Discovery and introgression of the wild sunflower-derived novel downy mildew resistance gene <Emphasis Type="Italic">Pl</Emphasis><Subscript><Emphasis Type="Italic">19</Emphasis></Subscript> in confection sunflower (<Emphasis Type="Italic">Helianthus annuus</Emphasis> L.)

Key message

A new downy mildew resistance gene, Pl ₁₉ , was identified from wild Helianthus annuus accession PI 435414, introduced to confection sunflower, and genetically mapped to linkage group 4 of the sunflower genome.

Abstract

Wild Helianthus annuus accession PI 435414 exhibited resistance to downy mildew, which is one of the most destructive diseases to sunflower production globally. Evaluation of the 140 BC₁F_2:3 families derived from the cross of CMS CONFSCLB1 and PI 435414 against Plasmopara halstedii race 734 revealed that a single dominant gene controls downy mildew resistance in the population. Bulked segregant analysis conducted in the BC₁F₂ population with 860 simple sequence repeat (SSR) markers indicated that the resistance derived from wild H. annuus was associated with SSR markers located on linkage group (LG) 4 of the sunflower genome. To map and tag this resistance locus, designated Pl ₁₉ , 140 BC₁F₂ individuals were used to construct a linkage map of the gene region. Two SSR markers, ORS963 and HT298, were linked to Pl ₁₉ within a distance of 4.7 cM. After screening 27 additional single nucleotide polymorphism (SNP) markers previously mapped to this region, two flanking SNP markers, NSA_003564 and NSA_006089, were identified as surrounding the Pl ₁₉ gene at a distance of 0.6 cM from each side. Genetic analysis indicated that Pl ₁₉ is different from Pl ₁₇ , which had previously been mapped to LG4, but is closely linked to Pl ₁₇ . This new gene is highly effective against the most predominant and virulent races of P. halstedii currently identified in North America and is the first downy mildew resistance gene that has been transferred to confection sunflower. The selected resistant germplasm derived from homozygous BC₂F₃ progeny provides a novel gene for use in confection sunflower breeding programs.

     

Development of PCR markers for the<Emphasis Type="Italic"> Pl5/Pl8</Emphasis> locus for resistance to<Emphasis Type="Italic"> Plasmopara halstedii</Emphasis> in sunflower,<Emphasis Type="Italic"> Helianthus annuus</Emphasis> L. from complete CC-NBS-LRR sequences

Sunflower downy mildew, caused by Plasmopara halstedii, is one of the major diseases of this crop. Development of elite sunflower lines resistant to different races of this oomycete seems to be the most efficient method to limit downy mildew damage. At least two different gene clusters conferring resistance to different races of P. halstedii have been described. In this work we report the cloning and mapping of two full-length resistance gene analogs (RGA) belonging to the CC-NBC-LRR class of plant resistance genes. The two sequences were then used to develop 14 sequence tagged sites (STS) within the Pl5/Pl8 locus conferring resistance to a wide range of P. halstedii races. These STSs will be useful in marker-assisted selection programs.Communicated by C. Möllers     

<Emphasis Type="Italic">Pm37</Emphasis>, a new broadly effective powdery mildew resistance gene from <Emphasis Type="Italic">Triticum </Emphasis><Emphasis Type="Italic">timopheevii</Emphasis>

Powdery mildew is an important foliar disease in wheat, especially in areas with a cool or maritime climate. A dominant powdery mildew resistance gene transferred to the hexaploid germplasm line NC99BGTAG11 from T. timopheevii subsp. armeniacum was mapped distally on the long arm of chromosome 7A. Differential reactions were observed between the resistance gene in NC99BGTAG11 and the alleles of the Pm1 locus that is also located on chromosome arm 7AL. Observed segregation in F_2:3 lines from the cross NC99BGTAG11 × Axminster (Pm1a) demonstrate that germplasm line NC99BGTAG11 carries a novel powdery mildew resistance gene, which is now designated as Pm37. This new gene is highly effective against all powdery mildew isolates tested so far. Analyses of the population with molecular markers indicate that Pm37 is located 16 cM proximal to the Pm1 complex. Simple sequence repeat (SSR) markers Xgwm332 and Xwmc790 were located 0.5 cM proximal and distal, respectively, to Pm37. In order to identify new markers in the region, wheat expressed sequence tags (ESTs) located in the distal 10% of 7AL that were orthologous to sequences from chromosome 6 of rice were targeted. The two new EST-derived STS markers were located distal to Pm37 and one marker was closely linked to the Pm1a region. These new markers can be used in marker-assisted selection schemes to develop wheat cultivars with pyramids of powdery mildew resistance genes, including combinations of Pm37 in coupling linkage with alleles of the Pm1 locus.     

<Emphasis Type="Italic">MlAG12</Emphasis>: a <Emphasis Type="Italic">Triticum timopheevii</Emphasis>-derived powdery mildew resistance gene in common wheat on chromosome 7AL

Wheat powdery mildew is an economically important disease in cool and humid environments. Powdery mildew causes yield losses as high as 48% through a reduction in tiller survival, kernels per head, and kernel size. Race-specific host resistance is the most consistent, environmentally friendly and, economical method of control. The wheat (Triticum aestivum L.) germplasm line NC06BGTAG12 possesses genetic resistance to powdery mildew introgressed from the AAGG tetraploid genome Triticum timopheevii subsp. armeniacum. Phenotypic evaluation of F₃ families derived from the cross NC06BGTAG12/‘Jagger’ and phenotypic evaluation of an F₂ population from the cross NC06BGTAG12/‘Saluda’ indicated that resistance to the ‘Yuma’ isolate of powdery mildew was controlled by a single dominant gene in NC06BGTAG12. Bulk segregant analysis (BSA) revealed simple sequence repeat (SSR) markers specific for chromosome 7AL segregating with the resistance gene. The SSR markers Xwmc273 and Xwmc346 mapped 8.3 cM distal and 6.6 cM proximal, respectively, in NC06BGTAG12/Jagger. The multiallelic Pm1 locus maps to this region of chromosome 7AL. No susceptible phenotypes were observed in an evaluation of 967 F₂ individuals in the cross NC06BGTAG12/‘Axminster’ (Pm1a) which indicated that the NC06BGTAG12 resistance gene was allelic or in close linkage with the Pm1 locus. A detached leaf test with ten differential powdery mildew isolates indicated the resistance in NC06BGTAG12 was different from all designated alleles at the Pm1 locus. Further linkage and allelism tests with five other temporarily designated genes in this very complex region will be required before giving a permanent designation to this gene. At this time the gene is given the temporary gene designation MlAG12.     

Inheritance and mapping of powdery mildew resistance gene <Emphasis Type="Italic">Pm43</Emphasis> introgressed from <Emphasis Type="Italic">Thinopyrum</Emphasis><Emphasis Type="Italic">intermedium</Emphasis> into wheat

Powdery mildew resistance from Thinopyrum intermedium was introgressed into common wheat (Triticum aestivum L.). Genetic analysis of the F₁, F₂, F₃ and BC₁ populations from powdery mildew resistant line CH5025 revealed that resistance was controlled by a single dominant allele. The gene responsible for powdery mildew resistance was mapped by the linkage analysis of a segregating F₂ population. The resistance gene was linked to five co-dominant genomic SSR markers (Xcfd233, Xwmc41, Xbarc11, Xgwm539 and Xwmc175) and their most likely order was Xcfd233–Xwmc41–Pm43–Xbarc11–Xgwm539–Xwmc175 at 2.6, 2.3, 4.2, 3.5 and 7.0 cM, respectively. Using the Chinese Spring nullisomic-tetrasomic and ditelosomic lines, the polymorphic markers and the resistance gene were assigned to chromosome 2DL. As no powdery mildew resistance gene was previously assigned to chromosome 2DL, this new resistance gene was designated Pm43. Pm43, together with the identified closely linked markers, could be useful in marker-assisted selection for pyramiding powdery mildew resistance genes. Runli He and Zhijian Chang contributed equally to this work.     

Mapping of the <Emphasis Type="Italic">multifoliate pinna</Emphasis> (<Emphasis Type="Italic">mfp</Emphasis>) leaf-blade morphology mutation in grain pea <Emphasis Type="Italic">Pisum sativum</Emphasis>

The multifoliate pinna (mfp) mutation alters the leaf-blade architecture of pea, such that simple tendril pinnae of distal domain are replaced by compound pinna blades of tendrilled leaflets in mfp homozygotes. The MFP locus was mapped with reference to DNA markers using F₂ and F_2:5 RIL as mapping populations. Among 205 RAPD, 27 ISSR and 35 SSR markers that demonstrated polymorphism between the parents of mapping populations, three RAPD markers were found linked to the MFP locus by bulk segregant analyses on mfp/mfp and MFP/MFP bulks assembled from the F_2:5 population. The segregational analysis of mfp and 267 DNA markers on 96 F₂ plants allowed placement of 26 DNA markers with reference to MFP on a linkage group. The existence of common markers on reference genetic maps and MFP linkage group developed here showed that MFP is located on linkage group IV of the consensus genetic map of pea.     

Molecular mapping of the rust resistance gene <Emphasis Type="Italic">R</Emphasis><Subscript><Emphasis Type="Italic">4</Emphasis></Subscript> to a large NBS-LRR cluster on linkage group 13 of sunflower

Rust is a serious fungal disease in the sunflower growing areas worldwide with increasing importance in North America in recent years. Several genes conferring resistance to rust have been identified in sunflower, but few of them have been genetically mapped and linked to molecular markers. The rust resistance gene R ₄ in the germplasm line HA-R3 was derived from an Argentinean open-pollinated variety and is still one of most effective genes. The objectives of this study were to determine the chromosome location of the R ₄ gene and the allelic relationship of R ₄ with the R _adv rust resistance gene. A total of 63 DNA markers previously mapped to linkage group (LG) 13 were used to screen for polymorphisms between two parental lines HA 89 and HA-R3. A genetic map of LG 13 was constructed with 21 markers, resulting in a total map length of 93.8 cM and an average distance of 4.5 cM between markers. Two markers, ZVG61 and ORS581, flanked the R ₄ gene at 2.1 and 0.8 cM, respectively, and were located on the lower end of LG 13 within a large NBS-LRR cluster identified previously. The PCR pattern generated by primer pair ZVG61 was unique in the HA-R3 line, compared to lines HA-R1, HA-R4, and HA-R5, which carry other R ₄ alleles. A SCAR marker linked to the rust resistance gene R _adv mapped to LG 13 at 13.9 cM from the R ₄ locus, indicating that R _adv is not an allele of the R ₄ locus. The markers tightly linked to the R ₄ gene will facilitate gene pyramiding for rust resistance breeding of sunflower.     

<Emphasis Type="Italic">Vr</Emphasis><Subscript><Emphasis Type="Italic">2</Emphasis></Subscript>: a new apple scab resistance gene

Reports from several European countries of the breakdown of the Vf resistance, the most frequently used source of resistance in breeding programs against apple scab, emphasize the urgency of diversifying the basis of apple scab resistance and pyramiding different apple scab resistances with the use of their associated molecular markers. GMAL 2473 is an apple scab resistant selection thought to carry the resistance gene Vr. We report the identification by BSA of three AFLP markers and one RAPD marker associated with the GMAL 2473 resistance gene. SSRs associated with the resistance gene were found by (1) identifying the linkage group carrying the apple scab resistance and (2) testing the SSRs previously mapped in the same region. One such SSR, CH02c02a, mapped on linkage group 2, co-segregates with the resistance gene. GMAL 2473 was tested with molecular markers associated with other apple scab resistance genes, and accessions carrying known apple scab resistance genes were tested with the SSR linked to the resistance gene found in GMAL 2473. The results indicate that GMAL 2473 does not carry Vr, and that a new apple scab resistance gene, named Vr ₂, has been identified.     

<Emphasis Type="Italic">Rpv10</Emphasis>: a new locus from the Asian <Emphasis Type="Italic">Vitis</Emphasis> gene pool for pyramiding downy mildew resistance loci in grapevine

A population derived from a cross between grapevine breeding strain Gf.Ga-52-42 and cultivar ‘Solaris’ consisting of 265 F1-individuals was genetically mapped using SSR markers and screened for downy mildew resistance. Quantitative trait locus (QTL) analysis revealed two strong QTLs on linkage groups (LGs) 18 and 09. The locus on LG 18 was found to be identical with the previously described locus Rpv3 and is transmitted by Gf.Ga-52-42. ‘Solaris’ transmitted the resistance-related locus on LG 09 explaining up to 50% of the phenotypic variation in the population. This downy mildew resistance locus is named Rpv10 for resistance to Plasmopara viticola. Rpv10 was initially introgressed from Vitis amurensis, a wild species of the Asian Vitis gene pool. The one-LOD supported confidence interval of the QTL spans a section of 2.1 centi Morgan (cM) corresponding to 314 kb in the reference genome PN40024 (12x). Eight resistance gene analogues (RGAs) of the NBS–LRR type and additional resistance-linked genes are located in this region of PN40024. The F1 sub-population which contains the Rpv3 as well as the Rpv10 locus showed a significantly higher degree of resistance, indicating additive effects by pyramiding of resistance loci. Possibilities for using the resistance locus Rpv10 in a grapevine breeding programme are discussed. Furthermore, the marker data revealed ‘Severnyi’ × ‘Muscat Ottonel’ as the true parentage for the male parent of ‘Solaris’.     

Identification and fine mapping of <Emphasis Type="Italic">Pi39</Emphasis>(t), a major gene conferring the broad-spectrum resistance to <Emphasis Type="Italic">Magnaporthe oryzae</Emphasis>

Blast, caused by the ascomycete fungus Magnaporthe oryzae, is one of the most devastating diseases of rice worldwide. The Chinese native cultivar (cv.) Q15 expresses the broad-spectrum resistance to most of the isolates collected from China. To effectively utilize the resistance, three rounds of linkage analysis were performed in an F₂ population derived from a cross of Q15 and a susceptible cv. Tsuyuake, which segregated into 3:1 (resistant/susceptible) ratio. The first round of linkage analysis employing simple sequence repeat (SSR) markers was carried out in the F₂ population through bulked-segregant assay. A total of 180 SSR markers selected from each chromosome equally were surveyed. The results revealed that only two polymorphic markers, RM247 and RM463, located on chromosome 12, were linked to the resistance (R) gene. To further define the chromosomal location of the R gene locus, the second round of linkage analysis was performed using additional five SSR markers, which located in the region anchored by markers RM247 and RM463. The locus was further mapped to a 0.27 cM region bounded by markers RM27933 and RM27940 in the pericentromeric region towards the short arm. For fine mapping of the R locus, seven new markers were developed in the smaller region for the third round of linkage analysis, based on the reference sequences. The R locus was further mapped to a 0.18 cM region flanked by marker clusters 39M11 and 39M22, which is closest to, but away from the Pita/Pita ² locus by 0.09 cM. To physically map the locus, all the linked markers were landed on the respective bacterial artificial chromosome clones of the reference cv. Nipponbare. Sequence information of these clones was used to construct a physical map of the locus, in silico, by bioinformatics analysis. The locus was physically defined to an interval of ≈37 kb. To further characterize the R gene, five R genes mapped near the locus, as well as 10 main R genes those might be exploited in the resistance breeding programs, were selected for differential tests with 475 Chinese isolates. The R gene carrier Q15 conveys resistances distinct from those conditioned by the carriers of the 15 R genes. Together, this valuable R gene was, therefore, designated as Pi39(t). The sequence information of the R gene locus could be used for further marker-based selection and cloning. Xinqiong Liu and Qinzhong Yang contributed equally to this work.     

Inheritance and molecular mapping of a downy mildew resistance gene, Pl 13 in cultivated sunflower (Helianthus annuus L.)

The inheritance of resistance to sunflower downy mildew (SDM) derived from HA-R5 conferring resistance to nine races of the pathogen has been determined and the new source has been designated as Pl ₁₃ . The F₂ individuals and F₃ families of the cross HA-R5 (resistant) × HA 821 (susceptible) were screened against the four predominant SDM races 300, 700, 730, and 770 in separate tests which indicated dominant control by a single locus or a cluster of tightly linked genes. Bulked segregant analysis (BSA) was carried out on 116 F₂ individuals with 500 SSR primer pairs that resulted in the identification of 10 SSR markers of linkage groups 1 (9 markers) and 10 (1 marker) of the genetic map (Tang et al. in Theor Appl Genet 105:1124–1136, 2002) that distinguished the bulks. Of these, the SSR marker ORS 1008 of linkage group 10 was tightly linked (0.9 cM) to the Pl ₁₃ gene. Genotyping the F₂ population and linkage analysis with 20 polymorphic primer pairs located on linkage group 10 failed to show linkage of the markers with downy mildew resistance and the ORS 1008 marker. