Molecular markers linked to white rust resistance in mustard Brassica juncea

 White rust, caused by Albugo candida (Pers.) Kuntze, is an economically important disease of Brassica juncea (L.) Czern. and Coss mustard, particularly in India. The most efficient and cost-effective way of protecting mustard plants from white rust disease is through genetic resistance. The objective of this study was to identify RAPD markers for white rust resistance in an F₁-derived doubled-haploid (DH) population originating from a cross between white rust-susceptible and white rust-resistant breeding lines of B. juncea from the canola-quality B. juncea breeding project of the Agriculture and Agri-Food Canada-Saskatoon Research Centre. The DH population was used to screen for RAPD markers associated with white rust resistance/susceptibility using bulked segregant analysis. Two markers, WR2 and WR3, linked to white rust resistance, flanked the resistance locus Ac2 ₁ and were highly effective in identifying the presence or absence of the resistance gene in the DH population. These two markers were shown to be specific to the Russian source of white rust resistance utilized in this project. It is concluded that the availability of these RAPD markers will enhance the breeding for white rust resistance in B. juncea. Received: 17 December 1997 / Accepted: 7 April 1998     

Characterization and fine mapping of <Emphasis Type="Italic">RppQ</Emphasis>, a resistance gene to southern corn rust in maize

Southern corn rust (SCR) is a fungal disease caused by Puccinia polysora Underw, which can infect maize and may result in substantial yield losses in maize production. The maize inbred line Qi319 carries the SCR resistance gene RppQ. In order to identify molecular markers linked to the RppQ gene, several techniques were utilized including random amplified polymorphic DNA (RAPD), simple sequence repeat (SSR), and amplified fragment length polymorphism (AFLP). In addition, sequence characterized amplified region (SCAR) techniques combined with bulked segregant analysis (BSA) were used. Seven RAPD markers, eight SSR markers, and sixty-three AFLP primer combinations amplified polymorphisms between two parents and two bulk populations. A large F₂ population was used for genetic analysis and for fine mapping of the RppQ gene region. One AFLP polymorphic band, M-CAA/E-AGC₃₂₄, was converted to a SCAR marker, MA7, which was mapped to a position 0.46 cM from RppQ. Finally, the RppQ gene was mapped between the SCAR marker MA7 and the AFLP marker M-CCG/E-AGA₁₅₇ with distances of 0.46 and 1.71 cM, respectively.     

Identification of molecular markers linked to the Yr15 stripe rust resistance gene of wheat originated in wild emmer wheat, Triticum dicoccoides

 The Yr15 gene of wheat confers resistance to the stripe rust pathogen Puccinia striiformis West., which is one of the most devastating diseases of wheat throughout the world. In the present study, molecular markers flanking the Yr15 gene of wheat have been identified using the near-isogenic-lines approach. RFLP screening of 76 probe-enzyme combinations revealed one polymorphic marker (Nor/TaqI) between the susceptible and the resistant lines. In addition, out of 340 RAPD primers tested, six produced polymorphic RAPD bands between the susceptible and the resistant lines. The genetic linkage of the polymorphic markers was tested on segregating F₂ population (123 plants) derived from crosses between stripe rust-susceptible Triticum durum wheat, cv D447, and a BC₃F₉ resistant line carrying Yr15 in a D447 background. A 2.8-kb fragment produced by the Nor RFLP probe and a 1420-bp PCR product generated by the RAPD primer OPB13 showed linkage, in coupling, with the Yr15 gene. Employing the standard maximum-likelihood technique it was found that the order OPB13 ₁₄₂₀ –Yr15–Nor1 on chromosome 1B appeared to be no less than 1000-times more probable than the closest alternative. The map distances between OPB13 ₁₄₂₀ –Yr15–Nor1 are 27.1 cM and 11.0 cM for the first and second intervals, respectively. The application of marker-assisted selection for the breeding of new wheat cultivars with the stripe rust resistance gene is discussed. Received: 27 February 1997/Accepted: 7 March 1997     

Genetic linkage maps of white birches (<Emphasis Type="Italic">Betula platyphylla</Emphasis> Suk. and <Emphasis Type="Italic">B. pendula</Emphasis> Roth) based on RAPD and AFLP markers

A pseudo-testcross mapping strategy was used in combination with the random amplified polymorphism DNA (RAPD) and amplified fragment length polymorphism (AFLP) genotyping methods to develop two moderately dense genetic linkage maps for Betula platyphylla Suk. (Asian white birch) and B. pendula Roth (European white birch). Eighty F₁ progenies were screened with 291 RAPD markers and 451 AFLP markers. We selected 230 RAPD and 362 AFLP markers with 1:1 segregation and used them for constructing the parent-specific linkage maps. The resultant map for B. platyphylla was composed of 226 markers in 24 linkage groups (LGs), and spanned 2864.5 cM with an average of 14.3 cM between adjacent markers. The linkage map for B. pendula was composed of 226 markers in 23 LGs, covering 2489.7 cM. The average map distance between adjacent markers was 13.1 cM. Clustering of AFLP markers was observed on several LGs. The availability of these white birch linkage maps will contribute to the molecular genetics and the implementation of marker-assisted selection in these important forest species.     

Marker-assisted selection for two rust resistance genes in sunflower

In this study we report on the identification of molecular markers, OX20₆₀₀ and OO04₉₅₀, linked to the geneR _Adv in the proprietary inbred line P2. This gene confers resistance to most of the pathotypes of Puccinia helianthi identified in Australia. Analysis indicates these RAPD markers are linked to the resistance locus at 0.0 cM and 11 cM respectively. SCAR markers SCX20₆₀₀ and SCO04₉₅₀ derived from these two RAPD markers, and SCT06₉₅₀ derived from a previously reported RAPD marker linked at 4.5 cM from the R ₁ rust resistance gene were developed. SCX20₆₀₀ and SCO04₉₅₀ were linked at similar distances from their resistance locus as the RAPD markers. SCTO6₉₅₀ co-segregated completely with rust resistance. The robustness of the R ₁ SCAR marker was demonstrated through the amplification of the marker in a diverse range of sunflower germplasm considered to possess the R ₁ gene. The SCAR markers forR _Adv were not amplified in the sunflower rust differential set thereby supporting the contention that this is a novel resistance gene. They did amplify in a number of proprietary lines closely related to the line P2. This locus is under further investigation as it will be useful in our attempts to use molecular-assisted breeding to produce durable resistance in sunflower to P. helianthi.     

Identification and mapping of resistance gene analogs and a white rust resistance locus in<Emphasis Type="Italic"> Brassica rapa</Emphasis> ssp.<Emphasis Type="Italic"> oleifera</Emphasis>

The objective of this investigation was to tag a locus for white rust resistance in a Brassica rapa ssp. oleifera F₂ population segregating for this trait, using bulked segregant analysis with random amplified polymorphic DNA (RAPD) markers, linkage mapping and a candidate gene approach based on resistance gene analogs (RGAs). The resistance source was the Finnish line Bor4109. The reaction against white rust races 7a and 7v was scored in 20 seedlings from each self-pollinated F₂ individual. The proportion of resistant plants among these F₃ families varied from 0 to 67%. Bulked segregant analysis did not reveal any markers linked with resistance and, therefore, a linkage map with 81 markers was created. A locus that accounted for 18.4% of the variation in resistance to white rust was mapped to linkage group (LG) 2 near the RAPD marker Z19a. During the study, a bacterial resistance gene homologous to Arabidopsis RPS2 and six different RGAs were sequenced. RPS2 and five of the RGAs were mapped to linkage groups LG1, LG4 and LG9. Unfortunately, none of the RGAs could be shown to be associated with white rust resistance.Communicated by H.C. BeckerThe nucleotide sequence data reported has been deposited in the Genbank under the accession numbers AF315081–AF315087.     

Development of leucine-rich repeat polymorphism, amplified fragment length polymorphism, and sequence characterized amplified region markers to the Cronartium ribicola resistance gene Cr2 in western white pine ( Pinus monticola )

White pine blister rust (WPBR), caused by Cronartium ribicola, is a devastating disease in Pinus monticola and other five-needle pines. Pyramiding a major resistance gene (Cr2) with other resistance genes is an important component of integrated strategies to control WPBR in P. monticola. To facilitate this strategy, the objective of the present study was to identify leucine-rich repeat (LRR) polymorphisms, amplified fragment length polymorphisms (AFLPs), and sequence characterized amplified region (SCAR) markers linked to the western white pine Cr2 (BSA) gene for precise gene mapping. Bulked segregant analysis and haploid segregation analysis allowed the identification of 11 LRR polymorphisms and five AFLP markers in the Cr2 linkage. The closest LRR markers were 0.53 Kosambi cM from Cr2 at either end. After marker cloning and sequencing, AFLP marker EacccMccgat-365 and random polymorphic DNA marker U570–843 were converted successfully into SCAR markers. For a potential application in marker-assisted selection (MAS), these two SCAR markers were verified in two western white pine families. This study represents the first report of LRR-related DNA markers linked to C. ribicola resistance in five-needle pines. These findings may help further candidate gene identification for disease resistance in a conifer species.     

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.     

Brassica napus DNA markers linked to white rust resistance in Brassica juncea

White rust, caused by Albugo candida, is an economically important disease of Brassica juncea mustard. The most efficient and cost effective way of protecting mustard plants from white rust is through genetic resistance. The development of canola quality B. juncea through interspecific crosses of B. juncea with Brassica napus has lead to the introgression of white rust resistance from B. napus into B. juncea. The objective of this study was to identify DNA markers for white rust resistance, derived from the introgressed B. napus chromosome segment, in a BC(3)F(2) population of condiment B. juncea mustard. This segregating population was phenotyped for white rust reaction and used to screen for AFLP markers associated with white rust resistance using bulked segregant analysis. Segregation data indicated that a single dominant gene controlled resistance to white rust. Eight AFLP markers linked to white rust resistance were identified, all derived from B. napus. The B. napus chromosome segment, carrying the white rust resistance gene ( Ac2V(1)), appeared to have recombined with the B. juncea DNA since recombinant individuals were identified. Comparative mapping of the eight B. napus-derived AFLP markers in a typical B. napus mapping population was inconclusive; therefore, the size of the introgressed B. napus fragment could not be determined.     

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

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

Rust mite resistance in apple assessed by quantitative trait loci analysis

The aim of this study was to assess the genetic basis of rust mite (Aculus schlechtendali) resistance in apple (Malus × domestica). A. schlechtendali infestation of apple trees has increased as a consequence of reduced side effects of modern fungicides on rust mites. An analysis of quantitative trait loci (QTLs) was carried out using linkage map data available for F₁ progeny plants of the cultivars ‘Fiesta’ × ‘Discovery’. Apple trees representing 160 different genotypes were surveyed for rust mite infestation, each at three different sites in two consecutive years. The distribution of rust mites on the individual apple genotypes was aggregated and significantly affected by apple genotype and site. We identified two QTLs for A. schlechtendali resistance on linkage group 7 of ‘Fiesta’. The AFLP marker E35M42-0146 (20.2 cM) and the RAPD marker AE10-400 (45.8 cM) were closest positioned to the QTLs and explained between 11.0% and 16.6% of the phenotypic variability. Additionally, putative QTLs on the ‘Discovery’ chromosomes 4, 5 and 8 were detected. The SSR marker Hi03a10 identified to be associated to one of the QTLs (AFLP marker E35M42-0146) was traced back in the ‘Fiesta’ pedigree to the apple cultivar ‘Wagener’. This marker may facilitate the breeding of resistant apple cultivars by marker assisted selection. Furthermore, the genetic background of rust mite resistance in existing cultivars can be evaluated by testing them for the identified SSR marker. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Development of RAPD and SCAR markers linked to the Russian wheat aphid resistance gene Dn2 in wheat

 RAPD (random amplified polymorphic DNA) analysis was used to identify molecular markers linked to the Dn2 gene conferring resistance to the Russian wheat aphid (Diuraphis noxia Mordvilko). A set of near-isogenic lines (NILs) was screened with 300 RAPD primers for polymorphisms linked to the Dn2 gene. A total of 2700 RAPD loci were screened for linkage to the resistance locus. Four polymorphic RAPD fragments, two in coupling phase and two in repulsion phase, were identified as putative RAPD markers for the Dn2 gene. Segregation analysis of these markers in an F₂ population segregating for the resistance gene revealed that all four markers were closely linked to the Dn2 locus. Linkage distances ranged from 3.3 cM to 4.4 cM. Southern analysis of the RAPD products using the cloned RAPD markers as probes confirmed the homology of the RAPD amplification products. The coupling-phase marker OPB10_880c and the repulsion-phase marker OPN1_400r were converted to sequence characterized amplified region (SCAR) markers. SCAR analysis of the F₂ population and other resistant and susceptible South African wheat cultivars corroborated the observed linkage of the RAPD markers to the Dn2 resistance locus. These markers will be useful for marker-assisted selection of the Dn2 gene for resistance breeding and gene pyramiding. Received: 1 July 1997 / Accepted: 20 October 1997     

Linkage analysis of a fertility restoring mutant generated from CMS rice

 DNA polymorphism between a cytoplasmic male-sterile rice line II-32A, the male-fertile maintainer counterpart II-32B, a fertile revertant (T24), as well as two commercial indica restorers, was analyzed with randomly amplified polymorphic DNA (RAPD). A very low degree of polymorphism was found between the revertant T24 and II-32A compared with that of indica rice varieties. This result, together with agronomic and genetic evidence, suggests the revertant to be a product of a nuclear mutation. An analysis of polymorphism between II-32A and the revertant T24 with 510 RAPD decamer primers identified the co-segregating markers OPB07₆₄₀ and OPB18₁₀₀₀ to be linked to a sterile allele of the restoring locus in the revertant T24, at a distance of 5.3 cM. RAPD analysis of a mapping population of Tesanai2/CB with primer OPB07 revealed linkage of OPB07₆₄₀ with RG374 (10.8 cM) and RG394 (8.8 cM) on chromosome 1. Thus the restorer gene, designated Rf ₅, was tentatively localized between RG374 and RG394 on chromosome 1 and appears to be independent of other mapped restorer genes in rice. Received: 11 November 1997 / Accepted: 17 December 1997     

Linkage of a RAPD marker with powdery mildew resistance <Emphasis Type="Italic">er-1</Emphasis> gene in <Emphasis Type="Italic">Pisum sativum</Emphasis> L.

The aim of this study was to investigate the inheritance of powdery mildew disease and to tag it with a DNA marker to utilize for the marker-assisted selection (MAS) breeding program. The powdery mildew resistant genotype Fallon ^er and susceptible genotype 11760-3 ^ER were selected from 177 genotypes by heavy infestation of germplasm with Erysiphe pisi through artificial inoculation The F₁ plants of the cross Fallon/11760-3 indicated the dominance of the susceptible allele, while F₂ plants segregated in 3: 1 ratio (susceptible: resistant) that fit for goodness of fitness by χ² (P > 0.07), indicating monogenic recessive inheritance for powdery mildew resistance in Pisum sativum. A novel RAPD marker OPB18 (5′-CCACAGCAGT-3′) was linked to the er-1 gene with 83% probability with a LOD score of 4.13, and was located at a distance of 11.2 cM from the er-1 gene.     

Identification and molecular tagging of a gene from PI 289824 conferring resistance to leaf rust (Puccinia triticina) in wheat

Host-plant resistance is the most economically viable and environmentally responsible method of control for Puccinia triticina, the causal agent of leaf rust in wheat (Triticum aestivum L.). The identification and utilization of new resistance sources is critical to the continued development of improved cultivars as shifts in pathogen races cause the effectiveness of widely deployed genes to be short lived. The objectives of this research were to identify and tag new leaf rust resistance genes. Forty landraces from Afghanistan and Iran were obtained from the National Plant Germplasm System and evaluated under field conditions at two locations in Texas. PI 289824, a landrace from Iran, was highly resistant under field infection. Further evaluation revealed that PI 289824 is highly resistant to a broad spectrum of leaf rust races, including the currently prevalent races of leaf rust in the Great Plains area of the USA. Eight F₁ plants, 176 F₂ individuals and 139 F_2:3 families of a cross between PI 289824 and T112 (susceptible) were evaluated for resistance to leaf rust at the seedling stage. Genetic analysis indicated resistance in PI 289824 is controlled by a single dominant gene. The AFLP analyses resulted in the identification of a marker (P39 M48-367) linked to resistance. The diagnostic AFLP band was sequenced and that sequence information was used to develop an STS marker (TXW₂₀₀) linked to the gene at a distance of 2.3 cM. The addition of microsatellite markers allowed the gene to be mapped to the short arm of Chromosome 5B. The only resistance gene to be assigned to Chr 5BS is Lr52. The Lr52 gene was reported to be 16.5 cM distal to Xgwm443 while the gene in PI 289824 mapped 16.7 cM proximal to Xgwm443. Allelism tests are needed to determine the relationship between the gene in PI 289824 and Lr52. If the reported map positions are correct, the gene in PI 289824 is unique.     

Mapping of QTLs for oil content and fatty acid composition in Indian mustard [Brassica juncea (L.) Czern. and Coss.]

An AFLP linkage map of Brassica juncea (L.) Czern and Coss was constructed using 88 recombinant inbred lines (RILs) from a cross between an Indian cultivar ‘Varuna’ and an accession from Poland ‘BEC-144’. The map included 91 AFLP markers organized on 19 linkage groups covering a total map distance of 1679.1 cM. A total of 14 QTLs were detected for oil content (2 QTLs), erucic acid (2 QTLs), eicosenoic acid (2 QTLs), linolenic acid (3 QTLs), linoleic acid (3 QTLs) and palmitic acid (2 QTLs). A specific genomic region on LG2 was associated with contents of three fatty acids: erucic acid, eicosenoic acid and linoleic acid. Some of the markers showed absolute linkage with the QTLs associated with the levels of linolenic acid, linoleic acid and oil content. These markers may be used for improvement of fatty acid profile of B. juncea.     

Molecular mapping of <Emphasis Type="Italic">Stb1</Emphasis>, a potentially durable gene for resistance to septoria tritici blotch in wheat

Septoria tritici blotch (STB), caused by the ascomycete Mycosphaerella graminicola (anamorph Septoria tritici), was the most destructive disease of wheat in Indiana and adjacent states before deployment of the resistance gene Stb1 during the early 1970s. Since then, Stb1 has provided durable protection against STB in widely grown wheat cultivars. However, its chromosomal location and allelic relationships to most other STB genes are not known, so the molecular mapping of Stb1 is of great interest. Genetic analyses and molecular mapping were performed for two mapping populations. A total of 148 F₁ plants (mapping population I) were derived from a three-way cross between the resistant line P881072-75-1 and the susceptible lines P881072-75-2 and Monon, and 106 F₆ recombinant-inbred lines (mapping population II) were developed from a cross between the resistant line 72626E2-12-9-1 and the susceptible cultivar Arthur. Bulked-segregant analysis with random amplified polymorphic DNA (RAPD), amplified fragment length polymorphism (AFLP), and microsatellite or simple-sequence repeat (SSR) markers was conducted to identify those that were putatively linked to the Stb1 gene. Segregation analyses confirmed that a single dominant gene controls the resistance to M. graminicola in each mapping population. Two RAPD markers, G7₁₂₀₀ and H19₅₂₀, were tightly linked to Stb1 in wheat line P881072-75-1 at distances of less than 0.68 cM and 1.4 cM, respectively. In mapping population II, the most closely linked marker was SSR Xbarc74, which was 2.8 cM proximal to Stb1 on chromosome 5BL. Microsatellite loci Xgwm335 and Xgwm213 also were proximal to Stb1 at distances of 7.4 cM and 8.3 cM, respectively. The flanking AFLP marker, EcoRI-AGC/MseI-CTA-1, was 8.4 cM distal to Stb1. The two RAPD markers, G7₁₂₀₀ and H19₅₂₀, and AFLP EcoRI-AGC/MseI-CTA-1, were cloned and sequenced for conversion into sequence-characterized amplified region (SCAR) markers. Only RAPD allele H19₅₂₀ could be converted successfully, and none of the SCAR markers was diagnostic for the Stb1 locus. Analysis of SSR and the original RAPD primers on several 5BL deletion stocks positioned the Stb1 locus in the region delineated by chromosome breakpoints at fraction lengths 0.59 and 0.75. The molecular markers tightly linked to Stb1 could be useful for marker-assisted selection and for pyramiding of Stb1 with other genes for resistance to M. graminicola in wheat.     

Genetic linkage map of melon (<Emphasis Type="Italic">Cucumis melo</Emphasis> L.) and localization of a major QTL for powdery mildew resistance

Powdery mildew caused by Podosphaera xanthii has become a major problem in melon since it occurs all year round irrespective of the growing system. The TGR-1551 melon genotype was found to be resistant to several melon diseases, among them powdery mildew. However, the corresponding resistance genes have been never mapped. We constructed an integrated genetic linkage map using an F2 population derived from a cross between the multi-resistant genotype TGR-1551 and the susceptible Spanish cultivar ‘Bola de Oro’. The map spans 1,284.9 cM, with an average distance of 3.6 cM among markers, and consists of 354 loci (188 AFLP, 39 RAPD, 111 SSR, 14 SCAR/CAPS/dCAPS, and two phenotypic traits) distributed in 14 linkage groups. QTL analysis identified one major QTL (Pm-R) on LG V for resistance to races 1, 2, and 5 of powdery mildew. The PM4-CAPS marker is closely linked to the Pm-R QTL at a genetic distance of 1.9 cM, and the PM3-CAPS marker is located within the support interval of this QTL. These codominant markers, together with the map information reported here, could be used for melon breeding, and particularly for genotyping selection of resistance to powdery mildew in this vegetable crop species.     

Mapping of southern corn rust-resistant genes in the W2D inbred line of maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.)

Southern corn rust, caused by Puccinia polysora Underw., has destructive potential on the susceptible host. In this study, the resistance inheritance was investigated in an F ₂ and its F _2:3 populations derived from a cross from two inbred lines W2D (resistant) and W222 (susceptible). The 3:1 ratio of resistant to susceptible plants indicated that the resistance is controlled by one dominant gene (named as RppD). The gene RppD was located by means of the F ₂ population. Total of 11 markers, including five SSR markers, five sequence-tagged site markers and one cleaved-amplified polymorphic sequence (CAPS) marker, were identified to narrow the gene RppD down to a smaller interval. The closest markers flanking RppD were SSR marker umc1291 and CAPS marker CAPS858, with genetic distances of 2.9 and 0.8 cM, respectively. Moreover, RppD might be a novel Rpp resistance gene or haplotype differing from RppQ and RppP25 according to an allelism test among the three crosses W2D × Qi319, W2D × P25 and Qi319 × P25. As a result, RppD haplotype might be helpful to maize germplasm enhancement and disease-resistant breeding.     

Development and mapping of AFLP markers linked to the sorghum fertility restorer gene <Emphasis Type="Italic">rf4</Emphasis>

The restoration of male fertility in the sorghum IS1112 C (A3) male-sterile cytoplasm is through a two-gene gametophytic system involving complementary action of the restoring alleles Rf3 and Rf4. To develop markers suitable for mapping rf4, AFLP technology was applied to bulks of sterile and fertile individuals from a segregating BC₃F₁ population. Three AFLP markers linked to rf4 were identified and subsequently converted to STS/CAPS markers, two of which are co-dominant. Based on a population of 378 BC₁F₁ individuals, two STS/CAPS markers, LW7 and LW8, mapped to within 5.31 and 3.18 cM, respectively, of rf4, while an STS marker, LW9, was positioned 0.79 cM on the flanking side of rf4. Markers LW8 and LW9 were used to screen sorghum BAC libraries to identify the genomic region encoding rf4. A series of BAC clones shown to represent a genomic region of linkage group E were identified by the rf4-linked markers. A contig of BAC clones flanking the LW9 marker represent seed clones on linkage group E, from which fine mapping of the rf4 locus and chromosome walking can be initiated. Received: 20 June 2001 / Accepted: 3 August 2001