Mapping tightly linked genes controlling potyvirus infection at the Rsv1 and Rpv1 region in soybean.

Soybean mosaic virus (SMV) and peanut mottle virus (PMV) are two potyviruses that cause yield losses and reduce seed quality in infested soybean (Glycine max (L.) Merr.) fields throughout the world. Rsv1 and Rpv1 are genes that provide soybean with resistance to SMV and PMV, respectively. Isolating and characterizing Rsv1 and Rpv1 are instrumental in providing insight into the molecular mechanism of potyvirus recognition in soybean. A population of 1056 F2 individuals from a cross between SMV- and PMV-resistant line PI 96983 (Rsv1 and Rpv1) and the susceptible cultivar 'Lee 68' (rsv1 and rpv1) was used in this study. Disease reaction and molecular-marker data were collected to determine the linkage relationship between Rsv1, Rpv1, and markers that target candidate disease-resistance genes. F2 lines showing a recombination between two of three Rsv1-flanking microsatellite markers were selected for fine mapping. Over 20 RFLP, RAPD, and microsatellite markers were used to map 38 loci at high-resolution to a 6.8-cM region around Rsv1 and Rpv1. This study demonstrates that Rsv1 and Rpv1 are tightly linked at a distance of 1.1 cM. In addition, resistance-gene candidate sequences were mapped to positions flanking and cosegregating with these resistance loci. Based on comparisons of genetic markers and disease reactions, it appears likely that several tightly linked genes are conditioning a resistance response to SMV. We discuss the specifics of these findings and investigate the utility of two disease resistance related probes for the screening of SMV or PMV resistance in soybean.     

Fitness penalty in susceptible host is associated with virulence of Soybean mosaic virus on Rsv1‐genotype soybean: a consequence of perturbation of HC‐Pro and not P3

The multigenic Rsv1 locus in the soybean plant introduction (PI) ‘PI96983’ confers extreme resistance against the majority of Soybean mosaic virus (SMV) strains, including SMV‐N, but not SMV‐G7 and SMV‐G7d. In contrast, in susceptible soybean cultivars lacking a functional Rsv1 locus, such as ‘Williams82’ (rsv1), SMV‐N induces severe disease symptoms and accumulates to a high level, whereas both SMV‐G7 and SMV‐G7d induce mild symptoms and accumulate to a significantly lower level. Gain of virulence by SMV‐N on Rsv1‐genotype soybean requires concurrent mutations in both the helper‐component proteinase (HC‐Pro) and P3 cistrons. This is because of the presence of at least two resistance (R) genes, probably belonging to the nucleotide‐binding leucine‐rich repeat (NB‐LRR) class, within the Rsv1 locus, independently mediating the recognition of HC‐Pro or P3. In this study, we show that the majority of experimentally evolved mutational pathways that disrupt the avirulence functions of SMV‐N on Rsv1‐genotype soybean also result in mild symptoms and reduced accumulation, relative to parental SMV‐N, in Williams82 (rsv1). Furthermore, the evaluation of SMV‐N‐derived HC‐Pro and P3 chimeras, containing homologous sequences from virulent SMV‐G7 or SMV‐G7d strains, as well as SMV‐N‐derived variants containing HC‐Pro or P3 point mutation(s) associated with gain of virulence, reveals a direct correlation between the perturbation of HC‐Pro and a fitness penalty in Williams82 (rsv1). Collectively, these data demonstrate that gain of virulence by SMV on Rsv1‐genotype soybean results in fitness loss in a previously susceptible soybean genotype, this being a consequence of mutations in HC‐Pro, but not in P3.     

