the breeding for rust resistance in the local programme are discussed. Received: 2 June 1999 / Accepted: 22 June 1999     

The maize rp1 rust resistance gene identifies homologues in barley that have been subjected to diversifying selection

A number of agronomically important grasses (sorghum, wheat, panicum, sugar cane, oats, rice and barley) are shown to contain sequences homologous to rp1, a maize gene that confers race-specific resistance to the rust fungus Puccinia sorghi. Mapping of rp1-related sequences in barley identified three unlinked loci on chromosomes 1HL, 3HL and 7HS. The locus located on chromosome 7HS comprises a small gene family of at least four members, two of which were isolated and are predicted to encode nucleotide binding site-leucine-rich repeat (NBS-LRR) proteins that are respectively 58% and 60% identical to the maize rp1 protein. Evidence of positive selection for sequence diversification acting upon these two barley genes was observed; however, diversifying selection was restricted to the carboxy terminal half of the LRR domain. One of these rp1 homologous genes cosegregated with the barley Rpg1 stem rust resistance gene amongst 148 members of the Steptoe × Morex double haploid mapping family. Three other unrelated resistance gene-like sequences, potentially encoding NBS-LRR proteins, are also shown to be linked to the Rpg1 locus but not cosegregating with the gene. Received: 2 August 1999 / Accepted: 28 September 1999     

Sources of resistance to Exserohilum turcicum (Pass.) and Puccinia polysora (Underw.) incitant of Turcicum leaf blight and polysora rust of maize

A study was carried out to identify the sources of resistance to Turcicum leaf blight (TLB) and polysora rust in maize. Sixty indigenous and exotic inbred lines were evaluated under artificial epiphytotic conditions at two identified hot spot locations, Almora and Nagenahalli. After two years continuous screening, a total of 20 inbred lines were identified as a sources of resistance at both the locations for TLB, whereas 10 inbred lines were identified resistant against Polysora rust at Nagenahalli. A clear cut variation in virulence pattern in case of E. turcicum was observed. Some of the inbred inbred lines, i.e CM 138, CM 212, IML 235, NAI 135, showed resistant reaction in both the locations in the year 2005, whereas they acted as susceptible in 2006 at both locations. It was also observed that the Nagenahalli isolate of E. turcicum was more virulent than the Almora isolate.     

Gene-for-gene-mediated resistance to southern corn rust in maize

Southern corn rust (SCR) severely threatens maize production worldwide. Achieving durable control of SCR requires efficient breeding and deployment of resistant hybrids. Recently, two research teams (Chen et al. and Deng et al.) cloned two SCR resistance genes (RppC and RppK) and the cognate Avr genes (AvrRppC and AvrRppK), which will accelerate SCR resistance breeding.     

QTL mapping of resistance to Sporisorium reiliana in maize

We mapped and characterized quantitative trait loci (QTL) for resistance to Sporisorium reiliana. A population of 220 F₃ families produced from the cross of two European elite inbreds (D32, D145) was evaluated with two replications at a French location with high natural incidence of S. reiliana and at a Chinese location employing artificial inoculation. The 220 F₃ families were genotyped with 87 RFLP and seven SSR markers. Using composite interval mapping, we identified two different sets of 3 and 8 QTL for the French and the Chinese locations explaining 13% and 44% of respectively. Individual QTL explained up to 14% of σ^² _p. The 11 QTL mapped to eight maize chromosomes and displayed mostly additive or partial dominant gene action. Significant digenic epistatic interactions were detected for one pair of these QTL. Only a few QTL for S. reiliana were in common with QTL for resistance to Ustilago maydis and Puccinia sorghi, identified at a German location for the same population. Consequently, in our materials resistance to these three fungal pathogens of maize seems to be inherited independently. Received: 14. December 1998 / Accepted: 30 January 1999     

Identification of QTLs for partial resistance to leaf rust (Puccinia hordei) in barley

