Identification and mapping of a gene conferring resistance to the spot form of net blotch (Pyrenophora teres f maculata) in barley

 Spot form of net blotch (SFNB) (Pyrenophora teres f maculata) is an economically damaging foliar disease of barley in many of the world’s cereal growing areas. The development of SFNB-resistant cultivars may be accelerated through the use of molecular markers. A screen for SFNB resistance in 96 lines identified four new sources of resistance, including a feed variety, ‘Galleon’, for which a fully mapped doubled haploid population was available. Segregation data indicated SFNB resistance was conferred by a single gene in the ‘Galleon’בHaruna Nijo’ cross, positioned on the long arm of chromosome 7H. This gene is designated Rpt4 and is flanked by the RFLP loci Xpsr117(D) and Xcdo673 at distances of 6.9 cM and 25.9 cM, respectively. The marker Xpsr117(D) was validated using another population segregating for Rpt4, correctly predicting SFNB resistance with more than 90% accuracy. Received: 24 September 1998 / Accepted: 19 December 1998     

Molecular mapping of genes conferring aluminum tolerance in rice (Oryza sativa L.)

Crop productivity on acid soil is restricted by multiple abiotic stress factors. Aluminum (Al) tolerance seems to be a key to productivity on soil with a pH below 5.0, but other factors such as Mn toxicity and the deficiency of P, Ca and Mg also play a role. The development of Al-tolerant genotypes of rice is an urgent necessity for improving crop productivity in developing countries. Inhibition of root growth is a primary and early symptom of Al toxicity. The present study was conducted to identify genetic factors controlling the aluminum tolerance of rice. Several parameters related to Al tolerance, most importantly the relative root growth under Al stress versus non-stress conditions, were scored in 188 F₃ selfed families from a cross between an Al-tolerant Vietnamese local variety, Chiembau, and an Al-susceptible improved variety, Omon269–65. The two varieties are both Oryza sativa ssp. indica, but showed a relatively high level of DNA polymorphism, permitting the assembly of an RFLP map consisting of 164 loci spanning 1,715.8 cM, and covering most of the rice genome. A total of nine different genomic regions on eight chromosomes have been implicated in the genetic control of root and shoot growth under aluminum stress. By far the greatest effects on aluminum tolerance were associated with the region near WG110 on chromosome 1. This region does not seem to correspond to most of the genes that have been mapped for aluminum tolerance in other species, nor do they correspond closely to one another. Most results, both from physiological studies and from molecular mapping studies, tend to suggest that aluminum tolerance is a complex multi-genic trait. The identification of DNA markers (such as WG110) that are diagnostic for aluminum tolerance in particular gene pools provides an important starting point for transferring and pyramiding genes that may contribute to the sustainable improvement of crop productivity in aluminum-rich soils. The isolation of genes responsible for aluminum tolerance is likely to be necessary to gain a comprehensive understanding of this complex trait. Received: 29 March 2000 / Accepted: 16 August 2000     

Identification and mapping of a new leaf stripe resistance gene in barley (Hordeum vulgare L.)

Pyrenophora graminea is the seed-borne pathogen causal agent of barley leaf stripe disease. Near-isogenic lines (NILs) carrying resistance of the cv ”Thibaut” against the highly virulent isolate Dg2 were obtained by introgressing the resistance into the genetic background of the susceptible cv ”Mirco”. The segregation of the resistance gene was followed in a F₂ population of 128 plants as well as on the F₃ lines derived from the F₂ plants; the segregation fitted the 1:2:1 ratio for a single gene. By using NILs, a RAPD marker associated with the resistance gene was identified; sequence-specific (STS) primers were designed on the basis of the amplicon sequence and a RILs mapping population with an AFLP-based map were used to position this molecular marker to barley chromosome 1 S (7HS). STS and CAPS markers were developed from RFLPs mapped to the telomeric region of barley chromosome 7HS and three polymorphic PCR-based markers were developed. The segregation of these markers was followed in the F₂ population and their map position with respect to the resistance gene was determined. Our results indicate that the Thibaut resistance gene, which we designated as Rdg2a, maps to the telomeric region of barley chromosome 7HS and is flanked by the markers OPQ-9₇₀₀ and MWG 2018 at distances of 3.1 and 2.5 cM respectively. The suitability of the PCR-based marker MWG2018 in selection- assisted barley breeding programs is discussed. Received: 22 June 2000 / Accepted: 16 October 2000     

