Location Of Resistance Genes Research Articles

Molecular characterization of accessions of crabgrass (Digitaria nuda) and response to ametryn

Digitaria species are sugar cane crop weeds in Brazil and are being controlled with herbicides, although there are some reports of control failure, notably to the triazine group. Molecular techniques are recommended to analyze the genetic variability in weeds. RAPD (Random Amplified Polymorphic DNA), PCR-RFLP (Polymerase Chain Reaction - Restriction Fragment Length Polymorphism) and, in combination with sequencing, allow the localization of resistance genes, as well as possible mutations related to the onset of resistant individuals in some species. Thus, the objective of this work was to characterize ten accessions of Digitaria spp. by RAPD and PCR-RFLP markers, to sequence a conserved region of the psbA gene and evaluate the accessions response to ametryn. As showed by molecular analysis there was high genetic similarity among the accessions, all of them presented similar genetics profiles and were susceptible to ametryn.

Development of an STS Marker Linked to Powdery Mildew Resistance Genes PmLK906 and Pm4a by Gene Chip Hybridization

Wheat ( Triticum aestivum L.) line Lankao 90(6) carries a recessive powdery mildew resistance gene temporarily named PmLK906 on chromosome 2AL. Near PmLK906 there is another known powdery mildew resistance gene locus Pm4. To track the two powdery mildew resistance genes in wheat breeding program by marker assisted selection (MAS), a linked molecular marker was developed in this study. Wheat gene chip hybridization combined with bulked segregant analysis (BSA) was used to develop an sequence-tagged sites (STS) marker for PmLK906 and Pm4. A new 2 125 bp full-length cDNA clone (GenBank accession no. EU082094) similar to csAtPR5 of Aegilops tauschii was isolated from Lankao 90(6) 21-12, and temporarily named TaAetPR5. Specific products could be amplified from cultivars or lines possessing Pm4a, Pm4b and PmLK906 with primers p9-7p1 and p9-7p2 derived from TaAetPR5. TaAetPR5 was linked to PmLK906 at a genetic distance of 7.62 cM, and cosegregated with Pm4a. The p9-7p1 and p9-7p2 could be used as an STS marker for these resistance genes in wheat breeding. Because this marker was cosegregated with Pm4a, it can be used in map-based cloning of the alleles at Pm4 locus also.

Developing Specific Markers and Improving Genetic Mapping for a Major Locus <em>Tyr1</em> of Citrus Nematode Resistance

The NBS-LRR class resistance-gene candidate sequences (Pt8a and Pt9a) were used to develop new specific markers to the citrus nematode resistance gene locus Tyr1 . By high-density colony screening, over 200 positive clones were pulled out from the BAC library. A few of the clones were found to be closely linked with the Tyr1 region, because the primers from these clones insert sequence produced polymorphism which matched up with the phenotype after bulked segregant analysis. By primer walking approach, three integrate NBS-LRR class resistance-gene sequences were tagged and identified separately in three clones (7A4, 4L17 and 29F20). More specific markers were developed from these tagged sequences and relatively high-density genetic maps were constructed by incorporating the newly developed markers and previously developed markers in the ‘9145 family’. New markers were applied in ‘9401 family’ trying to estimate roughly the genetic distance between the Ctv and Tyr1 region.

Recruitment of closely linked genes for divergent functions: the seed storage protein (Glu-3) and powdery mildew (Pm3) genes in wheat (Triticum aestivum L.)

Wheat seed storage protein gene loci (Glu-3) and powdery mildew resistance gene loci (Pm3 and Pm3-like) are closely linked on the short arms of homoeologous group 1 chromosomes. To study the structural organization of the Glu-3/Pm3 loci, three bacterial artificial chromosome clones were sequenced from the A, B, and D genomes of hexaploid wheat. The A and B genome clones contained a Glu-3 adjacent to a Pm3-like gene organized in a conserved Glu-3/SFR159/Pm3-like structure. The D genome clone contained clusters of resistance gene analogs but no Pm3. Its similarity to the A and B genome was limited to the Glu-3/SFR159 region. Comparison of the B genome PM3-like deduced amino acid sequence with known PM3 functional isotypes reinforced the hypothesis of allelic evolution via block exchange by gene conversion/recombination. The advent of glutenin genes and the formation of the Glu-3/SFR159/Pm3 locus occurred after divergence of wheat from rice and Brachypodium. Comparison of the A genome homologous sequences permitted an estimate of time of divergence of approximately 0.3 million years ago. The B genome sequences were not colinear indicating that they could either be paralogs or represent different B genome progenitors. Analysis of the 11 complete retrotransposons indicated a time of divergence ranging from 0.29 to 5.62 million years ago, consistent with their complex nested structure.