 The partial resistance to leaf rust in barley is a quantitative resistance that is not based on hypersensitivity. To map the quantitative trait loci (QTLs) for partial resistance to leaf rust, we obtained 103 recombinant inbred lines (RILs) by single-seed descent from a cross between the susceptible parent L94 and the partially resistant parent Vada. These RILs were evaluated at the seedling and adult plant stages in the greenhouse for the latent period (LP) of the rust fungus, and in the field for the level of infection, measured as area under the disease progress curve (AUDPC). A dense genetic map based on 561 AFLP markers had been generated previously for this set of RILs. QTLs for partial resistance to leaf rust were mapped using the “Multiple Interval Mapping” method with the putative QTL markers as cofactors. Six QTLs for partial resistance were identified in this population. Three QTLs, Rphq1, Rphq2 and Rphq3, were effective at the seedling stage and contributed approximately 55% to the phenotypic variance. Five QTLs, Rph2, Rphq3, Rphq4, Rphq5, and/or Rphq6 contributed approximtely. 60% of the phenotypic variance and were effective at the adult plant stage. Therefore, only the QTLs Rphq2 and Rhpq3 were not plant-stage dependent. The identified QTLs showed mainly additive effects and only one significant interaction was detected, i.e. between Rphq1 and Rphq2. The map positions of these QTLs did not coincide with those of the race-specific resistance genes, suggesting that genes for partial resistance and genes for hypersensitive resistance represent entirely different gene families. Also, three QTLs for days to heading, of which two were also involved in plant height, were identified in the present recombinant inbred population. These QTLs had been mapped previously on the same positions in different populations. The perspectives of these results for breeding for durable resistance to leaf rust are discussed. Received: 15 July 1997 / Accepted: 30 December 1997     

Molecular markers linked to white rust resistance in mustard Brassica juncea

 White rust, caused by Albugo candida (Pers.) Kuntze, is an economically important disease of Brassica juncea (L.) Czern. and Coss mustard, particularly in India. The most efficient and cost-effective way of protecting mustard plants from white rust disease is through genetic resistance. The objective of this study was to identify RAPD markers for white rust resistance in an F₁-derived doubled-haploid (DH) population originating from a cross between white rust-susceptible and white rust-resistant breeding lines of B. juncea from the canola-quality B. juncea breeding project of the Agriculture and Agri-Food Canada-Saskatoon Research Centre. The DH population was used to screen for RAPD markers associated with white rust resistance/susceptibility using bulked segregant analysis. Two markers, WR2 and WR3, linked to white rust resistance, flanked the resistance locus Ac2 ₁ and were highly effective in identifying the presence or absence of the resistance gene in the DH population. These two markers were shown to be specific to the Russian source of white rust resistance utilized in this project. It is concluded that the availability of these RAPD markers will enhance the breeding for white rust resistance in B. juncea. Received: 17 December 1997 / Accepted: 7 April 1998     

Application of synteny across Poaceae to determine the map location of a sugarcane rust resistance gene

A major rust resistance gene has been identified in a self-progeny of the sugarcane cultivar R570. Until now, this gene was known to be linked to a marker revealed by the sugarcane probe CDSR29 but unassigned to any linkage group of the current genetic map. We used synteny relationships between sugarcane and three other grasses in an attempt to saturate the region around this rust resistance gene. Comparison of sugarcane, sorghum, maize and rice genetic maps led to the identification of homoeologous chromosome segments at the extremity of sorghum linkage group D, rice linkage group 2, maize linkage group 4 and in the centromeric region of maize linkage group 5. One hundred and eighty-four heterologous probes were selected and tested for cross-hybridization with sugarcane DNA; 106 produced a good hybridization signal and were hybridized on 88 individuals of the R570 selfed progeny. Two hundred and seventeen single-dose markers were added to the R570 genetic map, of which 66% mapped to linkage group VII, together with the rust resistance gene. This gene has now been mapped to the end of a co-segregating group consisting of 19 RFLP markers. None of the mapped loci were located closer to the gene than CDSR29. The gene thus appears to reside at the edge of a ’’synteny cluster’’ used to describe the different grass genomes. Received: 12 January 2000 / Accepted: 21 March 2000     

