Genetic mapping revealed two loci for soybean aphid resistance in PI 567301B

The soybean aphid (Aphis glycines Matsumura) is the most damaging insect pest of soybean [Glycine max (L.) Merr.] in North America. New soybean aphid biotypes have been evolving quickly and at least three confirmed biotypes have been reported in USA. These biotypes are capable of defeating most known aphid resistant soybean genes indicating the need for identification of new genes. Plant Introduction (PI) 567301B was earlier identified to have antixenosis resistance against biotype 1 and 2 of the soybean aphid. Two hundred and three F_7:9 recombinant inbred lines (RILs) developed from a cross of soybean aphid susceptible cultivar Wyandot and resistant PI 567301B were used for mapping aphid resistance genes using the quantitative trait loci (QTL) mapping approach. A subset of 94 RILs and 516 polymorphic SNP makers were used to construct a genome-wide molecular linkage map. Two candidate QTL regions for aphid resistance were identified on this linkage map. Fine mapping of the QTL regions was conducted with SSR markers using all 203 RILs. A major gene on chromosome 13 was mapped near the previously identified Rag2 gene. However, an earlier study revealed that the detached leaves of PI 567301B had no resistance against the soybean aphids while the detached leaves of PI 243540 (source of Rag2) maintained aphid resistance. These results and the earlier finding that PI 243540 showed antibiosis resistance and PI 567301B showed antixenosis type resistance, indicating that the aphid resistances in the two PIs are not controlled by the same gene. Thus, we have mapped a new gene near the Rag2 locus for soybean aphid resistance that should be useful in breeding for new aphid-resistant soybean cultivars. Molecular markers closely linked to this gene are available for marker-assisted breeding. Also, the minor locus found on chromosome 8 represents the first reported soybean aphid-resistant locus on this chromosome.     

Genetic linkage mapping of the soybean aphid resistance gene in PI 243540

The soybean aphid (Aphis glycines Matsumura) is a pest of soybean [Glycine max (L.) Merr.] in many soybean growing countries of the world, mainly in Asia and North America. A single dominant gene in PI 243540 confers resistance to the soybean aphid. The objectives of this study were to identify simple sequence repeat (SSR) markers closely linked to the gene in PI 243540 and to position the gene on the consensus soybean genetic map. One hundred eighty-four F(2) plants and their F(2:3) families from a cross between the susceptible cultivar Wyandot and PI 243540, and the two parental lines were screened with the Ohio biotype of soybean aphid using greenhouse choice tests. A SSR marker from each 10-cM section of the consensus soybean map was selected for bulked segregant analysis (BSA) to identify the tentative genomic location of the gene. The BSA technique was useful to localize the gene to a genomic region in soybean linkage group (LG) F. The entire F(2) population was then screened with polymorphic SSR markers from this genomic region and a linkage map with nine SSR markers flanking the gene was constructed. The aphid resistance gene was positioned in the interval between SSR markers Satt334 and Sct_033 on LG F. These SSR markers will be useful for marker assisted selection of this gene. The aphid resistance gene from PI 243540 mapped to a different linkage group than the only named soybean aphid resistance gene, Rag1, from 'Dowling'. Also, the responses of the two known biotypes of the soybean aphid to the gene from PI 243540 and Rag1 were different. Thus, the aphid resistance gene from PI 243540 was determined to be a new and independent gene that has been named Rag2.     

Identification and mapping QTL for high-temperature adult-plant resistance to stripe rust in winter wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) cultivar ‘Stephens’

High-temperature adult-plant (HTAP) resistance from the winter wheat (Triticum aestivum) cultivar 'Stephens' has protected wheat crops from stripe rust caused by Puccinia striiformis f. sp. tritici for 30 years. The objectives of this study were to identify quantitative trait loci (QTL) for HTAP resistance in Stephens through genetic linkage analysis and identify DNA markers linked to the QTL for use in marker-assisted breeding. Mapping populations consisted of 101 recombinant inbred lines (RILs) through single-seed descent from 'Stephens' (resistant) x 'Michigan Amber' (susceptible). F(5), F(6) and F(7) RILs were evaluated for stripe rust resistance at Pullman, WA in 1996, 1997 and 1998, respectively, whereas F(8) RILs were evaluated at Mt Vernon, WA, USA in 2005. The 101 F(8) RILs were evaluated with 250 resistance gene analog polymorphism (RGAP), 245 simple sequence repeat (SSR) and 1 sequence tagged site (STS) markers for genetic linkage map construction. Two QTL, which explained 48-61% of the total phenotypic variation of the HTAP resistance in Stephens, were identified. QYrst.wgp-6BS.1 was within a 3.9-cM region flanked by Xbarc101 and Xbarc136. QYrst.wgp-6BS.2 was mapped in a 17.5-cM region flanked by Xgwm132 and Xgdm113. Both two QTL were physically mapped to the short arm of chromosome 6B, but in different bins. Validation and polymorphism tests of the flanking markers in 43 wheat genotypes indicated that the molecular markers associated with these QTL should be useful in marker-assisted breeding programs to efficiently incorporate HTAP resistance into new wheat cultivars.     