Pyramiding multiple genes for resistance to soybean mosaic virus in soybean using molecular markers

Seven strains of Soybean mosaic virus (SMV) and three independent resistance loci (Rsv1, Rsv3, and Rsv4) have been identified in soybean. The objective of this research was to pyramid Rsv1, Rsv3, and Rsv4 for SMV resistance using molecular markers. J05 carrying Rsv1 and Rsv3 and V94-5152 carrying Rsv4 were used as the donor parents for gene pyramiding. A series of F_2:3, F_3:4, and F_4:5 lines derived from J05 × V94-5152 were developed for selecting individuals carrying all three genes. Eight PCR-based markers linked to the three SMV resistance genes were used for marker-assisted selection. Two SSR markers (Sat_154 and Satt510) and one gene-specific marker (Rsv1-f/r) were used for selecting plants containing Rsv1; Satt560 and Satt063 for Rsv3; and Satt266, AI856415, and AI856415-g for Rsv4. Five F_4:5 lines were homozygous for all eight marker alleles and presumably carry all three SMV resistance genes that would potentially provide multiple and durable resistance to SMV.     

Inheritance and allelism of resistance to soybean mosaic virus in Zao18 soybean from China

Soybean mosaic disease caused by soybean mosaic virus (SMV) occurs wherever soybean [Glycine max (L.) Merr.] is grown and is considered one of the most important soybean diseases in many areas of the world. Use of soybean cultivars with resistance to SMV is a very effective way of controlling the disease. China has rich soybean germplasm, but there is very limited information on genetics of SMV resistance in Chinese soybean germplasm and reaction of the resistance genes to SMV strains G1-G7. There also is no report on allelic relationships of resistance genes in Chinese soybeans with other named genes at the three identified loci Rsv1, Rsv3, and Rsv4. The objectives of this study were to examine reactions of Chinese soybean cultivar Zao18 to SMV strains G1-G3 and G5-G7, to reveal the inheritance of SMV resistance in Zao18 and to determine the allelic relationship of resistance genes in Zao18 with previously reported resistance genes. Zao18 was crossed with the SMV-susceptible cultivar Lee 68 to study the inheritance of resistance. Zao18 was also crossed with the resistant lines PI96983, L29, and V94-5152, which possess Rsv1, Rsv3, and Rsv4, respectively, to examine the allelic relationship between the genes in Zao18 and genes at these three loci. Our research results indicated that Zao18 possesses two independent dominant genes for SMV resistance, one of which is allelic to the Rsv3 locus; the other is allelic with Rsv1. The presence of both genes (Rsv1 and Rsv3) in Zao18 confers resistance to SMV strains G1-G7.     

Multiplex single nucleotide polymorphism (SNP) assay for detection of soybean mosaic virus resistance genes in soybean

Soybean mosaic virus (SMV) is one of the most destructive viral diseases in soybean (Glycine max). Three independent loci for SMV resistance have been identified in soybean germplasm. The use of genetic resistance is the most effective method of controlling this disease. Marker assisted selection (MAS) has become very important and useful in the effort of selecting genes for SMV resistance. Single nucleotide polymorphism (SNP), because of its abundance and high-throughput potential, is a powerful tool in genome mapping, association studies, diversity analysis, and tagging of important genes in plant genomics. In this study, a 10 SNPs plus one insert/deletion (InDel) multiplex assay was developed for SMV resistance: two SNPs were developed from the candidate gene 3gG2 at Rsv1 locus, two SNPs selected from the clone N11PF linked to Rsv1, one ‘BARC’ SNP screened from soybean chromosome 13 [linkage group (LG) F] near Rsv1, two ‘BARC’ SNPs from probe A519 linked to Rsv3, one ‘BARC’ SNP from chromosome 14 (LG B2) near Rsv3, and two ‘BARC’ SNPs from chromosome 2 (LG D1b) near Rsv4, plus one InDel marker from expressed sequence tag (EST) AW307114 linked to Rsv4. This 11 SNP/InDel multiplex assay showed polymorphism among 47 diverse soybean germplasm, indicating this assay can be used to investigate the mode of inheritance in a SMV resistant soybean line carrying Rsv1, Rsv3, and/or Rsv4 through a segregating population with phenotypic data, and to select a specific gene or pyramid two or three genes for SMV resistance through MAS in soybean breeding program. The presence of two SMV resistance genes (Rsv1 and Rsv3) in J05 soybean was confirmed by the SNP assay.     

Simultaneous mutations in multi-viral proteins are required for soybean mosaic virus to gain virulence on soybean genotypes carrying different R genes

Background

Genetic resistance is the most effective and sustainable approach to the control of plant pathogens that are a major constraint to agriculture worldwide. In soybean, three dominant R genes, i.e., Rsv1, Rsv3 and Rsv4, have been identified and deployed against Soybean mosaic virus (SMV) with strain-specificities. Molecular identification of virulent determinants of SMV on these resistance genes will provide essential information for the proper utilization of these resistance genes to protect soybean against SMV, and advance knowledge of virus-host interactions in general.

Methodology/Principal Findings

To study the gain and loss of SMV virulence on all the three resistance loci, SMV strains G7 and two G2 isolates L and LRB were used as parental viruses. SMV chimeras and mutants were created by partial genome swapping and point mutagenesis and then assessed for virulence on soybean cultivars PI96983 (Rsv1), L-29 (Rsv3), V94-5152 (Rsv4) and Williams 82 (rsv). It was found that P3 played an essential role in virulence determination on all three resistance loci and CI was required for virulence on Rsv1- and Rsv3-genotype soybeans. In addition, essential mutations in HC-Pro were also required for the gain of virulence on Rsv1-genotype soybean. To our best knowledge, this is the first report that CI and P3 are involved in virulence on Rsv1- and Rsv3-mediated resistance, respectively.

Conclusions/Significance

Multiple viral proteins, i.e., HC-Pro, P3 and CI, are involved in virulence on the three resistance loci and simultaneous mutations at essential positions of different viral proteins are required for an avirulent SMV strain to gain virulence on all three resistance loci. The likelihood of such mutations occurring naturally and concurrently on multiple viral proteins is low. Thus, incorporation of all three resistance genes in a soybean cultivar through gene pyramiding may provide durable resistance to SMV.     

Recombination within a nucleotide-binding-site/leucine-rich-repeat gene cluster produces new variants conditioning resistance to soybean mosaic virus in soybeans

The soybean Rsv1 gene for resistance to soybean mosaic virus (SMV; Potyvirus) has previously been described as a single-locus multi-allelic gene mapping to molecular linkage group (MLG) F. Various Rsv1 alleles condition different responses to the seven (G1-G7) described strains of SMV, including extreme resistance, localized and systemic necrosis, and mosaic symptoms. We describe the cloning of a cluster of NBS-LRR resistance gene candidates from MLG F of the virus-resistant soybean line PI96983 and demonstrate that multiple genes within this cluster interact to condition unique responses to SMV strains. In addition to cloning 3gG2, a strong candidate for the major Rsv1 resistance gene from PI96983, we describe various unique resistant and necrotic reactions coincident with the presence or absence of other members of this gene cluster. Responses of recombinant lines from a high-resolution mapping population of PI96983 (resistant) x Lee 68 (susceptible) demonstrate that more than one gene in this region of the PI96983 chromosome conditions resistance and/or necrosis to SMV. In addition, the soybean cultivars Marshall and Ogden, which carry other previously described Rsv1 alleles, are shown to possess the 3gG2 gene in a NBS-LRR gene cluster background distinct from PI96983. These observations suggest that two or more related non-TIR-NBS-LRR gene products are likely involved in the allelic response of several Rsv1-containing lines to SMV.     

Evaluation of North American isolates of Soybean mosaic virus for gain of virulence on Rsv‐genotype soybeans with special emphasis on resistance‐breaking determinants on Rsv4

Resistance to Soybean mosaic virus (SMV) in soybean is conferred by three dominant genes: Rsv1, Rsv3 and Rsv4. Over the years, scientists in the USA have utilized a set of standard pathotypes, SMV‐G1 to SMV‐G7, to study interaction with Rsv‐genotype soybeans. However, these pathotypes were isolated from a collection of imported soybean germplasm over 30 years ago. In this study, 35 SMV field isolates collected in recent years from 11 states were evaluated for gain of virulence on soybean genotypes containing individual Rsv genes. All isolates were avirulent on L78‐379 (Rsv1), whereas 19 were virulent on L29 (Rsv3). On PI88788 (Rsv4), 14 of 15 isolates tested were virulent; however, only one was capable of systemically infecting all of the inoculated V94‐5152 (Rsv4). Nevertheless, virulent variants from 11 other field isolates were rapidly selected on initial inoculation onto V94‐5152 (Rsv4). The P3 cistrons of the original isolates and their variants on Rsv4‐genotype soybeans were sequenced. Analysis showed that virulence on PI88788 (Rsv4) was not associated, in general, with selection of any new amino acid, whereas Q1033K and G1054R substitutions were consistently selected on V94‐5152 (Rsv4). The role of Q1033K and G1054R substitutions, individually or in combination, in virulence on V94‐5152 (Rsv4) was confirmed on reconstruction in the P3 cistron of avirulent SMV‐N, followed by biolistic inoculation. Collectively, our data demonstrate that SMV has evolved virulence towards Rsv3 and Rsv4, but not Rsv1, in the USA. Furthermore, they confirm that SMV virulence determinants on V94‐5152 (Rsv4) reside on P3.     

Single nucleotide polymorphism markers for rapid detection of the <Emphasis Type="Italic">Rsv4</Emphasis> locus for soybean mosaic virus resistance in diverse germplasm

Soybean mosaic virus (SMV) causes a substantial decrease in soybean yield and reduction of seed quality. The most effective management strategy to control the virus is the deployment of host resistance. Seven SMV strains and three independent multi-allelic loci for SMV resistance have been identified previously. The goal of this research was to detect single nucleotide polymorphisms (SNPs) associated with SMV resistance at the Rsv4 locus. Ten soybean accessions, with confirmed resistance genes, were used for sequencing the candidate gene Glyma.02g121400. Alignment of these sequences revealed three SNPs displaying 100% consistency for genotypes carrying the Rsv4 gene. These SNPs were applied for a rapid screen of diverse soybean germplasm using the Sequenom iPLEX Gold platform, phenotyped with SMV-G1 and G7 strains to determine phenotype and classified into several groups carrying the proposed R-gene. The population of V94-5152 (Rsv4) × Lee 68 (rsv) was screened using novel SNPs to create a genetic map with improved resolution to determine the location of the Rsv4. To observe the recombination frequencies within the population, three additional SNPs on both sides of the Glyma.02g121400 gene were added. A linkage map revealed a distance of 3.6 cM between the Rsv4 locus and the closest SNP, thus shifting the putative Rsv4 region downstream on chromosome 2. With this region, five candidate genes have been proposed. The genomic position of the discovered SNPs, linked to the Rsv4, could increase screening precision and accelerate breeding efforts to develop multi-strain-resistant crops.     

Genetic diversity and association mapping in a collection of selected Chinese soybean accessions based on SSR marker analysis

For broadening the narrow genetic base of modern soybean cultivars, 159 accessions were selected from the Chinese soybean collection which contained at least one of seven important agronomic traits: resistance to soybean cyst nematode (SCN) or soybean mosaic virus (SMV), tolerance to salt, cold, or drought, high seed oil content or high protein content. Genetic diversity evaluation using 55 microsatellite loci distributed across the genome indicated that a large amount of genetic diversity (0.806) and allelic variation (781) were conserved in this selected set, which captured 65.6% of the alleles present in Chinese soybean collection (1,863 accessions). On average, 9.4 rare alleles (frequency <5%) per locus were present, which were highly informative. Using model-based Bayesian clustering in STRUCTURE we distinguished four main clusters and a set of accessions with admixed ancestry. The four clusters reflected different geographic regions of origin of the accessions. Since the clusters were also clearly different with respect to the seven agronomic traits, the inferred population structure was introduced when association analysis was conducted. A total of 21 SSR markers on 16 chromosomes were identified as significantly (P < 0.01) associated with high oil content (6), high protein content (1), drought tolerance (5), SCN resistance (6) and SMV resistance (3). Twelve of these markers were located in or near previously identified quantitative trait loci (QTL). The results for both genetic relationship and trait-related markers will be useful for effective conservation and utilization of soybean germplasm.     

Genetic analysis and mapping of genes for resistance to multiple strains of Soybean mosaic virus in a single resistant soybean accession PI 96983

Soybean mosaic virus (SMV) is one of the most broadly distributed soybean (Glycine max (L.) Merr.) diseases and causes severe yield loss and seed quality deficiency. Multiple studies have proved that a single dominant gene can confer resistance to several SMV strains. Plant introduction (PI) 96983 has been reported to contain SMV resistance genes (e.g., Rsv1 and Rsc14) on chromosome 13. The objective of this study was to delineate the genetics of resistance to SMV in PI 96983 and determine whether one gene can control resistance to more than one Chinese SMV strain. In this study, PI 96983 was identified as resistant and Nannong 1138-2 was identified as susceptible to four SMV strains SC3, SC6, SC7, and SC17. Genetic maps based on 783 F₂ individuals from the cross of PI 96983 × Nannong 1138-2 showed that the gene(s) conferring resistance to SC3, SC6, and SC17 were between SSR markers BARCSOYSSR_13_1114 and BARCSOYSSR_13_1136, whereas SC7 was between markers BARCSOYSSR_13_1140 and BARCSOYSSR_13_1185. The physical map based on 58 recombinant lines confirmed these results. The resistance gene for SC7 was positioned between BARCSOYSSR_13_1140 and BARCSOYSSR_13_1155, while the resistance gene(s) for SC3, SC6, and SC17 were between BARCSOYSSR_13_1128 and BARCSOYSSR_13_1136. We concluded that, there were two dominant resistance genes flanking Rsv1 or one of them at the reported genomic location of Rsv1. One of them (designated as “Rsc-pm”) conditions resistance for SC3, SC6, and SC17 and another (designated as “Rsc-ps”) confers resistance for SC7. The two tightly linked genes identified in this study would be helpful to cloning of resistance genes and breeding of multiple resistances soybean cultivars to SMV through marker-assisted selection (MAS).     

Gain of virulence by Soybean mosaic virus on Rsv4‐genotype soybeans is associated with a relative fitness loss in a susceptible host

‘Gene‐for‐gene’ theory predicts that gain of virulence by an avirulent pathogen on plants expressing resistance (R) genes is associated with fitness loss in susceptible hosts. However, the validity of this prediction has been studied in only a few plant viral pathosystems. In this study, the Soybean mosaic virus (SMV)–Rsv4 pathosystem was exploited to test this prediction. In Rsv4‐genotype soybeans, P3 of avirulent SMV strains provokes an as yet uncharacterized resistance mechanism that restricts the invading virus to the inoculated leaves. A single amino acid substitution in P3 functionally converts an avirulent to a virulent strain, suggesting that the genetic composition of P3 plays a crucial role in virulence on Rsv4‐genotype soybeans. In this study, we examined the impact of gain of virulence mutation(s) on the fitness of virulent variants derived from three avirulent SMV strains in a soybean genotype lacking the Rsv4 gene. Our data demonstrate that gain of virulence mutation(s) by all avirulent viruses on Rsv4‐genotype soybean is associated with a relative fitness loss in a susceptible host. The implications of this finding on the durable deployment of the Rsv4 gene in soybean are discussed.     

Amplified fragment length polymorphism (AFLP) in soybean: species diversity, inheritance, and near-isogenic line analysis

Amplified fragment length polymorphism (AFLP) analysis is a PCR-based technique capable of detecting more than 50 independent loci in a single PCR reaction. The objectives of the present study were to: (1) assess the extent of AFLP variation in cultivated (Gycine max L. Merr.) and wild soybean (G. soja Siebold & Zucc.), (2) determine genetic relationships among soybean accessions using AFLP data, and (3) evaluate the usefulness of AFLPs as genetic markers. Fifteen AFLP primer pairs detected a total of 759 AFLP fragments in a sample of 23 accessions of wild and cultivated soybean, with an average of 51 fragments produced per primer pair per accession. Two-hundred and seventy four fragments (36% of the total observed) were polymorphic, among which 127 (17%) were polymorphic in G. max and 237 (31%) were polymorphic in G. soja. F₂ segregation analysis of six AFLP fragments indicated that they segregate as stable Mendelian loci. The number of polymorphic loci detected per AFLP primer pair in a sample of 23 accessions ranged from 9 to 27. The AFLP phenotypic diversity values were greater in wild than in cultivated soybean. Cluster and principal component analyses using AFLP data clearly separated G. max and G. soja accessions. Within the G. max group, adapted soybean cultivars were tightly clustered, illustrating the relatively low genetic diversity present in cultivated soybean. AFLP analysis of four soybean near-isogenic lines (NILs) identified three AFLP markers putatively linked to a virus resistance gene from two sources. The capacity of AFLP analysis to detect thousands of independent genetic loci with minimal cost and time requirements makes them an ideal marker for a wide array of genetic investigations.     

Inheritance and allelism tests of Raiden soybean for resistance to soybean mosaic virus

The gene symbol Rsv2 was previously assigned to the gene in the soybean [Glycine max (L.) Merr.] line OX670 for resistance to soybean mosaic virus (SMV). The Rsv2 gene was reported to be derived from the Raiden soybean (PI 360844) and to be independent of Rsv1. Accumulated data from our genetic experiments were in disagreement with this conclusion. In this study, Raiden and L88-8431, a Williams BC5 isoline with SMV resistance derived from Raiden, were crossed with two SMV-susceptible cultivars to investigate the mode of inheritance of SMV resistance in Raiden. They were also crossed with five resistant cultivars to examine the allelomorphic relationships of the Raiden gene with other reported genes at the Rsv1 locus. F1 plants, F2 populations, and F2-derived F3 (F2:3) lines were tested with SMV strains G1 or G7 in the greenhouse or in the field. The individual plant reactions were classified as resistant (R, symptomless), necrotic (N, systemic necrosis), or susceptible (S, mosaic). The F2 populations from R x S crosses segregated in a ratio of 3 (R + N):1 S and the F2:3 lines from Lee 68 (S) x Raiden (R) exhibited a segregation pattern of 1 (all R):2 segregating:1 (all S). The F2 populations and F2:3 progenies from all R x R crosses did not show any segregation for susceptibility. These results demonstrate that the resistance to SMV in Raiden and L88-8431 is controlled by a single dominant gene and the gene is allelic to Rsv1. The heterozygous plants from R x S and R x N crosses exhibited systemic necrosis when inoculated with SMV G7, indicating a partial dominance nature of the resistance gene. Raiden and L88-8431 are both resistant to SMV G1-G4 and G7, but necrotic to G5, G6, and G7A. Since the resistance gene in Raiden is clearly an allele at the Rsv1 locus and it exhibits a unique reaction to the SMV strain groups, assignment of a new gene symbol, Rsv1-r, to replace Rsv2 would seem appropriate. Further research is ongoing to investigate the possible existence of the Rsv2 locus in OX670 and its relatives.     

The comparison of RFLP,RAPD, AFLP and SSR (microsatellite) markers for germplasm analysis

The utility of RFLP (restriction fragment length polymorphism), RAPD (random-amplified polymorphic DNA), AFLP (amplified fragment length polymorphism) and SSR (simple sequence repeat, microsatellite) markers in soybean germplasm analysis was determined by evaluating information content (expected heterozygosity), number of loci simultaneously analyzed per experiment (multiplex ratio) and effectiveness in assessing relationships between accessions. SSR markers have the highest expected heterozygosity (0.60), while AFLP markers have the highest effective multiplex ratio (19). A single parameter, defined as the marker index, which is the product of expected heterozygosity and multiplex ratio, may be used to evaluate overall utility of a marker system. A comparison of genetic similarity matrices revealed that, if the comparison involved both cultivated (Glycine max) and wild soybean (Glycine soja) accessions, estimates based on RFLPs, AFLPs and SSRs are highly correlated, indicating congruence between these assays. However, correlations of RAPD marker data with those obtained using other marker systems were lower. This is because RAPDs produce higher estimates of interspecific similarities. If the comparisons involvedG. max only, then overall correlations between marker systems are significantly lower. WithinG. max, RAPD and AFLP similarity estimates are more closely correlated than those involving other marker systems.Abbreviations RFLP restriction fragment length plymorphism - RAPD random-amplified polymorphic DNA - AFLP amplified fragment length polymorphism - SSR simple sequence repeat - PCR polymerase chain reaction - TBE Tris-borate-EDTA buffer - MI marker index - SENA sum of effective numbers of alleles     

Amino acid substitution in P3 of Soybean mosaic virus to convert avirulence to virulence on Rsv4‐genotype soybean is influenced by the genetic composition of P3

The modification of avirulence factors of plant viruses by one or more amino acid substitutions converts avirulence to virulence on hosts containing resistance genes. Limited experimental studies have been conducted on avirulence/virulence factors of plant viruses, in particular those of potyviruses, to determine whether avirulence/virulence sites are conserved among strains. In this study, the Soybean mosaic virus (SMV)–Rsv4 pathosystem was exploited to determine whether: (i) avirulence/virulence determinants of SMV reside exclusively on P3 regardless of virus strain; and (ii) the sites residing on P3 and crucial for avirulence/virulence of isolates belonging to strain G2 are also involved in virulence of avirulent isolates belonging to strain G7. The results confirm that avirulence/virulence determinants of SMV on Rsv4‐genotype soybean reside exclusively on P3. Furthermore, the data show that sites involved in the virulence of SMV on Rsv4‐genotype soybean vary among strains, with the genetic composition of P3 playing a crucial role.     

Genetic characteristics of two genes for resistance to soybean mosaic virus in PI486355 soybean

Soybean [Glycine max (L.) Merr.] PI486355 is resistant to all the identified strains of soybean mosaic virus (SMV) and possesses two independently inherited resistance genes. To characterize the two genes, PI486355 was crossed with the susceptible cultivars Lee 68 and Essex and with cultivars Ogden and Marshall, which are resistant to SMV-G1 but systemically necrotic to SMV-G7. The F₂ populations and F₂₃ progenies from these crosses were inoculated with SMV-G7 in the greenhouse. The two resistance genes were separated in two F₃₄ lines, LR1 and LR2, derived from Essex x PI486355. F₁ individuals from the crosses of LR1 and LR2 with Lee 68, Ogden, and York were tested with SMV-G7 in the greenhouse; the F₂ populations were tested with SMV-G1 and G7. The results revealed that expression of the gene in LR1 is gene-dosage dependent, with the homozygotes conferring resistance but the heterozygotes showing systemic necrosis to SMV-G7. This gene was shown to be an allele of the Rsv1 locus and was designated as Rsv1-s. It is the only allele identified so far at the Rsv1 locus which confers resistance to SMV-G7. Rsv1-s also confers resistance to SMV-G1 through G4, but results in systemic necrosis with SMV-G5 and G6. The gene in LR2 confers resistance to strains SMV-G1 through G7 and exhibits complete dominance. It appears to be epistatic to genes at the Rsv1 locus, inhibiting the expression of the systemic necrosis conditioned by the Rsv1 alleles. SMV-G7 induced a pin-point necrotic reaction on the inoculated primary leaves in LR1 but not in LR2. The unique genetic features of the two resistance genes from PI486355 will facilitate their proper use and identification in breeding and contribute to a better understanding of the interaction of SMV strains with soybean resistance genes.     

Development and genetic mapping of 127 new microsatellite markers in barley

To enhance the marker density of existing genetic maps of barley (Hordeum vulgare L.), a new set of microsatellite markers containing dinucleotide motifs was developed from genomic clones. Out of 254 primer pairs tested, a total of 167 primer pairs were classifed as functional in a panel of six barley cultivars and three H. spontaneum accessions, and of those, 127 primer pairs resulting in 133 loci were either mapped or located onto chromosomes. The polymorphism information content (PIC) ranged from 0.05 to 0.94 with an average of 0.60. The number of alleles per locus varied from 1 to 9. On average, 3.9 alleles per primer pair were observed. The RFLP frameworks of two previously published linkage maps were used to locate a total of 115 new microsatellite loci on at least one mapping population. The chromosomal assignment of 48 mapped loci was corroborated on a set of wheat-barley chromosome addition lines; 18 additional loci which were not polymorphic in the mapping populations were assigned to chromosomes by this method. The microsatellites were located on all seven linkage groups with four significant clusters in the centromeric regions of 2H, 3H, 6H and 7H. These newly developed microsatellites improve the density of existing barley microsatellite maps and can be used in genetic studies and breeding research.Communicated by G. Wenzel