Isolate-specific QTLs for partial resistance to Puccinia hordei in barley

By using a high-density AFLP marker linkage map, six QTLs for partial resistance to barley leaf rust (Puccinia hordei) isolate 1.2.1. have been identified in the RIL offspring of a cross between the partially resistant cultivar ’Vada’ and the susceptible line L94. Three QTLs were effective at the seedling stage, and five QTLs were effective at the adult plant stage. To study possible isolate specificity of the resistance, seedlings and adult plants of the 103 RILs from the cross L94×’Vada’ were also inoculated with another leaf rust isolate, isolate 24. In addition to the two QTLs that were effective against isolate 1.2.1. at the seedling stage, an additional QTL for seedling resistance to isolate 24 was identified on the long arm of chromosome 7. Of the eight detected QTLs effective at the adult plant stage, three were effective in both isolates and five were effective in only one of the two isolates. Only one QTL had a substantial effect at both the seedling and the adult plant stages. The expression of the other QTLs was developmental-stage specific. The isolate specificity of the QTLs supports the hypothesis of Parlevliet and Zadoks (1977) that partial resistance may be based on a minor-gene-for-minor-gene interaction. Received: 16 February 1999 / Accepted: 20 February 1999     

Genetic basis of resistance to sugarcane mosaic virus in European maize germplasm

 Sugarcane mosaic virus (SCMV) causes considerable damage to maize (Zea mays L.) in Europe. The objective of the present study was to determine the genetic basis of resistance to SCMV in European maize germplasm and to compare it with that of U.S. inbred Pa405. Three resistant European inbreds D21, D32, and FAP1360A were crossed with four susceptible inbreds F7, KW1292, D408, and D145 to produce four F₂ populations and three backcrosses to the susceptible parent. Screening for SCMV resistance in parental inbreds and segregating generations was done in two field trials as well as under greenhouse conditions. RFLP markers umc85, bnl6.29, umc10, umc44, and SSR marker phi075 were used in F₂ populations or F₃ lines to locate the resistance gene(s) in the maize genome. Segregation in the F₂ and backcross generations fitted to different gene models depending on the environmental conditions and the genotype of the susceptible parent. In the field tests, resistance in the three resistant European inbreds seems to be controlled by two to three genes. Under greenhouse conditions, susceptibility to SCMV in D32 appears to be governed by one dominant and one recessive gene. Allelism tests indicated the presence of a common dominant gene (denoted as Scm1) in all three resistant European inbreds and Pa405. Marker analyses mapped two dominant genes: Scm1 on chromosome 6S and Scm2 on chromosome 3. Received: 17 November 1997 / Accepted: 25 November 1997     

Quantitative trait loci for resistance against Yellow rust in two wheat-derived recombinant inbred line populations

Yellow rust, which is a major disease in areas where cool temperatures prevail, can strongly influence grain yield. To control this disease, breeders have extensively used major specific resistance genes. Unfortunately this kind of resistance is rapidly lost due to pathogen adaptation. More-durable resistance against yellow rust can be achieved using quantitative resistance derived from cultivars with well-established durable resistance. The winter wheat Camp Remy has maintained a high level of resistance for over 20 years. In order to map quantitative trait loci (QTLs) for durable yellow rust resistance, we analysed a set of 98 F₈ recombinant inbred (RI) lines derived from the cross Camp Remy×Michigan Amber. We also mapped QTLs for adult resistance to yellow rust using the International Triticae Mapping Initiative RI population (114 lines derived from the cross Opata85×synthetic hexaploid). Two and five QTLs, respectively, were identified from these two populations. This work has highlighted the importance of the centromeric region of chromosome 2B and the telomeric regions of chromosomes 2AL and 7DS in durable yellow rust resistance. The same chromosomal regions are also implicated in resistance to other pathogens. Received: 8 December 2000 / Accepted: 17 April 2001     

Identification of molecular markers linked to quantitative trait loci for soybean resistance to corn earworm

 One hundred and thirty nine restriction fragment length polymorphisms (RFLPs) were used to construct a soybean (Glycine max L. Merr.) genetic linkage map and to identify quantitative trait loci (QTLs) associated with resistance to corn earworm (Helicoverpa zea Boddie) in a population of 103 F₂-derived lines from a cross of ‘Cobb’ (susceptible) and PI229358 (resistant). The genetic linkage map consisted of 128 markers which converged onto 30 linkage groups covering approximately 1325 cM. There were 11 unlinked markers. The F₂-derived lines and the two parents were grown in the field under a plastic mesh cage near Athens, Ga., in 1995. The plants were artificially infested with corn earworm and evaluated for the amount of defoliation. Using interval-mapping analysis for linked markers and single-factor analysis of variance (ANOVA), markers were tested for an association with resistance. One major and two minor QTLs for resistance were identified in this population. The PI229358 allele contributed insect resistance at all three QTLs. The major QTL is linked to the RFLP marker A584 on linkage group (LG) ‘M’ of the USDA/Iowa State University public soybean genetic map. It accounts for 37% of the total variation for resistance in this cross. The minor QTLs are linked to the RFLP markers R249 (LG ‘H’) and Bng047 (LG ‘D1’). These markers explain 16% and 10% of variation, respectively. The heritability (h²) for resistance was estimated as 64% in this population. Received: 15 October 1997 / Accepted: 4 November 1997     

Localising QTLs for leaf rust resistance and agronomic traits in barley (Hordeum vulgare L.)

