Development of RAPD and SCAR markers linked to the Pvr4 locus for resistance to PVY in pepper ( Capsicum annuum L.)

Potato Virus Y (PVY) is the only potyvirus infecting pepper ( Capsicum annuum L.) in Europe. Currently, the development of pepper varieties resistant to PVY seems to be the most-efficient method to control PVY damage. Among the sources of resistance, a monogenic dominant gene Pvr4 confers resistance against all known PVY pathotypes. In this work, bulked segregant analysis (BSA) was used to search for randomly amplified polymorphic DNA (RAPD) markers linked to the Pvr4 gene, using segregating progenies obtained by crossing a homozygous resistant ('Serrano Criollo de Morelos-334') with a homozygous susceptible ('Yolo Wonder') cultivar. Eight hundred decamer primers were screened to identify one RAPD marker (UBC19(1432)) linked in repulsion phase to Pvr4. This marker was converted into a dominant sequence characterised amplified region (SCAR) marker (SCUBC19(1423)). This marker was mapped into a dense Capsicum genetic map in a region where several genes for resistance to different diseases are located. This marker can be useful to identify PVY-resistant genotypes in segregating progenies of pepper in marker-assisted selection (MAS) breeding programs.     

RNA-Dependent RNA Polymerase (NIb) of the Potyviruses Is an Avirulence Factor for the Broad-Spectrum Resistance Gene Pvr4 in Capsicum annuum cv. CM334

Potyviruses are one of the most destructive viral pathogens of Solanaceae plants. In Capsicum annuum landrace CM334, a broad-spectrum gene, Pvr4 is known to be involved in resistance against multiple potyviruses, including Pepper mottle virus (PepMoV), Pepper severe mosaic virus (PepSMV), and Potato virus Y (PVY). However, a potyvirus avirulence factor against Pvr4 has not been identified. To identify the avirulence factor corresponding to Pvr4 in potyviruses, we performed Agrobacterium-mediated transient expressions of potyvirus protein coding regions in potyvirus-resistant (Pvr4) and -susceptible (pvr4) pepper plants. Hypersensitive response (HR) was observed only when a RNA-dependent RNA polymerase (NIb) of PepMoV, PepSMV, or PVY was expressed in Pvr4-bearing pepper leaves in a genotype-specific manner. In contrast, HR was not observed when the NIb of Tobacco etch virus (TEV), a virulent potyvirus, was expressed in Pvr4-bearing pepper leaves. Our results clearly demonstrate that NIbs of PepMoV, PepSMV, and PVY serve as avirulence factors for Pvr4 in pepper plants.     

Development of a CAPS marker for the Pvr4 locus: a tool for pyramiding potyvirus resistance genes in pepper.

The Pvr4 resistance gene in pepper confers a complete resistance to the three pathotypes of potato virus Y (PVY) and to pepper mottle virus (PepMoV). In order to use this gene in a marker-assisted selection (MAS) program and to permit the pyramiding of several potyvirus resistance genes in the same cultivar, tightly linked amplified fragment length polymorphism (AFLP) markers were obtained by the bulked segregant analysis method. Eight linked AFLP markers were mapped in an interval from 2.1 +/- 0.8 to 13.8 +/- 2.9 cM around this locus. The closest codominant AFLP marker was converted into a codominant CAPS (cleaved amplified polymorphic sequence) marker using data from the alignment of the two allele sequences. We have further characterized the relevance of the CAPS marker for MAS programs in different pepper breeding lines.     