Identification and Utility of Markers Linked to the Zucchini Yellow Mosaic Virus Resistance Gene in Watermelon

Zucchini yellow mosaic virus (ZYMV) is one of the most economically important viruses affecting watermelon [Citrullus lanatus (Thunb.) Matsun & Nakai var. lanatus] in the United States. The ZYMV-Florida strain (ZYMV-FL) is considered a major limitation to commercial watermelon production in the United States. Inheritance of resistance to ZYMV-FL is conferred by a recessive gene. This report describes the identification of single-reaction, polymerase chain reaction-based markers linked to the ZYMV-FL resistance gene in watermelon. In this study, we identified a marker ZYMV-resistant polymorphism (ZYRP) linked to the ZYMV-FL resistance gene locus (genetic distance of 8 cM) in an F2 population, and in a backcross one to the resistant parent population (BC1R) (genetic distance of 13 cM). The identification of a single nucleotide polymorphism within the ZYRP marker for the parental genotypes allowed the development of a sequence-characterized amplification region marker linked to the ZYMV-FL resistance gene. Experiments using a BC2F2 population derived from the U.S. Plant Introduction 595203 (C. lanatus var. lanatus) and the recurrent parent ‘Charleston Gray’ indicated that the ZYRP marker can be used in marker-assisted selection to identify genotypes containing the gene conferring ZYMV-FL resistance in watermelon.

Research advances in molecular breeding of powdery mildew resistance of wheat

小麦白粉病是小麦生产的主要病害之一，应用抗病品种是防治该病的十分经济、有效的措施。近年来分子标记技术的发展为小麦抗白粉病基因的研究提供了极大的方便。目前为止，小麦基因组中已定名抗白粉病主效基因有38个，其中有41个基因位点的57个抗白粉病基因被标记和作图。本文详细叙述了小麦抗白粉病基因的来源、抗病基因利用及其分子标记研究现状，介绍了国内外分子育种研究最新进展，旨在为我国抗小麦白粉病分子育种提供参考。

The Frequent Transcriptional Readthrough of the Tobacco Tnt1 Retrotransposon and Its Possible Implications for the Control of Resistance Genes

Retrotransposons are a major component of eukaryote genomes, being especially abundant in plant genomes. They are frequently found inserted in gene-rich regions and have greatly contributed to the evolution of gene coding capacity and regulation. Retrotransposon insertions can influence the expression of neighboring genes in many ways, such as modifying their promoter or terminator sequences and altering their epigenetic control. Plant retrotransposons are highly regulated and their expression is usually associated with stress situations. While the control of transcription of some plant retrotransposons has been analyzed in some detail, little is known about the transcriptional termination of these elements. Here we show that the transcripts of the tobacco retrotransposon Tnt1 display a high variability of polyadenylation sites, only a fraction of them terminating at the major termination site. We also report on the ability of Tnt1 to extend its transcription into flanking genomic sequences and we analyze a particular case in which Tnt1 transcripts include sequences of an oppositely oriented resistance-like gene. The expression of this gene and the neighboring Tnt1 copy generate transcripts overlapping in more that 800 nucleotides, which could anneal and form dsRNAs and enter into silencing regulatory pathways. Resistance gene loci are usually composed of tandem arrays of resistance-like genes, a number of which contain mutations, including retrotransposon insertions, and are considered as to be pseudogenes. Given that plant retrotransposons are usually regulated by stress, the convergent expression of these resistance-like pseudogenes and the interleaving inducible retrotransposons may contribute to the control of plant responses to stress.

Genetic Analysis and Mapping of Resistance Gene to Seed Coat Mottle in Soybean

Soybean mosaic virus(SMV) causes a worldwide disease in soybean,reducing yield and making mottling of soybean seeds,and so largely losing soybean economy value. Resistant variety breeding is the most effective way to protect against the seed coat mottle. In this study,an F2:3 population,derived from Dongnong 3C624(susceptible) × Dongnong 8143(resistant) ,was used to study the genetic mechanism of the resistance,and map the resistant gene of seed coat mottle by simple sequence repeat(SSR) markers with bulked segregation analysis(BSA) method. Two parents and their F2,F3 population were inoculated with SMV strain No.1 in 2006 and 2007. Identification results in two years showed that resistance to SMV strain No.1 of Dongnong 8143 was controlled by one pair of dominant genes. Six hundred pairs of SSR primers were screened,and five SSR markers,Sat_297,Sat_229,Sat_317,Satt335,and Sct_188,were mapped near the resistance gene. Linkage analysis indicated that the re-sistance gene locus was located on the linkage group F. The order and genetic distance among the resistance gene and markers were Sat_297–12.4 cM–Sat_229–3.6 cM–SRSMV1–1.7 cM–Sat_317–2.4 cM–Satt335–13.8 cM–Sct_188. Sat_229,Sat_317,and Satt335 can be used for molecular assisted breeding.

Genetic mapping and transcription analyses of resistance gene loci in potato using NBS profiling

NBS profiling is a method for the identification of resistance gene analog (RGA) derived fragments. Here we report the use of NBS profiling for the genome wide mapping of RGA loci in potato. NBS profiling analyses on a minimal set of F1 genotypes of the diploid mapping population previously used to generate the ultra dense (UHD) genetic map of potato, allowed us to efficiently map polymorphic RGA fragments relative to 10,000 existing AFLP markers. In total, 34 RGA loci were mapped, of which only 13 contained RGA sequences homologous to RGAs genetically positioned at approximately similar positions in potato or tomato. The remaining RGA loci mapped either at approximate chromosomal regions previously shown to contain RGAs in potato or tomato without sharing homology to these RGAs, or mapped at positions not yet identified as RGA-containing regions. In addition to markers representing RGAs with unknown functions, segregating markers were detected that were closely linked to four functional R genes that segregate in the UHD mapping population. To explore the potential of NBS profiling in RGA transcription analyses, RNA isolated from different tissues was used as template for NBS profiling. Of all the fragments amplified approximately 15% showed putative intensity or absent/present differences between different tissues suggesting putative tissue specific RGA or R gene transcription. Putative absent/present differences between individuals were also found. In addition to being a powerful tool for generating candidate gene markers linked to R gene loci, NBS profiling, when applied to cDNA, can be instrumental in identifying those members of an R gene cluster that are transcribed, and thus putatively functional.

Targeting and genotyping RGAs in a mappingpopulation of the AA genome of wild Arachis

Peanut is an important legume crop. Although it has high levels of morphological diversity, it lacks genetic variability and sources of disease resistance. The transference of resistance genes from wild species is difficult due to the different ploidy level of the wild and cultivated species. Recently, amphidiploids have been produced that can be used as 'bridges' to introgress wild genes. Molecular markers are useful to pyramidize desirable genes and track them through generations of backcrossings. Molecular markers based on Resistance Gene Analogs have improved chances to be present in or linked to resistance gene loci. This study describes the development and genotyping of molecular markers based on resistance gene motifs. Specific primers were designed based on unique sequences of an Arachis RGA dataset. The identity of the amplified polymorphic bands was confirmed by sequencing. These markers were genotyped on a F2 population that segregates for resistance to biotic stress types.

The Mdm1 Locus and Maize Resistance to Maize dwarf mosaic virus.

Previously, Mdm1, a gene controlling resistance to Maize dwarf mosaic virus (MDMV), was identified in the inbred line Pa405. The gene was tightly linked to the restriction fragment length polymorphism marker umc85 on the short arm of chromosome 6. This chromosomal region is also the location of resistance genes to two other viruses in the family Potyviridae, Sugarcane mosaic virus (SCMV) and Wheat streak mosaic virus (WSMV). A diverse collection of 115 maize inbred lines was evaluated for resistance to MDMV and SCMV, and for MDMV resistance loci on chromosome 6S. Forty-six resistant inbred lines were crossed to three MDMVsusceptible inbred lines to develop F2 populations. The F2 populations were inoculated with MDMV and scored for infection and symptom type. Environmental factors influenced both the rate and type of symptom development. Bulked segregant analysis of each F2 population indicated that, in 42 of 43 MDMV-resistant lines, chromosome 6S markers found in the resistant parent also were present in the bulked resistant but not the susceptible tissue. Markers previously associated with resistance to both SCMV and WSMV on chromosome 3 and to WSMV on chromosome 10 were associated with resistance in nine and seven of the F2 populations, respectively. These data suggest that Mdm1 or closely linked genes on chromosome 6S are associated with MDMV resistance in most germplasm, but that other loci also may affect resistance.

Studies on the mechanisms of β-lactam resistance in Bordetella bronchiseptica

Little is currently known about beta-lactam resistance in Bordetella bronchiseptica. So far, only a single beta-lactamase gene, bla(BOR-1), has been identified. In a previous study, high MICs of ampicillin, cefalotin and ceftiofur were determined among 349 porcine B. bronchiseptica isolates. The aim of this study was to identify genes associated with elevated MICs of beta-lactams and their transferability. Selected isolates were investigated by PCR for commonly found bla genes and class 1 integrons; selected amplicons were sequenced. Plasmid location of resistance genes was confirmed by conjugation. Beta-lactamases were characterized by SDS-PAGE and isoelectric focusing. The genomic relatedness of the isolates was investigated by XbaI macrorestriction analysis. Inhibition studies with efflux pump inhibitors were conducted. The permeability of cephalosporins into intact cells was measured exemplarily for one isolate. Of the 349 B. bronchiseptica isolates, eight isolates carried a class 1 integron with a bla(OXA-2) cassette on a conjugative plasmid of ca. 50 kb. In addition, one plasmid-free isolate also carried this class 1 integron. Besides bla(BOR-1), no other beta-lactamase gene was detected in the remaining isolates with high MICs of ampicillin of >or= 32 mg/L. Inhibition experiments suggested that efflux does not play a role in beta-lactam resistance. Instead, membrane permeability for cephalosporins was reduced as shown for B. bronchiseptica isolate B543. This is to the best of our knowledge the first report of a mobile bla gene in B. bronchiseptica. Reduced membrane permeability of B. bronchiseptica seems to decrease susceptibility against cephalosporins.

Independent action and contrasting phenotypes of resistance genes against spotted alfalfa aphid and bluegreen aphid in Medicago truncatula

Host resistance to aphids is poorly understood. Medicago truncatula, a model legume and cultivated pasture species, was used to elucidate defense against two aphid species, Therioaphis trifolii f. maculata (spotted alfalfa aphid, SAA) and Acyrthosiphon kondoi (bluegreen aphid, BGA). Aphid performance and plant damage were compared between near-isogenic cultivars, Mogul and Borung, that differ in resistance to both aphids. Analyses of aphid resistance in Mogul x Borung F2 plants and their progeny revealed modes of action and chromosome locations of resistance genes. Separate genes were identified for SAA resistance (TTR) and BGA resistance (AKR); both mapped to chromosome 3 but were found to act independently to reduce survival and growth of their target aphid species. The TTR locus controls distinct, and contrasting, local and systemic plant responses between the near-isogenic cultivars. TTR-mediated plant responses imply interaction between a resistance factor(s) in vascular tissue and a bioactive component(s) of SAA saliva. Features of both resistance traits suggest homology to aphid resistance in other legumes; elucidation of their molecular mechanisms will likely apply to other aphid-plant interactions.

APPLICATION OF SCAR MARKERS LINKED TO A PUTATIVE PRSV-P RESISTANCE LOCUS FOR ASSISTED BREEDING OF RESISTANT C. PAPAYA CULTIVARS

Papaya ringspot virus type P (PRSV-P) is a serious disease of Carica papaya. Several wild relatives of the papaya, belonging to the genus Vasconcellea, exhibit complete resistance to PRSV-P, namely V. cundinamarcensis, V. cauliflora and V. quercifolia. Molecular genetic analysis of interspecific hybrids of V. cundinamarcensis and V. parviflora using Randomly Amplified DNA Fingerprinting (RAF) permitted the construction of genetic maps of these species. Identification and mapping of several DNA markers linked to a PRSV-P resistance gene locus in V. cundinamarcensis was possible, two of which (OPK41R and OPA115R), resulted from a Single Nucleotide Polymorphism (SNP), in close proximity to the PRSV-P resistance gene locus. The Sequence Characterised Amplified Regions (SCAR's), were shown to be present in both parents, and produced codominant markers, which were applied as reliable diagnostic markers for resistant and susceptible genotypes. The OPK41R SCAR was identified in 4 additional Vasconcellea species made available, including V. cauliflora and V. quercifolia, but not in C. papaya. The indel identified may be useful for the selection of PRSV-P resistant individuals in breeding programs with Vasconcellea and C. papaya to assist introgression of putative resistance genes. The markers were subsequently applied to an intergeneric hybrid population from C. papaya and V. quercifolia. Genetic characterisation of the hybrid population using RAF markers confirmed hybridity of the F1 and reduced V. quercifolia DNA content of back cross generations. The OPK41R and OPA115R SCARs were tested in the quercifolia hybrids, and did not identify linkage to PRSV-P resistance. From the results it can be inferred that Vasconcellea may contain multiple sources of resistance to PRSV-P. Overall these studies indicate that there is significant potential for the continued application of DNA markers in molecular breeding for PRSV-P resistant papaya genotypes.

Identification and chromosomal location of major genes for resistance toPyrenophorateresin a doubled-haploid barley population

Net blotch, caused by Pyrenophora teres, is one of the most economically important diseases of barley worldwide. Here, we used a barley doubled-haploid population derived from the lines SM89010 and Q21861 to identify major quantitative trait loci (QTLs) associated with seedling resistance to P. teres f. teres (net-type net blotch (NTNB)) and P. teres f. maculata (spot-type net blotch (STNB)). A map consisting of simple sequence repeat (SSR) and amplified fragment length polymorphism (AFLP) markers was used to identify chromosome locations of resistance loci. Major QTLs for NTNB and STNB resistance were located on chromosomes 6H and 4H, respectively. The 6H locus (NTNB) accounted for as much as 89% of the disease variation, whereas the 4H locus (STNB resistance) accounted for 64%. The markers closely linked to the resistance gene loci will be useful for marker-assisted selection.

Response of Selected Hard Red Wheat Lines to Imazamox as Affected by Number and Location of Resistance Genes, Parental Background, and Growth Habit

ABSTRACTImidazolinone‐resistant (IR) wheat (Triticum aestivum L.) was released for commercial production in portions of the USA in 2002 and has provided growers with a new technology to selectively control winter annual grass weeds. Imidazolinone herbicides inhibit acetolactate synthase (ALS) in susceptible plants; however, IR wheat has an altered target site which confers resistance to these herbicides. The mutation‐derived resistance trait of most commercially available IR winter wheat cultivars is located on the D‐genome; however, winter and spring wheat cultivars with the resistance trait on the A, B, or D genome or on multiple genomes are currently under development. Four groups of near‐isoline wheat with spring or winter growth habit and resistance genes on the B, D, or both B and D genomes were compared for whole plant and ALS enzyme response to imazamox. Biomass accumulation after treatment was similar among B‐ and D‐ genome resistant winter wheat biotypes and was always higher than B‐ and D‐genome resistant spring wheat biotypes. D‐genome resistant spring wheat was more resistant than B‐genome resistant spring wheat and the two‐gene resistant spring wheat had an additive level of tolerance to imazamox compared with single‐gene resistant spring wheat. Growth habit (spring vs. winter) did not affect in vitro ALS activity among B‐ or among D‐genome resistant cultivars; however, D‐genome resistant cultivars had significantly higher in vitro ALS activity in the presence of imazamox compared with B‐genome resistant cultivars regardless of growth habit. D‐genome resistance appears to provide greater tolerance to imazamox compared with B‐genome resistance; however, multiple‐genome resistance likely will be required to consistently avoid crop injury in spring wheat from labeled U.S. rates. Although ALS extracted from winter wheat and spring wheat responded similarly to imazamox, whole plant responses demonstrates that tolerance is affected by factors other than resistance gene location.

Genetic analysis and fine mapping of a rice brown planthopper (Nilaparvata lugens Stål) resistance gene bph19(t)

Genetic analysis and fine mapping of a resistance gene against brown planthopper (BPH) biotype 2 in rice was performed using two F(2) populations derived from two crosses between a resistant indica cultivar (cv.), AS20-1, and two susceptible japonica cvs., Aichi Asahi and Lijiangxintuanheigu. Insect resistance was evaluated using F(1) plants and the two F(2) populations. The results showed that a single recessive gene, tentatively designated as bph19(t), conditioned the resistance in AS20-1. A linkage analysis, mainly employing microsatellite markers, was carried out in the two F(2) populations through bulked segregant analysis and recessive class analysis (RCA), in combination with bioinformatics analysis (BIA). The resistance gene locus bph19(t) was finely mapped to a region of about 1.0 cM on the short arm of chromosome 3, flanked by markers RM6308 and RM3134, where one known marker RM1022, and four new markers, b1, b2, b3 and b4, developed in the present study were co-segregating with the locus. To physically map this locus, the bph19(t)-linked markers were landed on bacterial artificial chromosome or P1 artificial chromosome clones of the reference cv., Nipponbare, released by the International Rice Genome Sequencing Project. Sequence information of these clones was used to construct a physical map of the bph19(t) locus, in silico, by BIA. The bph19(t) locus was physically defined to an interval of about 60 kb. The detailed genetic and physical maps of the bph19(t) locus will facilitate marker-assisted gene pyramiding and cloning.

Rps8 Maps to a Resistance Gene Rich Region on Soybean Molecular Linkage Group F

Phytophthora root and stem rot caused by Phytophthora sojae M.J. Kaufmann & J.W. Gerdemann is a serious disease of soybean [Glycine max (L.) Merr.] worldwide. Recently, a new locus for resistance to P. sojae, Rps8, was identified and mapped in two small soybean populations. The objective of this study was to verify the genomic location of Rps8 in a larger population. One hundred thirty‐eight F2:3 families from a cross between ‘Williams’ (rps8/rps8) × PI 399073 (Rps8/Rps8) were genotyped by simple sequence repeat (SSR) and restriction fragment length polymorphism (RFLP) markers. From this set of families, 138 and 69 were phenotyped with P. sojae races 1 and 25, respectively. The segregation ratio for each race fit a 3:1 resistant:susceptible, confirming that Rps8 segregated as a single dominant gene. On the basis of linkage analysis with SSR and RFLP markers, Rps8 was located on molecular linkage group F in this population. This region of the soybean genome contains numerous other resistance gene loci as well as pathogen and pest resistance QTL.

DEVELOPMENT OF A CO-DOMINANT SCAR MARKER LINKED TO A PUTATIVE PRSV-P RESISTANCE LOCUS IN WILD “PAPAYA”

Molecular genetic analysis of interspecific hybrids of V. cundinamarcensis and V. parviflora using Randomly Amplified DNA Fingerprinting (RAF) has permitted the construction of genetic maps of these species. The maps presented provided a framework for the development of marker systems for detection of valuable trait loci. Subsequently, several DNA markers linked to a PRSV-P resistance gene locus were identified and mapped in V. cundinamarcensis. One of these markers (OPK41R) resulted from a Single Nucleotide Polymorphism (SNP) in close proximity to the PRSV-P resistance gene locus. The Sequence Characterised Amplified Region (SCAR) was shown to be present in both parents, and 4 additional Vasconcellea species including V. cauliflora and V. quercifolia, but not in papaya. Sequence analysis revealed several polymorphisms, one resulting in the creation of a restriction site within the OPK4IR region, which was used to convert the SCAR into an easily detected co-dominant marker. This marker presents a simple yet useful tool that may be applied as a diagnostic test for selection of homozygous PRSV-P resistant hybrids of V. cundinamarcensis. With the development of this marker there is new potential to bridge hybridisation between Hybrids of V. cundinamarcensis and C. papaya via marker assisted selection.

Organization and evolution of resistance gene analogs in peanut

The scarcity of genetic polymorphism in Arachis hypogaea (peanut), as in other monophyletic polyploid species, makes it especially vulnerable to nematode, bacterial, fungal, and viral pathogens. Although no disease resistance genes have been cloned from peanut itself, the conserved motifs in cloned resistance genes from other plant species provide a means to isolate and analyze similar genes from peanut. To survey the number, diversity, evolutionary history, and genomic organization of resistance gene-like sequences in peanut, we isolated 234 resistance gene analogs (RGAs) by using primers designed from conserved regions of different classes of resistance genes including NBS-LRR, and LRR-TM classes. Phylogenetic and sequence analyses were performed to explore evolutionary relationships both among peanut RGAs and with orthologous genes from other plant taxa. Fifty-six overgos designed from the RGA sequences on the basis of their phyletic association were applied to a peanut BAC library; 736 hybridizing BAC clones were fingerprinted and contigs were formed in order to gain insights into the genomic organization of these genes. All the fingerprinting gels were blotted and screened with the respective overgos in order to verify the authenticity of the hits from initial screens, and to explore the physical organization of these genes in terms of both copy number and distribution in the genome. As a result, we identified 250 putative resistance gene loci. A correlation was found between the phyletic positions of the sequences and their physical locations. The BACs isolated here will serve as a valuable resource for future applications, such as map-based cloning, and will help improve our understanding of the evolution and organization of these genes in the peanut genome.