Identification and Fine Mapping of rhm1 Locus for Resistance to Southern Corn Leaf Blight in Maize

rhm1 is a major recessive disease resistance locus for Southern corn leaf blight (SCLB).To further narrow down its genetic position,F 2 population and BC 1 F 1 population derived from the cross between resistant (H95 rhm) and susceptible parents (H95) of maize (Zea mays) were constructed.Using newly developed markers,rhm1 was initially delimited within an interval of 2.5 Mb,and then finally mapped to a 8.56 kb interval between InDel marker IDP961-503 and simple sequence repeat (SSR) marker A194149-1.Three polymorphic markers IDP961-504,IDP B2-3 and A194149-2 were shown to be co-segregated with the rhm1 locus.Sequence analysis of the 8.56 kb DNA fragment revealed that it contained only one putative gene with a predicted amino acid sequence identical to lysine histidine transporter 1 (LHT1).Comparative sequence analysis indicated that the LHT1 in H95 rhm harbors a 354 bp insertion in its third exon as compared with that of susceptible alleles in B73,H95 and Mo17.The 354 bp insertion resulted in a truncation of the predicted protein of candidate resistance allele (LHT1-H95 rhm).Our results strongly suggest LHT1 as the candidate gene for rhm1 against SCLB.The tightly linked molecular markers developed in this study can be directly used for molecular breeding of resistance to Southern corn leaf blight in maize.     

Molecular tagging of gene conferring leaf blight resistance using microsatellites in sorghum [Sorghum bicolor (L.) Moench

Resistance to leaf blight in sorghum [Sorghum bicolor (L.) Moench] accession G-118 was found to segregate as a single dominant trait in a cross to susceptible cultivar, HC-136. Molecular marker(s) linked to the locus for disease resistance was identified using simple sequence repeat (SSR) markers coupled with bulk segregant analysis. Genomic DNA from the parental cultivars and bulks were screened by PCR amplification with 50 simple sequence repeat primer pairs. Out of these, 38 SSR primers produced polymorphism between parents. After screening of these 38 SSRs with resistant and susceptible bulk, one SSR primer, Xtxp 309 produced a unique band of approximately 700 bp only in resistant parent and resistant bulk and a unique band of 450 bp only in susceptible parent and susceptible bulk. Upon screening with individual resistant and susceptible recombinant inbred lines (RILs), marker Xtxp 309 produced amplification in 23 of the 26 resistant RILs and no amplification was produced in any of the 25 susceptible RILs. The same marker Xtxp 309 produced amplification in 21 of the susceptible RILs and 3 of the resistant RILs of 450 bp band. This was found to be located at a distance of 3.12 cM away from the locus governing resistance to leaf blight which was considered to be closely linked and 7.95 cM away from the locus governing susceptibility to leaf blight. This marker may prove useful in MAS for gene introgression, plant genetic diagnostics and gene pyramiding for resistance via genetic transformation for disease resistance in plants.     

Soybean phytophthora resistance gene Rps8 maps closely to the Rps3 region

Root and stem rot is one of the major diseases of soybean. It is caused by the oomycete pathogen Phytophthora sojae. A series of resistance genes (Rps) have been providing soybean with reasonable protection against this pathogen. Among these genes, Rps8, which confers resistance to most P. sojae isolates, recently has been mapped. However, the most closely linked molecular marker was mapped at about 10 cM from Rps8. In this investigation, we attempted to develop a high-density genetic map of the Rps8 region and identify closely linked SSR markers for marker-assisted selection of this invaluable gene. Bulk segregant analysis was conducted for the identification of SSR markers that are tightly linked to Rps8. Polymorphic SSR markers selected from the Rps8 region failed to show cosegregation with Phytophthora resistance. Subsequently, bulk segregant analysis of the whole soybean genome and mapping experiments revealed that the Rps8 gene maps closely to the disease resistance gene-rich Rps3 region.     

Mapping novel sources of leaf rust and stripe rust resistance introgressed from Triticum dicoccoides in cultivated tetraploid wheat background

Wild emmer wheat, Triticum dicoccoides, the progenitor of modern tetraploid and hexaploid wheats, is an important resource for new variability for disease resistance genes. T. dicoccoides accession pau4656 showed resistance against prevailing leaf rust and stripe rust races in India and was used for developing stable introgression lines (IL) in T. durum cv Bijaga yellow and named as IL pau16068. F₅ Recombinant inbred lines (F₅ RILs) were developed by crossing IL pau16068 with T. durum cultivar PBW114 and RIL population was screened against highly virulent Pt and Pst pathotypes at the seedling and adult plant stages. Inheritance analyses revealed that population segregated for two genes for all stage resistance (ASR) against leaf rust, one ASR gene against stripe rust and three adult plant resistance (APR) genes for stripe rust resistance. For mapping these genes a set of 483 SSR marker was used for bulked segregant analysis. The markers showing diagnostic polymorphism in the resistant and susceptible bulks were amplified on all RILs. Single marker analysis placed all stage leaf rust resistance genes on chromosome 6A and 2A linked to the SSR markers Xwmc256 and Wpaus268, respectively. Likewise one all stage stripe rust resistance gene were mapped on long arm of chromosome 6A linked to markers 6AL-5833645 and 6AL-5824654 and two APR genes mapped on chromosomes 2A and 2B close to the SSR marker Wpaus268 and Xbarc70, respectively. The current study identified valuable leaf rust and stripe rust resistance genes effective against multiple rust races for deployment in the wheat breeding programme.

     

Identification and mapping of microsatellite markers linked to a root-knot nematode resistance gene (rkn1) in Acala NemX cotton (Gossypium hirsutum L.)

Host-plant resistance is the most economic and effective strategy for root-knot nematode (RKN) Meloidogyne incognita control in cotton (Gossypium hirsutum L.). Molecular markers linked to resistance are important for incorporating resistance genes into elite cultivars. To screen for microsatellite markers (SSR) closely linked to RKN resistance in G. hirsutum cv. Acala NemX, F₁, F₂, BC₁F₁, and F_2:7 recombinant inbred lines (RILs) from intraspecific crosses and an F₂ from an interspecific cross with G. barbadense cv. Pima S-7 were used. Screening of 284 SSR markers, which cover all the known identified chromosomes and most linkage groups of cotton, was performed by bulked segregant analysis, revealing informative SSRs. The informative SSRs were then mapped on the above populations. One co-dominant SSR marker CIR316 was identified tightly linked to a major resistance gene (designated as rkn1), producing amplified DNA fragments of approximately 221 bp (CIR316a) and 210 bp (CIR316c) in Acala NemX and susceptible Acala SJ-2, respectively. The linkage between CIR316a marker and resistance gene rkn1 in Acala NemX had an estimated distance of 2.1–3.3 cM depending on the population used. Additional markers, including BNL1231 with loose linkage to rkn1 (map distance 25.1–27.4 cM), BNL1066, and CIR003 allowed the rkn1 gene to be mapped to cotton linkage group A03. This is the first report in cotton with a closely linked major gene locus determining nematode resistance, and informative SSRs may be used for marker-assisted selection.     

Characterization and molecular mapping of Yr52 for high-temperature adult-plant resistance to stripe rust in spring wheat germplasm PI 183527

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the most destructive diseases of wheat worldwide. Resistance is the best approach to control the disease. High-temperature adult-plant (HTAP) stripe rust resistance has proven to be race non-specific and durable. However, genes conferring high-levels of HTAP resistance are limited in number and new genes are urgently needed for breeding programs to develop cultivars with durable high-level resistance to stripe rust. Spring wheat germplasm PI 183527 showed a high-level of HTAP resistance against stripe rust in our germplasm evaluations over several years. To elucidate the genetic basis of resistance, we crossed PI 183527 and susceptible wheat line Avocet S. Adult plants of parents, F(1), F(2) and F(2:3) progeny were tested with selected races under the controlled greenhouse conditions and in fields under natural infection. PI 183527 has a single dominant gene conferring HTAP resistance. Resistance gene analog polymorphism (RGAP) and simple sequence repeat (SSR) markers in combination with bulked segregant analysis (BSA) were used to identify markers linked to the resistance gene. A linkage map consisting of 4 RGAP and 7 SSR markers was constructed for the resistance gene using data from 175 F(2) plants and their derived F(2:3) lines. Amplification of nulli-tetrasomic, ditelosomic and deletion lines of Chinese Spring with three RGAP markers mapped the gene to the distal region (0.86-1.0) of chromosome 7BL. The molecular map spanned a genetic distance of 27.3?cM, and the resistance gene was narrowed to a 2.3-cM interval flanked by markers Xbarc182 and Xwgp5258. The polymorphism rates of the flanking markers in 74 wheat lines were 74 and 30?%, respectively; and the two markers in combination could distinguish the alleles at the resistance locus in 82?% of tested genotypes. To determine the genetic relationship between this resistance gene and Yr39, a gene also on 7BL conferring HTAP resistance in Alpowa, a cross was made between PI 183527 and Alpowa. F(2) segregation indicated that the genes were 36.5?±?6.75?cM apart. The gene in PI 183527 was therefore designed as Yr52. This new gene and flanking markers should be useful in developing wheat cultivars with high-level and possible durable resistance to stripe rust.     

Identification of simple sequence repeat markers linked to lipoxygenase-1 gene in soybean

Off-flavour generated in soy products is ascribed to soybean seed lipoxygenase-1, lipoxygenase-2 and lipoxygenase-3, controlled by single dominant genes Lox1, Lox2 and Lox3, respectively. Lox2 locus has already been mapped and reported to be tightly linked with Lox1 locus. The objective of the present study was to map Lox1 locus by investigating the SSR markers reported to be linked with Lox2 locus and the neighbouring SSR markers in two mapping populations of 116 and 91 plants developed from LSb1 × PI408251 and JS335 × PI408251, respectively. Parental polymorphism was surveyed using SSR markers Sat_074, Satt522 reported to be linked with Lox2 locus and the SSR markers in its proximity. F_2:3 seeds were used for assaying lipoxygenase-1 to identify the genotype of the F₂ individuals. SSR marker Satt656 was found to be tightly linked with Lox1 locus at distance of 3.6 and 4.8 cM in the mapping population of LSb1 × PI408251 and JS335 × PI408251, respectively. SSR marker Satt656 can be useful for marker assisted selection for transferring recessive allele of lipoxygenase-1 in the background of high yielding soybean genotypes.     

Recessive resistance genes against potyviruses are localized in colinear genomic regions of the tomato ( Lycopersicon spp.) and pepper ( Capsicum spp.) genomes

Resistance against both Potato virus Y (PVY) and Tobacco etch virus (TEV) was identified in the wild tomato relative Lycopersicon hirsutum PI247087. Analysis of the segregation ratio in F(2)/F(3) and BC(1) interspecific progenies indicated that a single recessive gene, or two very tightly linked recessive loci, are involved in resistance to both potyviruses. This locus was named pot-1. Using amplified fragment length polymorphism markers and a set of L. hirsutum introgression lines, pot-1 was mapped to the short arm of tomato chromosome 3, in the vicinity of the recessive py-1 locus for resistance to corky root rot. Because of the occurrence of phenotypically similar genes in pepper ( Capsicum spp.), the comparative genetics of resistance to potyviruses between tomato and pepper was investigated. Unlike most of the comparative genetic studies on resistance genes, pot-1 was tightly flanked by the same restriction fragment length polymorphism (RFLP) markers than the pvr2/pvr5 locus for resistance to PVY and TEV from pepper. These results may indicate that recessive resistance genes against potyviruses evolve less rapidly than the majority of the dominant genes cloned so far, and consequently may belong to a different family of resistance genes.     

Mapping of a resistance gene effective against Karnal bunt pathogen of wheat

A set of 130 wheat recombinant inbred lines (RILs) developed from a cross between parents susceptible (WL711) and resistant (HD29) to Karnal bunt (caused by Tilletia indica), were screened for 3 years with the pathogen populations prevalent in northern India. When 90 simple sequence repeats (SSRs) and 81 amplified fragment length polymorphism (AFLP) loci were mapped on the RILs, markers on chromosomes 2A, 4B and 7B accounted collectively for about one-third of the variation in the disease reaction. The genomic region of largest effect, identified on the long arm of chromosome 4B, reduced Karnal bunt disease by half in three different experiments and accounted for up to 25% of the phenotypic variation for KB reaction. A closely linked SSR marker, GWM538, may be useful in marker-assisted selection for Karnal bunt resistance in wheat.     

Saturation of two chromosome regions conferring resistance to SCMV with SSR and AFLP markers by targeted BSA

Quantitative trait loci (QTLs) and bulked segregant analyses (BSA) identified the major genes Scmv1 on chromosome 6 and Scmv2 on chromosome 3, conferring resistance against sugarcane mosaic virus (SCMV) in maize. Both chromosome regions were further enriched for SSR and AFLP markers by targeted bulked segregant analysis (tBSA) in order to identify and map only markers closely linked to either Scmv1 or Scmv2. For identification of markers closely linked to the target genes, symptomless individuals of advanced backcross generations BC5 to BC9 were employed. All AFLP markers, identified by tBSA using 400 EcoRI/ MseI primer combinations, mapped within both targeted marker intervals. Fourteen SSR and six AFLP markers mapped to the Scmv1 region. Eleven SSR and 18 AFLP markers were located in the Scmv2 region. Whereas the linear order of SSR markers and the window size for the Scmv2 region fitted well with publicly available genetic maps, map distances and window size differed substantially for the Scmv1 region on chromosome 6. A possible explanation for the observed discrepancies is the presence of two closely linked resistance genes in the Scmv1 region.     

Inheritance and mapping of the ore gene controlling the quantity of β-carotene in cucumber (Cucumis sativus L.) endocarp

The metabolic precursor of vitamin A, ??-carotene, is essential for human health. The gene(s) controlling ??-carotene quantity (Q??C) has been introgressed from Xishuangbanna gourd (XIS, possessing ??-carotene; Cucumis sativus L. var. xishuangbannanesis Qi et Yuan; 2n?=?2x?=?14) into cultivated cucumber (no ??-carotene; Cucumis sativus L.). To determine the inheritance of Q??C in cucumber fruit endocarp, F1 progeny and a set of 124 F7 recombinant inbred lines (RILs) derived from the cultivated cucumber line CC3 and XIS line SWCC8 were evaluated for Q??C during 2009 and 2010 in Nanjing, China. Segregation analysis revealed that endocarp Q??C of greenhouse-grown fruit was controlled by a single recessive gene. Further, marker analysis indicated the gene controlling Q??C was linked to seven SSR markers on linkage group 3, where their order was SSR20710?CSSR19511?CSSR15419?CSSR07706?Core?CSSR23231?CSSR11633?CSSR20270. These markers and the putative candidate gene were mapped to cucumber chromosome 3DS. An evaluation of 30 genetically diverse cucumber lines indicated that marker SSR07706 has utility in further genetic analyses of the Q??C orange endocarp gene, designated ore. Moreover, the markers defined herein may have utility for marker-assisted selection directed towards the development of cucumber germplasm with high fruit ??-carotene content.     

Molecular mapping of Rxp conditioning reaction to bacterial pustule in soybean.

The Rxp locus in soybean [Glycine max (L.) Merr.] that conditions reaction to bacterial pustule was mapped by simple sequence repeat (SSR) marker analysis. A population of 116 F4-derived lines from a cross between the resistant parent Young and the susceptible parent PI 416937 was used for mapping. The Rxp locus was mapped 3.9 cM from Satt372 and 12.4 cM from Satt014 on linkage group D2. Linkage associations were confirmed by identifying a close association between the SSR genotype at each locus identified as flanking Rxp and the bacterial pustule reaction of individual lines derived from a population different from the one used for mapping. A molecular pedigree analysis showed that bacterial pustule-resistant cultivars inherited the resistance gene rxp from the ancestral cultivar CNS based on their consistent genotypic pattern at flanking marker loci. Based on the results of the study, marker-assisted selection for rxp would be very effective.     

Restriction fragment length polymorphisms linked to genes for resistance to crown rust (Puccinia coronata) in near-isogenic lines of hexaploid oat (Avena sativa).

Crown rust, perhaps the most important fungal disease of oat, is caused by Puccinia coronata. An examination of near-isogenic lines (NILs) of hexaploid oat (Avena sativa) was conducted to identify markers linked to genes for resistance to crown rust. These lines were created such that a unique resistance gene is present in each of the two recurrent parent backgrounds. The six NILs of the current study, X434-II, X466-I, and Y345 (recurrent parent C237-89) and D486, D494, and D526 (recurrent parent Lang), thus provide a pair of lines to study each of three resistance genes. Restriction fragment length polymorphisms and resistance loci were mapped using BC1F2 populations. Three markers were found linked to a locus for resistance to crown rust race 203, the closest at 1.9 cM in line D494 and 3.8 cM in line X466-I. In lines D526 and Y345 a marker was placed 1.0 and 1.9 cM, respectively, from the locus conferring resistance to crown rust race 345, and in D486 and X434-II a marker mapped at 8.0 and 10.2 cM from the locus for resistance to rust race 264B.     

Molecular mapping of the Pl 16 downy mildew resistance gene from HA-R4 to facilitate marker-assisted selection in sunflower

The major genes controlling sunflower downy mildew resistance have been designated as Pl genes. Ten of the more than 20 Pl genes reported have been mapped. In this study, we report the molecular mapping of gene Pl(16) in a sunflower downy mildew differential line, HA-R4. It was mapped on the lower end of linkage group (LG) 1 of the sunflower reference map, with 12 markers covering a distance of 78.9 cM. One dominant simple sequence repeat (SSR) marker, ORS1008, co-segregated with Pl(16), and another co-dominant expressed sequence tag (EST)-SSR marker, HT636, was located 0.3 cM proximal to the Pl(16) gene. The HT636 marker was also closely linked to the Pl(13) gene in another sunflower differential line, HA-R5. Thus the Pl(16) and Pl(13) genes were mapped to a similar position on LG 1 that is different from the previously reported Pl(14) gene. When the co-segregating and tightly linked markers for the Pl(16) gene were applied to other germplasms or hybrids, a unique band pattern for the ORS1008 marker was detected in HA-R4 and HA-R5 and their F(1) hybrids. This is the first report to provide two tightly linked markers for both the Pl(16) and Pl(13) genes, which will facilitate marker-assisted selection in sunflower resistance breeding, and provide a basis for the cloning of these genes.     

Genetics of resistance to anthracnose and identification of AFLP and RAPD markers linked to the resistance gene in PI 320937 germplasm of lentil (<Emphasis Type="Italic">Lens culinaris</Emphasis> Medikus)

Anthracnose, caused by Colletotrichum truncatum, is a major disease problem and production constraint of lentil in North America. The research was conducted to examine the resistance to anthracnose in PI 320937 lentil and to identify molecular markers linked to the resistance gene in a recombinant inbred line (RIL) population developed from a cross of Eston lentil, the susceptible parent, and PI 320937, the resistant parent. A total of 147 F(5:6) RILs were evaluated for resistance to anthracnose in the greenhouse using isolate 95B36 of C. truncatum. Bulked segregant analysis (BSA) strategy was employed and two contrasting DNA bulks were constructed based on greenhouse inoculation of F(5)-derived F(6) RILs. DNA from the parents and bulks were screened with 700 RAPD primers and seven AFLP primer combinations. Analysis of segregation data indicated that a major dominant gene was responsible for resistance to anthracnose while variations in the resistance level among RILs could be the influences of minor genes. We designate the major gene as LCt-2. MapMaker analysis produced two flanking RAPD markers OPEO6(1250) and UBC-704(700) linked to LCt-2 locus in repulsion (6.4 cM) and in coupling (10.5 cM), respectively. Also, three AFLP markers, EMCTTACA(350) and EMCTTAGG(375) in coupling, and EMCTAAAG(175) in repulsion, were linked to the LCt-2 locus. These markers could be used to tag the LCt-2 locus and facilitate marker-assisted selection for resistance to anthracnose in segregating populations of lentil in which PI 320937 was used as the source of resistance. Also, a broader application of the linked RAPD markers was also demonstrated in Indianhead lentil, widely used as a source of resistance to anthracnose in the breeding program at the Crop Development Centre, University of Saskatchewan. Further selection within the few F(5:6) lines should be effective in pyramiding one or several of the minor genes into the working germplasm of lentil, resulting in a more durable and higher level of resistance.     

Genetic and physical mapping of<Emphasis Type="Italic"> Pi5(t)</Emphasis>, a locus associated with broad-spectrum resistance to rice blast

To gain an understanding of the molecular basis for resistance to rice blast (Magnaporthe grisea), we have initiated a project to clone Pi5(t), a locus associated with broad-spectrum resistance to diverse blast isolates. AFLP-derived markers linked to Pi5(t)-mediated resistance were isolated using bulked segregant analysis of F(2) populations generated by crossing three recombinant inbred lines (RILs), RIL125, RIL249, and RIL260 with the susceptible line CO39. The most tightly linked AFLP marker, S04G03, was positioned on chromosome 9 of the fingerprint-based physical map of Nipponbare, a well-characterized rice genotype. Flanking BAC-based Nipponbare markers were generated for saturation mapping using four populations, the three initial RILs and an additional one derived from a cross between M202 and RIL260. A BIBAC (binary BAC) library was constructed from RIL260. Using these resources Pi5(t) was mapped to a 170-kb interval, and a contiguous set of BIBAC clones spanning this region was constructed. It had previously been suggested that Pi3(t) and Pi5(t) might be allelic, due to their identical resistance spectrum and tight linkage. We therefore compared genomic regions for lines containing Pi3(t) using the Pi5(t)-linked markers. DNA gel-blot analyses indicated that the region around Pi3(t) is identical to that of Pi5(t), suggesting that Pi3(t) and Pi5(t) are the same resistance gene.     

Inheritance and Gene Mapping of Resistance to Soybean Mosaic Virus Strain SC14 in Soybean

Soybean mosaic virus (SMV) is one of the most broadly distributed diseases worldwide. It causes severe yield loss and seed quality deficiency in soybean (Glycine max (L.) Merr.). SMV Strain SC14 isolated from Shanxi Province, China, was a newly identified virulent strain and can infect Kefeng No. 1, a source with wide spectrum resistance. In the present study, soybean accessions, PI96983, Qihuang No. 1 and Qihuang No. 22 were identified to be resistant (R) and Nannong 1138‐2, Pixianchadou susceptible (S) to SC14. Segregation analysis of PI96983 x Nannong 1138‐2 indicated that a single dominant gene (designated as R_SC14) controlled the resistance to SC14 at both V₂ and R₁ developmental stages. The same results were obtained for the crosses of Qihuang No. 1 × Nannong 1138‐2 and Qihuang No. 22 × Nannong 1138‐2 as in PI96983 × Nannong 1138‐2 at V₂ stage, but at R₁ stage, the F₁ performed as necrosis (a susceptible symptom other than mosaic), F₂ segregated in a ratio of 1R:2N:1S, and the progenies of necrotic (N) F₂ individuals segregated also in R, N and S. It indicated that a single gene (designated as R_SC14Q, to be different from that of PI96983) controlled the resistance to SC14, its dominance was the same as in PI96983 × Nannong 1138‐2 (without symptoms) at V₂ stage and not the same at R₁ stage. The tightly linked co‐dominant simple sequence repeat (SSR) marker Satt334 indicated that all the heterozygous bands were completely corresponding to the necrotic F₂ individuals, or all the necrotic F₂ individuals were heterozygotes. It was inferred that necrosis might be due to the interaction among SMV strains, resistance genes, genetic background of the resistance genes, and plant development stage. Furthermore, the bulked segregant analysis (BSA) of SSR markers was conducted to map the resistance genes. In F2of PI96983 × Nannong 1138‐2, five SSR markers, Sat_297, Sat_234, Sat_154, Sct_033 and Sat_120, were found closely linked to RSC14, with genetic distances of 14.5 cM, 11.3cM, 4.3cM,3.2cM and 6cM, respectively. In F₂ of Qihuang No. 1 × Nannong 1138‐2, three SSR markers, Sat_234, Satt334 and Sct_033, tightly linked to R_SC14Q with genetic distances of 7.2 cM, 1.4 cM and 2.8 cM, respectively. Based on the integrated joint map by Cregan et al. (1999), both RScMand R_SC14Q were located between Sat_234 and Sct_033 on linkage with group F of soybean, with their distances from Sct_033 at the same side being 3.2 cM and 2.8 cM, respectively. Therefore, RSC14and R_SC14Q might be on a same locus. The obtained information provides a basic knowledge for marker‐assisted selection of the resistance gene in soybean breeding programs and fine mapping and map‐based cloning of the resistance gene. (Managing editor: Li‐Hui Zhao)