Characterization of sources of resistance to the watermelon strain of Papaya ringspot virus in cucumber: allelism and co-segregation with other potyvirus resistances

At least three sources of resistance to the watermelon strain of Papaya ringspot virus (PRSV-W) have been identified in cucumber (Cucumis sativus L.) including: ’TMG-1’, an inbred line derived from the Taiwanese cultivar, ’Taichung Mou Gua’; ’Dina-1’, an inbred line derived from the Dutch hybrid ’Dina’; and the South American cultivar ’Surinam’. In this investigation we sought to determine the inheritance of resistance to PRSV-W in ’Dina-1’, the allelic relationships among the three sources of PRSV-W resistance, and the relationship between PRSV-W resistance and known resistances to other cucurbit potyviruses. Like ’Surinam’ and ’TMG-1’, resistance in ’Dina-1’ is controlled by a single gene. Despite differences in dominance vs recessive performance and patterns of virus accumulation, all three sources of resistance complemented each other. ’TMG-1’ and ’Dina-1’ also possess co-segregating, single-gene resistances to Zucchini yellow mosaic virus (ZYMV), Watermelon mosaic virus and Moroccan watermelon mosaic virus. Sequential inoculations and F₃ family analysis indicated that resistance to PRSV-W completely co- segregated with resistance to ZYMV in ’TMG-1’. Although PRSV-W resistances are at the same locus in both ’TMG-1’ and ’Surinam’, ’Surinam’ is only resistant to PRSV-W, and progeny of ’TMG-1’×’Surinam’ were resistant to PRSV-W but susceptible to ZYMV. Susceptibility to ZYMV and resistance to PRSV-W in ’Surinam’ was not influenced by co-inoculation or sequential in- oculations of the two viruses. Collectively, the co- segregation of resistances to PRSV-W, ZYMV, WMV and MWMV in ’TMG-1’ (within 1 cM), allelism of PRSV-W resistances in ’TMG-1’ and ’Surinam’, and resistance to only PRSV-W in ’Surinam’, suggest that multiple potyvirus resistance in cucumber may be due to different alleles of a single potyvirus resistance gene with differing viral specificities, or that the multiple resistances are conferred by a tightly linked cluster of resistance genes, of which ’Surinam’ only possesses one member. Received: 22 July 1999 / Accepted: 2 December 1999     

Extreme resistance to potato virus V in clones of Solanum tuberosum that are also resistant to potato viruses Y and A: evidence for a locus conferring broad-spectrum potyvirus resistance

 Extreme resistance to the potato V potyvirus (PVV) was found in four potato cultivars that contain Ry genes from Solanum stoloniferum. When plants of these cultivars, were inoculated by grafting in shoot tips from PVV-infected tomato plants, necrotic symptoms developed in some cultivars, although a full hypersensitive reaction was not elicited, while other cultivars were symptomless. PVV replication was not detected in any of the inoculated plants by ELISA, an infectivity assay of leaf extracts by manual inoculation to Nicotiana benthamiana indicator plants, or by ‘return grafting’ of shoot tips taken from newly developed shoots of the potato plants to virus-free indicator plants of tomato. These methods readily detected PVV infection in inoculated plants of cv ‘Flourball’, which does not contain an Ry gene and is susceptible, and in cvs ‘Maris Piper’ and ‘Dr Macintosh’, which contain gene Nv conditioning a hypersensitive reaction to inoculation. One of the Ry-containing cultivars, ‘Barbara’, has been previously shown to contain two genes that control extreme resistance, defined as no viral replication in intact plants, to the potyviruses potato viruses Y and A (PVY and PVA). These genes are: Ry _sto , which conditions resistance to PVY and PVA, and gene Ra, which conditions resistance to PVA only. It was found that in genotypes from a progeny of the cross ‘Barbara’ (Ry _sto /Ra)בFlourball’ (ry/ra), extreme resistance to PVV segregated with gene Ry _sto . It is proposed that either gene Ry _sto conditions broad-spectrum extreme resistance to the distinct potyviruses PVY, PVA, and PVV or that Ry _sto represents a family of genetically closely linked genes each controlling resistance to a specific virus. Received: 27 December 1996 / Accepted: 9 June 1997     

Molecular mapping of a resistance-specific PCR-based marker linked to a gall midge resistance gene (Gm4t) in rice