The Hordeum vulgare accession ’HOR 1063’ was crossed with the barley cultivar Krona, and 220 doubled haploid lines were produced based on this cross. A molecular map was constructed based on RFLP markers. Field trials were performed over 2 years and at two locations. In field trials, resistance to leaf rust by means of artificial infection, heading date, plant height and Kernel weight were assessed. For leaf rust resistance, 4 QTLs were localised, that explained 96.1% of the genetic variation. One QTL on chromosome 4H confirmed a position found in another genetic background and one mapped to the same position as Rph16 on chromosome 2H. All digenic effects decreased the effects of the respective QTLs. In addition to the denso-locus and the hex-v locus, other QTLs influencing heading date, plant length and kernel weight were found in this cross. Received: 16 July 1999 / Accepted: 7 September 1999     

Cytogenetic and molecular mapping of the leaf rust resistance gene Lr39 in wheat

Leaf rust, caused by the fungus Puccinia triticina Eriks,is one of the most serious diseases of wheat (Triticum aestivum AABBDD, 2n=6x=42) worldwide. Growing resistant cultivars is an efficient and economical method of reducing losses to leaf rust. Here we report a new leaf rust resistance gene, Lr39, transferred from Aegilops tauschii into common wheat. Lr39 conditions both seedling and adult plant resistance to the leaf rust pathogen. The inter- and intra-chromosomal mapping of the Lr39 gene showed that it is different from all previously described Lr genes. We used monosomic analysis for the inter-chromosomal mapping and wheat microsatellite markers for the intra-chromosomal mapping. The monosomic and ditelosomic analysis indicated that Lr39 is independent of the centromere on the short arm of chromosome 2D. Eight microsatellite markers for 2DS were used for linkage analysis on a population of 57 F₂ plants derived from a cross of an Ae. tauschii-derived wheat, cv. Wichita line TA4186 (possessing Lr39), with Wichita monosomics for the D-genome chromosomes. The microsatellite marker analysis confirmed the location of the gene on 2DS. Three markers were polymorphic and linked to the gene. The closest marker Xgwm210 mapped 10.7 cM from Lr39. The location of Lr39 near the telomere of 2DS distinguishes it from the Lr2 and Lr22 loci, which are located on 2DS proximal to Xgwm210. Received: 19 April 2000 / Accepted: 15 May 2000     

Fine mapping and DNA marker-assisted pyramiding of the three major genes for blast resistance in rice

Three major genes (Pi1, Piz-5 and Pita) for blast resistance on chromosomes 11, 6 and 12, respectively, were fine-mapped and closely linked RFLP markers identified. New markers for Pi1 and Pita were found that were flanking the genes. The three genes were pyramided using RFLP markers. A PCR-based SAP (sequence amplified polymorphism) marker was used to identify Piz-5 in the segregating population. The plants carrying the two- and three-gene combinations that were tested for resistance to leaf blast in the Philippines and India indicated that combinations including Piz-5 have enhanced resistance than when it is present alone. The genes from the pyramided lines are at present being deployed into agronomically superior ricevarieties by marker-aided selection (MAS). Received: 20 June 1997 / Accepted: 14 September 1999     

The detection and molecular mapping of a major gene for non-specific adult-plant disease resistance against stripe rust (Puccinia striiformis) in wheat

A major gene determining non-specific adult-plant disease resistance against stripe rust (Puccinia striiformis) designated Yrns-B1 was mapped by using a cross between ’Lgst.79–74’ (resistant) and ’Winzi’ (susceptible). Analyzing F₃ lines of two consecutive experimental years contrary modes of inheritance were observed due to the intermediate character of the gene and the difference in the disease pressure during the seasons. Using the disease scoring data of both experimental years independently two maps were constructed detecting Yrns-B1 20.5 and 21.7 cM, respectively, proximal to the wheat microsatellite (WMS) marker Xgwm493 on the short arm of chromosome 3BS. The genetic relationships to other major genes or to quantitative trait loci controlling adult plant disease resistance against rusts in wheat are discussed. Received: 27 May 1999 / Accepted: 28 September 1999     

Inheritance of adult field resistance to yellow rust disease among broad-based hexaploid spring wheat germplasm

 Complete F₁ and F₂ diallel crosses were used to investigate the inheritance of yellow rust resistance among eight bread wheat lines, developed by CIMMYT for the East African Highlands, which showed a wide response to this disease. Both diallel sets were grown at a site with a high incidence of yellow rust, although for one season, during which the F₁ diallel was grown, disease incidence was unusually low. Analyses disclosed the presence of additive, dominance and epistatic effects among those genes controlling rust resistance, with the former being the most important. At normal disease levels, excluding two arrays having resistant common parents removed non-allelic interactions from the F₁ diallels. For all F₂ diallels, and the remaining F₁ diallel, omitting two arrays based on susceptible parents removed these interactions. Local selection of material from a broadly based germplasm appears to be a feasible method of developing adapted cultivars resistant to endemic diseases. Received: 1 March 1998 / Accepted: 19 March 1998     

Relationships among maize inbred lines and populations from European and North-American origins as estimated using RFLP markers

RFLP markers have proven to be a reliable and highly informative tool for characterizing genetic diversity in maize. Joint analysis of inbred lines and populations should provide valuable information with respect to (1) a better understanding of the genetic basis of present elite germplasm and (2) the identification of populations that may prove to be useful sources of genetic diversity for breeding programs. Sixty-two inbred lines of known heterotic groups and ten maize populations, some of them significant contributors to the genetic basis of the heterotic groups, were assayed at 28 RFLP loci. Joint data analyses first underlined that the populations displayed a large number of alleles that were absent in the set of inbred lines. Associations among inbreds and populations further proved consistent with pedigree data of the inbreds and provided new information on the genetical basis of heterotic groups. In particular, European flint inbreds were revealed to be as close to the Northeastern U.S. flint population studied as to the typical European populations. These results advocate the analysis of larger sets of populations by means of molecular markers in order to (1) gain insight into the history of maize germplasm and (2) set up appropriate strategies for the use of genetic resources in breeding programs. Received: 23 February 1998 / Accepted: 5 February 1999     

Inheritance of resistance in maize silks to the corn earworm

The corn earworm,Helicoverpa zea (Boddie), is a perennial economic pest of field crops in the United States. Maize,Zea mays L., is the major host crop promoting the build-up of devastating corn earworm populations that limit full production of cotton, soybean, peanut, and grain sorghum. Resistance to the corn earworm in maize and in particular sweet maize, would provide an environmentally safe, economical method of control for this pest insect. Antibiotic effects of corn silks on this insect are: small larvae, extended developmental period, and reduced fecundity. Silks from individual maize plants of resistant and susceptible lines and progeny in six generations consisting of parents (P₁, P₂), F₁, F₂, and backcrosses BC_1.1 (F₁ × P₁) and BC_1.2 (F₁ × P₂) from each of four crosses were used to determine the genetic basis of the antibiotic resistance of silks to the corn earworm. In the cross of Zapalote Chico × PI340856, genes controlling resistance in the silks to the corn earworm larvae are dominant in PI340856 to those in Zapalote Chico. The cross of Zapalote Chico × GT114 involves parents differing in degree of resistance, and possibly differing for the genetic mechanism by which the resistance is inherited. The inheritance of resistance may involve non-additive (dominance and epistasis) genetic variance. A digenic 6-parameter model indicated (1) the resistance in this cross is controlled by more than one pair of genes and (2) some or all of the genes interact to cause non-allelic interaction. Thus, the resistance in this cross may be controlled by both dominant and recessive genes. The resistance of Zapalote Chico × CI64, an intermediate inbred, is influenced by additive gene effects. The digenic model adequately predicts all generation means of the cross of GT3 × PI340856 except for the F1. Thus, it appears that the additive-dominance model is not satisfactory for this cross involving susceptible and resistant parents. Generation mean analysis indicates that resistance to silk-feeding by corn earworm larvae is under genetic control, but gene action differs from one type of cross to another.