RFLP mapping of the ym4 virus resistance gene in barley

RFLP (restriction fragment length polymorphism) mapping of a recessive gene (ym4) conferring resistance to barley yellow mosaic and barley mild mosaic virus was performed using progeny of 86 F₁ anther-derived doubled haploid lines. Two closely linked RFLP markers that flank the gene at a distance of 1.2 centiMorgans were identified. Using one of these markers (MWG10) we obtained a clear differentiation between resistant and susceptible German cultivars. An analysis of a series of unrelated barley lines with probe MWG10 did not reveal additional RFLP fragments. The use of this probe for both marker-assisted selection and the generation of a high-density map around the resistance locus is discussed.     

Molecular mapping of a new gene in wild barley conferring complete resistance to leaf rust (Puccinia hordei Otth)

 A dominant gene conferring resistance to all known races of Puccinia hordei Otth was identified in two accessions of Hordeum vulgare ssp. spontaneum. Using restriction fragment length polymorphism (RFLP) markers the gene was mapped on chromosome 2HS in doubled-haploid populations derived from crosses of both accessions to the susceptible cultivar L94. Until now, complete leaf rust resistance was not known to be conditioned by genetic factors on this barley chromosome. Therefore, the designation Rph16 is proposed for the gene described in this study. A series of sequence tagged site (STS) and cleaved amplified polymorphic sequence (CAPS) markers were generated by conversion of RFLP probes which originate from the chromosomal region carrying the resistance gene. Two PCR-based markers were shown to co-segregate with the Rph16 gene in both populations thus providing the basis for marker-assisted selection. Received: 20 May 1998 / Accepted: 9 June 1998     

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     

Temporal changes in allele frequencies in two reciprocally selected maize populations

The effects of breeding on allele frequency changes at 82 restriction fragment length polymorphism (RFLP) loci were examined in two maize (Zea mays L.) populations undergoing reciprocal recurrent selection, Iowa Stiff Stalk Synthetic and Iowa Corn Borer Synthetic #1. After 12 cycles of selection, approximately 30% of the alleles were extinct and 10% near fixation in each population. A test of selective neutrality identified several loci in each population whose allele frequency changes cannot be explained by genetic drift; interpopulation mean expected heterozygosity increased for that subset of 28 loci but not for the remaining 54 loci. Mean expected heterozygosity within the two subpopulations decreased 39%, while the between-population component of genetic variation increased from 0.5% to 33.4% of the total. Effective population size is a key parameter for discerning allele frequency changes due to genetic drift versus those resulting from selection and genetic hitchhiking. Empirical estimates of effective population size for each population were within the range predicted by the breeding method. Received: 10 June 1998 / Accepted: 29 April 1999     

Response to potyvirus infection and genetic mapping of resistance loci to potyvirus infection in Lactuca

 We have investigated the interaction between two different potyviruses and resistant cultivars of Lactuca sativa. Turnip mosaic virus (TuMV) and lettuce mosaic virus (LMV) were used to inoculate several cultivars under different temperature regimes to characterize the resistance reaction. Resistance conferred by the recessive mo locus against LMV infection did not provide immunity. Virus accumulated in plant tissues to different levels depending on the genetic background of the cultivar, suggesting that several genes were involved in the resistance phenotype. Under temperature regimes that enhanced the hypersensitive reaction, resistant cultivars produced necrotic reactions. In contrast, resistance to TuMV infection conferred by the dominant Tu locus resulted in complete immunity in the plant. No virus accumulated in inoculated leaves nor was any necrotic reaction observed. The resistance loci were characterized at the genetic level by mapping them relative to molecular markers. Only weak linkages could be identified to mo, again supporting the hypothesis that several genes are involved. The Tu locus was mapped in two different crosses relative to several markers, the closest two linked at less than 1 cM. A high-resolution genetic map of the Tu locus was constructed by screening 500 F₂ individuals for recombinants around that locus. Received: 4 June 1996/Accepted: 15 November 1996     

Inheritance and genetic mapping of cucumber mosaic virus resistance introgressed from Lycopersicon chilense into tomato

Cucumber mosaic virus (CMV) infects a wide variety of crop plants and in tomato (Lycopersicon esculentum Mill.) causes significant economic losses in many growing regions, particularly the Mediterranean. The objective of the present study was to identify the number and map locations of genes controlling resistance to CMV in breeding lines (BC₁–inbreds) derived from the related wild species L. chilense. These lines also carried the gene Tm-2 ^a for resistance to ToMV, which facilitated the interpretation of disease symptoms. The segregation for CMV resistance in the BC₂F₁ and BC₂F₂ generations, following mechanical inoculation with subgroup-I isolates, was consistent with expectations for a single dominant gene, for which the symbol Cmr (cucumber mosaic resistance) was given. Resistant and susceptible BC₁-inbreds were analyzed with RFLP and isozyme markers to identify genomic regions introgressed from L. chilense. The only L. chilense-specific markers found were on chromosome 12; some resistant lines contained a single introgression comprising the entire short arm and part of the long arm of this chromosome, while others contained a recombinant derivative of this introgression. The chromosome 12 markers were significantly associated with CMV resistance in both qualitative and quantitative models of inheritance. The qualitative analysis, however, demonstrated that CMV resistance was not expressed as a reliable monogenic character, suggesting a lack of penetrance, significant environmental effects, or the existence of additional (undetected) resistance factors. In the quantitative analysis, the marker interval TG68 – CT79 showed the most significant association with CMV resistance. No association between CMV resistance and the Tm-2 ^a gene was observed. These breeding lines are potentially useful sources of CMV resistance for tomato improvement, in which context knowledge of the map location of Cmr should accelerate introgression by marker-assisted selection. Received: 9 August 1999 / Accepted: 22 December 1999     

Generation-means analysis and quantitative trait locus mapping of anthracnose stalk rot genes in maize

A generation-means analysis was performed on two maize populations, each segregating for genes conferring resistance to anthracnose stalk rot (ASR). The populations were derived from a cross of DE811ASR x DE811 and of DE811ASR x LH132. The resistant parent, DE811ASR, was obtained through introgression with MP305 as the donor and DE811 as the recurrent parent. The analysis revealed significant additive effects in both populations and a significant additive x dominant effect in the DES11ASR x DES11 population. Quantitative trait locus (QTL) mapping, using restriction fragment length polymorphism (RFLP)-based molecular markers, indicated a significant QTL on linkage group 4 in both populations. The QTL analysis confirmed additive inheritance in both populations. This work demonstrates a close correspondence between generation-means analysis and discrete observations using molecular markers. Linkage of a genetic marker to genes conferring resistance to ASR will be useful for the introgression of resistance into elite germplasm.This research was part of a thesis submitted by the first author in partial fulfillment of the requirements for a MS degree. Published as Miscellaneous Paper number 1491 of the Delaware Agricultural Experiment Station     

Molecular mapping and physical location of major gene conferring seedling resistance to Septoria tritici blotch in wheat

Septoria tritici blotch (STB) caused by Mycosphaerella graminicola (anamorph: Septoria tritici), is one of the most important foliar diseases of wheat. We assessed three doubled-haploid (DH) populations derived from Chara (STB-susceptible)/WW2449 (STB-resistant), Whistler (STB-susceptible)/WW1842 (STB-resistant) and Krichauff (STB susceptible)/WW2451 (STB-resistant) for resistance to a single-pycnidium isolate 79.2.1A of M. graminicola at the seedling stage. STB resistance in each of the three DH populations was conditioned by a single major gene designated as StbWW2449, StbWW1842 and StbWW2451. Linkage analyses and physical mapping indicated that the StbWW loci were located on the short arm of chromosome 1B (IBS). Four simple sequence repeat (SSR) markers linked with STB resistance: Xwmc230, Xbarc119b, Xksum045 and Xbarc008 were located to the distal bin of 1BS.sat1BS-4 (FL: 0.52–1.00) in the 1BS physical map. Xwmc230, Xbarc119b and Xksum045 markers, mapped within 7 cM from StbWW were validated for their linkage and predicted the STB resistance with over 94% accuracy in the 79 advanced breeding lines having WW2449 as one of the parents. The marker interval Xwmc230/Xksum045-Xbarc119b also explained up to 38% of the phenotypic variance at the adult plant stage in all three DH mapping populations. These results have proven that SSR markers are useful in monitoring STB resistance both at seedling and adult plant stages and hence are suitable for routine marker-assisted selection in the wheat breeding programs. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Characterisation of resistance to turnip mosaic virus in oilseed rape (Brassica napus) and genetic mapping of TuRB01

Turnip mosaic virus (TuMV) is the major virus infecting Brassica crops. A dominant gene, TuRB01, that confers extreme resistance to some isolates of TuMV on Brassica napus (oilseed rape), has been mapped genetically. The mapping employed a set of doubled-haploid lines extracted from a population used previously to develop a reference RFLP map of the B. napus genome. The positioning of TuRB01 on linkage group N6 of the B. napus A–genome indicated that the gene probably originated from Brassica rapa. Resistance phenotypes were confirmed by indirect plate-trapped antigen ELISA using a monoclonal antibody raised against TuMV. The specificity of TuRB01 was determined using a wide range of TuMV isolates, including representatives of the European and American/Taiwanese pathotyping systems. Some isolates of TuMV that did not normally infect B. napus plants possessing TuRB01 produced mutant viruses able to overcome the action of the resistance gene. TuRB01 is the first gene for host resistance to TuMV to be mapped in a Brassica crop. A second locus, TuRB02, that appeared to control the degree of susceptibility to the TuMV isolate CHN 1 in a quantitative manner, was identified on the C-genome linkage group N14. The mapping of other complementary genes and the selective combining of such genes, using marker-assisted breeding, will make durable resistance to TuMV a realisable breeding objective. Received: 14 December 1998 / Accepted: 10 April 1999     

Mapping of the QTL (quantitative trait locus) conferring partial resistance to leaf blast in rice cultivar Chubu 32

The rice cultivar Chubu 32 possesses a high level of partial resistance to leaf blast. The number and chromosomal location of genes conferring this resistance were detected by restriction fragment length polymorphism (RFLP) linkage mapping and quantitative trait locus (QTL) analysis. For the mapping, 149 F₃ lines derived from the cross between rice cultivar Norin 29, with a low level of partial resistance, and Chubu 32 were used, and their partial resistance to leaf blast was assessed in upland nurseries. A linkage map covering six chromosomes and consisting of 36 RFLP markers was constructed. In the map, only one significant QTL (LOD>2.0) for partial resistance was detected on chromosome 11. This QTL explained 45.6% of the phenotypic variation. The segregation ratio of the F3 lines was 3:1 for partial resistance to susceptibility. These results suggest that the partial resistance in Chubu 32 is controlled by a major gene. Received: 15 March 2001 / Accepted: 13 August 2001     

Localization by restriction fragment length polymorphism mapping in potato of a major dominant gene conferring resistance to the potato cyst nematode Globodera rostochiensis

Summary A major dominant locus conferring resistance against several pathotypes of the root cyst nematode Globodera rostochiensis was mapped on the linkage map of potato using restriction fragment length polymorphism (RFLP) markers. The assessment of resistance versus susceptibility of the plants in the experimental population considered was based on an in vivo (pot) and an in vitro (petri dish) test. By linkage to nine RFLP markers the resistance locus Gro1 was assigned to the potato linkage group IX which is homologous to the tomato linkage group 7. Deviations from the additivity of recombination frequencies between Gro1 and its neighbouring markers in the pot test led to the detection of a few phenotypic misclassifications of small plants with poor root systems that limited the observation of cysts on susceptible roots. Pooled data from both tests provided better estimates of recombination frequencies in the linkage interval defined by the markers flanking the resistance locus.     

Molecular markers for rust and pyricularia leaf spot disease resistance in pearl millet

 Pearl millet [Pennisetum glaucum (L.) R.Br.] is a warm-season grass used for food, feed, fodder and forage, primarily in countries of Africa and India but grown around the world. The two most-destructive diseases to pearl millet in the United States are rust (caused by Puccinia substriata var. indica) and pyricularia leaf spot (caused by Pyricularia grisea). Genes for disease resistance to both pathogens have been transferred into agronomically acceptable forage and grain cultivars. A study was undertaken to identify molecular markers for three rust loci and one pyricularia resistance locus. Three segregating populations were screened for RAPDs using random decamer primers and for RFLPs using a core set of probes detecting single-copy markers on the pearl millet map. The rust resistance gene Rr ₁ from the pearl millet subspecies P. glaucum ssp. monodii was linked 8.5 cM from the RAPD OP-G8₃₅₀. The linkage of two RFLP markers, Xpsm108 (15.5 cM) and Xpsm174 (17.7 cM), placed the Rr ₁ gene on linkage-group 3 of the pearl millet map. Rust resistance genes from both Tift 89D₂ and ICMP 83506 were placed on linkage-group 4 by determining genetic linkage to the RFLP marker Xpsm716 (4.9 and 0.0 cM, respectively). Resistance in ICMP 83506 was also linked to the RFLP marker Xpsm306 (10.0 cM), while resistance in Tift 89D₂ was linked to RAPD markers OP-K19₃₅₀ (8.8 cM) and OP-O8₃₅₀ (19.6 cM). Fragments from OP-K19 and OP-O8 in the ICMP 83506 population, and Xpsm306 in the Tift 89D₂ population, were monomorphic. Only one RAPD marker (OP-D11₇₀₀, 5.6 cM) was linked to pyricularia leaf spot resistance. Attempts to detect polymorphisms with rice RFLP probes linked to rice blast resistance (Pyricularia oryzae; syn=P. grisea) were unsuccessful. Received: 19 May 1997 / Accepted: 21 October 1997     

Genetic mapping of maize streak virus resistance from the Mascarene source. I. Resistance in line D211 and stability against different virus clones

Maize streak virus (MSV) disease may cause significant grain yield reductions in maize in Africa. Réunion island maize germplasm is a proven source of strong resistance. Its genetic control was investigated using 123 RFLP markers in an F₂ population of D211 (resistant) × B73 (susceptible). This population of 165 F_2:3 families was carefully evaluated in Harare (Zimbabwe) and in Réunion. Artificial infestation was done with viruliferous leafhoppers. Each plant was rated weekly six times after infestation on a 1–9 scale previously adjusted by image analysis. QTL analyses were conducted for each scoring date, and for the areas under the disease, incidence and severity progress curves. The composite interval mapping method used allowed the estimation of the additive and dominance effects and QTL × environment interactions. Heritabilities ranged from 73% to 98%, increasing with time after infestation. Resistance to streak virus in D211 was provided by one region on chromosome 1, with a major effect, and four other regions on chromosomes 2, 3 (two regions) and 10, with moderate or minor effects. Overall, they explained 48–62% of the phenotypic variation for the different variables. On chromosome 3, one of the two regions seemed to be more involved in early resistance, whereas the second was detected at the latest scoring date. Other QTLs were found to be stable over time and across environments. Mild QTL × environment interactions were detected. Global gene action appeared to be partially dominant, in favor of resistance, except at the earliest scoring dates, where it was additive. From this population, 32 families were chosen, representing the whole range of susceptibility to MSV. They were tested in Réunion against three MSV clones, along with a co-inoculation of two of them. Virulence differences between clones were significant. There were genotype × clone interactions, and these were more marked for disease incidence than for severity. Although these interactions were not significant for the mean disease scores, it is suggested that breeders should select for completely resistant genotypes. Received: 15 June 1998 / Accepted: 30 January 1999     

Identification of quantitative trait loci associated with resistance to cucumber mosaic virus in Capsicum annuum

QTL analysis for resistance to cucumber mosaic virus (CMV) was performed in an intraspecific Capsicum annuum population. A total of 180 F3 families were derived from a cross between the susceptible bell-type cultivar Maor and the resistant small-fruited Indian line Perennial and inoculated with CMV in three experiments carried out in the USA and Israel using two virus isolates. Mostly RFLP and AFLP markers were used to construct the genetic map, and interval analysis was used for QTL detection. Four QTL were significantly associated with resistance to CMV. Two digenic interactions involving markers with and without an individual effect on CMV resistance were also detected. The QTL controlling the largest percentage (16–33%) of the observed phenotypic variation (cmv11.1) was detected in all three experiments and was also involved in one of the digenic interactions. This QTL is linked to the L locus that confers resistance to tobacco mosaic virus (TMV), confirming earlier anecdotal observations of an association between resistance to CMV and susceptibility to TMV in Perennial. An advanced backcross breeding line from an unrelated population, 3990, selected for resistance to CMV was analyzed for markers covering the genome, allowing the identification of genomic regions introgressed from Perennial. Four of these introgressions included regions associated with QTL for CMV resistance. Markers in two genomic regions that were identified as linked to QTL for CMV resistance were also linked to QTL for fruit weight, confirming additional breeding observations of an association between resistance to CMV originating from Perennial and small fruit weight. Received: 17 July 2000 / Accepted: 16 October 2000     

RFLP mapping of QTLs conferring salt tolerance during the vegetative stage in tomato

 Quantitative trait loci (QTLs) contributing to salt tolerance during the vegetative stage in tomato were investigated using an interspecific backcross between a salt-sensitive Lycopersicon esculentum breeding line (NC84173, maternal and recurrent parent) and a salt-tolerant Lycopersicon pimpinellifolium accession (LA722). One hundred and nineteen BC₁ individuals were genotyped for 151 RFLP markers and a linkage map was constructed. The parental lines and 119 BC₁S₁ families (self-pollinated progeny of the BC₁ individuals) were evaluated for salt tolerance in aerated saline-solution cultures with the salt concentration gradually raised to 700 mM NaCl+70 mM CaCl₂ (equivalent to an electrical conductivity of approximately 64 dS/m and a water potential of approximately −35.2 bars). The two parental lines were distinctly different in salt tolerance: 80% of the LA722 plants versus 25% of the NC84173 plants survived for at least 2 weeks after the final salt concentration was reached. The BC₁S₁ population exhibited a continuous variation, typical of quantitative traits, with the survival rate of the BC₁S₁ families ranging from 9% to 94% with a mean of 51%. Two QTL mapping techniques, interval mapping (using MAPMAKER/QTL) and single-marker analysis (using QGENE), were used to identify QTLs. The results of both methods were similar and five QTLs were identified on chromosomes 1 (two QTLs), 3, 5 and 9. Each QTL accounted for between 5.7% and 17.7%, with the combined effects (of all five QTLs) exceeding 46%, of the total phenotypic variation. All QTLs had the positive QTL alleles from the salt-tolerant parent. Across QTLs, the effects were mainly additive in nature. Digenic epistatic interactions were evident among several QTL-linked and QTL-unlinked markers. The overall results indicate that tomato salt tolerance during the vegetative stage could be improved by marker-assisted selection using interspecific variation. Received: 4 January 1999 / Accepted: 4 January 1999     

Saturation mapping of a major gene for resistance to white pine blister rust in sugar pine

 The molecular basis of resistance to diseases in plants can be better understood if the genes coding for resistance can be cloned. The single major dominant gene (R) that confers resistance to the white pine blister rust fungus (Cronartium ribicola Fisch.) in sugar pine (Pinus lambertiana Dougl.) has been previously mapped. The objectives of the present study were to saturate the region flanking R with tightly linked markers and to construct genetic maps for each of four individual seed trees. Bulked segregant analysis (BSA) and haploid segregation analysis were employed to identify random amplified polymorphic DNA (RAPD) markers linked to R. Automated PCR analysis was used to assay 1115 primers with susceptible and resistant DNA pools from each of four seed trees (8920 PCR reactions). Thirteen RAPD loci were identified that were linked to R. The linkage analyses programs JoinMap 1.4 and Mapmaker 2.0 were used to order RAPD loci relative to R and to construct maps for each of the individual seed trees. Two seed trees, 5701 and 6000, had a large number of tightly linked markers flanking R. These trees will be used in subsequent high-resolution mapping experiments to identify very tightly linked markers to facilitate the eventual cloning of R. Received: 1 May 1998 / Accepted: 13 July 1998     

Molecular mapping of genes for resistance to rice blast (Pyricularia grisea Sacc.)

Two dominant genes conferring complete resistance to specific isolates of the rice blast fungus, Pyricularia grisea Sacc., were located on the molecular map of rice in this study. Pi-l(t) is a blast resistance gene derived from the cultivar LAC23. Its map location was determined using a pair of nearly isogenic lines (NILs) and a B₆F₃ segregating population from which the isoline was derived. RFLP analysis showed that Pi-l(t) is located near the end of chromosome 11, linked to RZ536 at a distance of 14.0±4.5 centiMorgans (cM). A second gene, derived from the cultivar Apura, was mapped using a rice doubled-haploid (DH) population. This gene was located on chromosome 12, flanked by RG457 and RG869, at a distance of 13.5+-4.3 cM and 17.7+-4.5 cM, respectively. The newly mapped gene on chromosome 12 may be allelic or closely linked toPi-ta. (=Pi-4(t)), a gene derived from Tetep that was previously reported to be linked to RG869 at a distance of 15.4±4.7 cM. The usefulness of markers linked to blast resistance genes will be discussed in the context of breeding for durable blast resistance.