 A PCR-based marker (E20₅₇₀) linked to the gene Gm4t, which confers resistance to a dipteran pest gall midge (Orseolia oryzae), has been mapped using the restriction fragment length polymorphism (RFLP) technique in rice. Gm4t is a dominant resistance gene. We initially failed to detect useful polymorphism for this marker in a F₃ mapping population derived from a cross between two indica parents, ‘Abhaya’בShyamala’, with as many as 35 restriction enzymes. ‘Abhaya’ carries the resistance gene Gm4t and ‘Shyamala’ is susceptible to gall midge. Subsequently, E20₅₇₀ was mapped using another mapping population represented by a F₂ progeny from a cross between ‘Nipponbare’, a japonica variety, and ‘Kasalath’, an indica variety, in which the gene Gm4t was not known to be present. Gm4t mapped onto chromosome 8 between markers R1813 and S1633B. Our method, thus, presents an alternative way of mapping genes which otherwise would be difficult to map because of a lack of polymorphism between closely related parents differing in desired agronomic traits. Received: 1 April 1997 / Accepted: 13 May 1997     

High-resolution genetic mapping of the pepper (Capsicum annuum L.) resistance loci Me3 and Me4 conferring heat-stable resistance to root-knot nematodes (Meloidogyne spp.)

The PM687 line of Capsicum annuum L. has a single dominant gene, Me ₃ , that confers heat-stable resistance to root-knot nematodes (RKN). Me ₃ was mapped using doubled-haploid (DH) lines and F₂ progeny from a cross between the susceptible cultivar ’Yolo Wonder’ (’YW’) and the highly resistant line ’PM687’. Bulked-segregant analysis with DNA pools, from susceptible or resistant DH lines, was performed to identify RAPD and AFLP markers linked to Me ₃ . There was no polymorphism between bulks of ten DH lines using over 800 RADP primers (4,000 amplified fragments analysed). Using 512 AFLP primers (74,000 amplified fragments analysed), and bulked DNA templates from 20 resistant and 20 susceptible plants, we identified eight repulsion-phase and four coupling-phase markers linked to Me _3. Analysed in 103 DH progeny, they defined a 56.1-cM interval containing the target gene. The nearest were located 0.5, 1.0, 1.5 and 3.0 centimorgans (cM) on both sides of the gene. Analysis of the F₂ progeny (162 plants) with the nearest coupling-phase marker confirmed its close position. Another resistance gene to RKN, present in ’PM687’ (Me ₄ ), was shown to be linked to Me ₃ , 10 cM from it. In order to localize Me ₃ and Me ₄ on our reference intraspecific pepper linkage map, two AFLP markers were mapped. The Me ₃ nearest marker was 10.1cM from a RAPD marker named Q04_0.3 and 2.7cM from a RFLP marker named CT135. We investigated map-position orthologies between Me ₃ and two other nematode resistance genes, the tomato Mi-3 and the potato Gpa ₂ genes, which mapped in the telomeric region of the short arm of the tomato and potato chromosome 12 (or XII for potato). Received: 23 March 2000 / Accepted: 2 January 2001     

Marker-assisted dissection of the oligogenic anthracnose resistance in the common bean cultivar, ‘G2333’

 Two independently assorting dominant genes conditioning resistance to bean anthracnose were identified in an F₂ population derived from the highly resistant bean differential cultivar, ‘G 2333’. One gene was allelic to the Co-4 gene in the differential cultivar ‘TO’ and was named Co-4 ² , whereas the second gene was assigned the temporary name Co-7 until a complete characterization with other known resistance genes can be conducted. Two RAPD markers linked to the Co-4 ² allele were identified. One RAPD, OAS13₉₅₀, co-segregated with no recombinants in two segregating populations of 143 F₂ individuals, whereas the second RAPD, OAL9₇₄₀, mapped at 3.9 cM from the Co-4 ² allele. Two 24-mer SCAR primers (SAS13), developed from the OAS13₉₅₀ RAPD marker, were dominant and polymorphic, similar to the original RAPD, and supported the tight linkage between the marker(s) and the Co-4 ² allele. The markers were present in germplasm with known resistance alleles at the Co-4 locus. The presence of the markers in two other differential cultivars not previously characterized and in four navy bean cultivars suggests the existence of a gene family for anthracnose resistance at or near the Co-4 locus. Since the Co-7 gene was present only in germplasm which also possessed the Co-4 ² and Co-5 genes, the SAS13 markers were used in combination with standard inoculation techniques to identify F₃ lines in which the Co-7 gene was homozygous and the Co-4 ² allele was absent. A similar strategy of marker-assisted dissection is proposed to identify resistant lines in which the Co-5 gene is absent and the Co-7 gene is present by selecting against the OAB3₄₅₀ marker, which has been shown previously to be linked to the Co-5 gene. These genes cannot be distinguished using traditional screening methods since all current races of the pathogen virulent to the Co-5 gene are avirulent to the Co-4 ² and Co-7 genes. We describe the use of molecular markers tightly linked to resistance genes to facilitate the identification of an uncharacterized resistance gene for which no discriminating race of the pathogen is known. Received: 22 March 1997 / Accepted: 15 July 1997     

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.     

Molecular examination of a chromosome region that controls resistance to potato Y and A potyviruses in potato

 The gene Ry _adg that confers resistance to potato Y potyvirus (PVY) in the cultivated potato [Solanum tuberosum subsp. andigena, line 2x(v-2)7] is located on chromosome XI in a segment that contains three other known resistance genes in other syntenic solanaceous species. One of them is the gene N that controls resistance to tobacco mosaic tobamovirus in tobacco and has previously been isolated and sequenced. Three sequence-related, resistance gene-like (RGL) DNA fragments (354–369 bp) highly homologous to the gene N were PCR-amplified from the potato line 2x(v-2)7. Two RGL fragments (79 and 81% homologous to the N gene) co-segregated with Ry _adg among the 77 F1 progeny tested. These RGLs may originate from a resistance gene family on chromosome XI. The potato line 2x(v-2)7 also expressed resistance to potato A potyvirus (PVA), which was controlled by another locus on chromosome XI mapped ca. 6.8 cM distal to Ry _adg . Received: 18 December 1997 / Accepted: 30 December 1997     

Molecular mapping and characterization of a single dominant gene controlling CMV resistance in peppers (Capsicum annuum L.)

Cucumber mosaic virus (CMV) is one of the most destructive viruses in the Solanaceae family. Simple inheritance of CMV resistance in peppers has not previously been documented; all previous studies have reported that resistance to this virus is mediated by several partially dominant and recessive genes. In this study, we showed that the Capsicum annuum cultivar ‘Bukang’ contains a single dominant resistance gene against CMV_Korean and CMV_FNY strains. We named this resistance gene Cmr1 (Cucumber mosaic resistance 1). Analysis of the cellular localization of CMV using a CMV green fluorescent protein construct showed that in ‘Bukang,’ systemic movement of the virus from the epidermal cell layer to mesophyll cells is inhibited. Genetic mapping and FISH analysis revealed that the Cmr1 gene is located at the centromeric region of LG2, a position syntenic to the ToMV resistance locus (Tm-1) in tomatoes. Three SNP markers were developed by comparative genetic mapping: one intron-based marker using a pepper homolog of Tm-1, and two SNP markers using tomato and pepper BAC sequences mapped near Cmr1. We expect that the SNP markers developed in this study will be useful for developing CMV-resistant cultivars and for fine mapping the Cmr1 gene.     

Microsatellite and sequence-tagged site markers diagnostic for the rice bacterial leaf blight resistance gene xa-5

 Microsatellite and sequence-tagged site (STS) markers tightly linked to the bacterial leaf blight (BLB) resistance gene xa-5 were identified in this study. A survey was conducted to find molecular markers that detected polymorphisms between the resistant (IRBB5) and susceptible (‘IR24’) nearly isogenic lines for xa-5, and between Chinsurah Boro II (CBII), an alternative source of xa-5, and a widely planted variety (‘IR64’) that lacks xa-5. Two F₂ populations, from the crosses ‘IR24’×IRBB5 and CBIIבIR64’, were used to estimate linkage based on marker genotype and reaction to disease inoculation with Xanthomonas oryzae pv. oryzae. Two RFLP clones, RZ390 and RG556, were found to co-segregate with xa-5 and were converted into STS markers. A microsatellite marker, RM390, was developed based on a simple sequence repeat in the 5′ untranslated region of the cDNA probe, RZ390, and found to co-segregate with resistance. Two other microsatellites, RM122 and RM13, were located 0.4 cM and 14.1 cM away from xa-5. A germplasm survey of diverse lines containing BLB resistance genes using automated fluorescent detection indicated the range of allelic diversity for each of the microsatellite loci linked to xa-5 and confirmed their usefulness in following genes through the narrow crosses typical of a breeding program. The limited number of alleles observed at the microsatellite loci linked to the resistance gene in 35 xa-5-containing accessions suggested either a single ancestral origin or a few independent origins of the xa-5 gene. PCR-based markers, like the ones developed in this study, are economical and easy to use, and have applicability in efforts to pyramid the recessive xa-5 gene with other BLB resistance genes. Received: 27 September 1996/Accepted: 7 February 1997     

Genetic variability of Potato virus Y (PVY) and Tobacco etch virus (TEV) from naturally infected pepper fields in the Hatay region of Turkey

The genetic structure of Potato virus Y (PVY) and Tobacco etch virus (TEV) (Potyvirus) populations was investigated in pepper fields in two regions in Turkey. The diversity of PVY and TEV populations according to coat protein (CP) and VPg coding regions showed some similarity. All the isolates built a monophyletic group due to a single introduction event or multiple introductions of genetically similar isolates. All the isolates of both viruses showed evidence to the diversification for a long time. Based on VPg and CP sequences, all PVY isolates corresponded to clade C1. Turkish potyvirus isolates were only able to break the pvr2¹ resistance allele and therefore belonged to pathotype (0,?1). The Pvr4 dominant gene was found to be efficient and durable against PVY but not at all efficient against TEV. Consequently, the pvr2² resistance allele, efficient resistance against PVY and TEV pathotype (0,?1) isolates, would be the most suitable strategy to control potyviruses.     

Molecular mapping of leaf rust resistance gene <Emphasis Type="Italic">Rph14</Emphasis> in <Emphasis Type="Italic">Hordeum vulgare</Emphasis>

An incompletely dominant gene conferring resistance to Puccinia hordei, Rph14, identified previously in an accession of Hordeum vulgare, confers resistance to all known pathotypes of P. hordei in Australia. Knowledge of the chromosomal location of Rph14 and the identification of DNA markers closely linked to it will facilitate combining it with other important leaf rust resistance genes to achieve long lasting resistance. The inheritance of Rph14 was confirmed using 146 and 106 F₃ lines derived from the crosses ‘Baudin’/‘PI 584760’ (Rph14) and ‘Ricardo’/‘PI 584760’ (Rph14), respectively. Bulk segregant analysis on DNA from the parental genotypes and resistant and susceptible DNA bulks using DArT markers located Rph14 to the short arm of chromosome 2H. DArT marker bPb-1664 was identified as having the closest genetic association with Rph14. PCR based marker analysis identified a single SSR marker, Bmag692, linked closely to Rph14 at a map distance of 2.1 and 3.8 cm in the ‘Baudin’/‘PI 584760’and ‘Ricardo’/‘PI 584760’ populations, respectively.     

Identification of RAPD markers for 11 Hessian fly resistance genes in wheat

 The pyramiding of genes that confer race- or biotype-specific resistance has become increasingly attractive as a breeding strategy now that DNA-based marker-assisted selection is feasible. Our objective here was to identify DNA markers closely linked to genes in wheat (Triticum aestivum L.) that condition resistance to Hessian fly [Mayetiola destructor (Say)]. We used a set of near-isogenic wheat lines, each carrying a resistance gene at 1 of 11 loci (H3, H5, H6, H9, H10, H11, H12, H13, H14, H16 or H17) and developed by backcrossing to the Hessian fly-susceptible wheat cultivar ‘Newton’. Using genomic DNA of these 11 lines and ‘Newton’, we have identified 18 randomly amplified polymorphic DNA (RAPD) markers linked to the 11 resistance genes. Seven of these markers were identified by denaturing gradient gel electrophoresis and the others by agarose gel electrophoresis. We confirmed linkage to the Hessian fly resistance loci by cosegregation analysis in F₂ populations of 50–120 plants for each different gene. Several of the DNA markers were used to determine the presence/absence of specific Hessian fly resistance genes in resistant wheat lines that have 1 or possibly multiple genes for resistance. The use of RAPD markers presents a valuable strategy for selection of single and combined Hessian fly resistance genes in wheat improvement. Received: 20 March 1996 / Accepted: 6 September 1996     

Multiple alleles for zucchini yellow mosaic virus resistance at the zym locus in cucumber

 Sources of resistance to several potyviruses have been identified and characterized within the cucumber (Cucumis sativus L.) germplasm. Resistance to zucchini yellow mosaic virus (ZYMV) is present in inbred lines derived from the Dutch hybrid Dina (Dina-1) and from the Chinese cultivar ‘Taichung Mou Gua’ (TMG-1). Tests of allelism indicated that the genes for resistance to ZYMV in TMG-1 and Dina-1 are at the same locus; however, the two genotypes exhibited different phenotypes in response to cotyledon inoculation with ZYMV. Dina-1 exhibited a distinct veinal chlorosis and accumulation of virus limited to the first and/or second true leaves, while TMG-1 remained symptom-free and did not accumulate virus. The distinct veinal chlorosis phenotype in Dina-1 was dominant to the symptom-free phenotype in TMG-1 and was shown not to be due to a separate gene. These results indicate that a series of alleles differing in effectiveness and dominance relationships occurs at the zym locus such that Zym>zym ^Dina>zym ^TMG-1. In addition to ZYMV resistance, TMG-1 is also resistant to watermelon mosaic virus (WMV), the watermelon strain of papaya ringspot virus (PRSV-W) and the Moroccan watermelon mosaic virus (MWMV); the WMV and MWMV resistances are at the same locus, or tightly linked to the zym locus. Dina-1 also was found to be resistant to PRSV-W and MWMV. The gene for MWMV resistance in Dina-1 appeared to be at the same locus or tightly linked (<1% recombination) to the gene for ZYMV resistance. In contrast to the response to ZYMV inoculation, Dina-1 does not exhibit distinct veinal chlorosis when inoculated with PRSV-W or MWMV. Collectively, these observations suggest that the gene(s) conferring resistance to ZYMV, WMV, and MWMV may be part of a gene cluster for potyvirus resistance in cucumber. Received: 12 November 1996 / Accepted: 25 April 1997     

Resistance to Phytophthora fragariae var. fragariae in strawberry: the Rpf2 gene

  Phytophthora fragariae var. fragariae is the causal agent of red stele (red core) root rot in strawberry (Fragaria spp.). The inheritance of resistance to one isolate of this fungus was studied in 12 segregating populations of F.×ananassa derived from crosses between four resistant cultivars (‘Climax’, ‘Redgauntlet’, ‘Siletz’, and ‘Sparkle’) and three susceptible cultivars (‘Blakemore’, ‘Glasa’, and ‘Senga’ Sengana’). The analysis clearly supports the hypothesis of a single segregating dominant resistance gene. It is proposed that this gene be designated Rpf2. Received 12 November 1996 / Accepted: 22 November 1996     

Localization of the rice stripe disease resistance gene, Stv-bi, by graphical genotyping and linkage analyses with molecular markers

 We used graphical genotyping and linkage analyses with molecular markers to determine the chromosomal location of the rice stripe disease resistance gene, Stv-b ⁱ . The stripe resistance gene from the indica rice (Oryza sativa) cv ‘Modan’ was introgressed into several Japanese rice varieties. We found 4 RFLP markers in ‘Modan’, five susceptible parental rice varieties (‘Norin No. 8’, ‘Sachihikari’, ‘Kanto No. 98’, ‘Hokuriku No.103’ and ‘Koganebare’) and four resistant progeny varieties (‘St. No. 1’, ‘Aichi No. 6’, ‘Aoisora’ and ‘Asanohikari’). Graphical genotyping of the resistant progeny revealed a chromosomal segment ascribable to ‘Modan’ and associated with stripe resistance. The chromosomal segment from ‘Modan’ was located at 35.85 cM on chromosome 11. Linkage analysis using 120 F₂ individuals from a cross between ‘Koshihikari’ (susceptible) and ‘Asanohikari’ (resistant) revealed another 8 RFLP markers in the same chromosome. We performed a bioassay for rice stripe resistance in F₃ lines of the F₂ individuals using infective small brown planthoppers and identified an 1.8-cM segment harboring the rice stripe disease resistance gene, Stv-b ⁱ , between XNpb220 and XNpb257/ XNpb254. Furthermore, Stv-b ⁱ was linked by 0.0 cM to a RFLP marker, ST10, which was developed on the basis of the results of RAPD analysis. These DNA markers near the Stv-b ⁱ locus may be useful in marker-assisted selection and map-based cloning of the Stv-b ⁱ gene. Received: 26 September 1997 / Accepted: 4 November 1997     

Genome mapping of three major resistance genes to woolly apple aphid (Eriosoma lanigerum Hausm.)

Woolly apple aphid (WAA; Eriosoma lanigerum Hausm.) can be a major economic problem to apple growers in most parts of the world, and resistance breeding provides a sustainable means to control this pest. We report molecular markers for three genes conferring WAA resistance and placing them on two linkage groups (LG) on the genetic map of apple. The Er1 and Er2 genes derived from ‘Northern Spy’ and ‘Robusta 5,’ respectively, are the two major genes that breeders have used to date to improve the resistance of apple rootstocks to this pest. The gene Er3, from ‘Aotea 1’ (an accession classified as Malus sieboldii), is a new major gene for WAA resistance. Genetic markers linked to the Er1 and Er3 genes were identified by screening random amplification of polymorphic deoxyribonucleic acid (DNA; RAPD) markers across DNA bulks from resistant and susceptible plants from populations segregating for these genes. The closest RAPD markers were converted into sequence-characterized amplified region markers and the genome location of these two genes was assigned to LG 08 by aligning the maps around the genes with a reference map of ‘Discovery’ using microsatellite markers. The Er2 gene was located on LG 17 of ‘Robusta 5’ using a genetic map developed in a M.9 × ‘Robusta 5’ progeny. Markers for each of the genes were validated for their usefulness for marker-assisted selection in separate populations. The potential use of the genetic markers for these genes in the breeding of apple cultivars with durable resistance to WAA is discussed.     

Mapping of four major rice blast resistance genes from ’Lemont’ and ’Teqing’ and evaluation of their combinatorial effect for field resistance

A framework linkage map was developed using 284 F₁₀ recombinant inbred lines (RILs) from a ’Lemont’×’Teqing’ rice cultivar cross. Evaluation of a subset of 245 of these RILs with five races of the rice blast pathogen permitted RFLP mapping of three major resistance genes from Teqing and one major gene from Lemont. All mapped genes were found to confer resistance to at least two blast races, but none conferred resistance to all five races evaluated. RFLP mapping showed that the three resistance genes from Teqing, designated Pi-tq5, Pi-tq1 and Pi-tq6, were present on chromosomes 2, 6 and 12, respectively. The resistance gene from Lemont, Pi-lm2, was located on chromosome 11. Pi-tq1 is considered a new gene, based on its reaction to these five races and its unique map location, while the other three genes may be allelic with previously reported genes. Lines with different gene combinations were evaluated for disease reaction in field plots. Some gene combinations showed both direct effects and non-linear interaction. The fact that some of the lines without any of the four tagged genes exhibited useful levels of resistance in the field plots suggests the presence of additional genes or QTLs affecting the blast reaction segregating in this population. Received: 16 December 1999 / Accepted: 28 February 2000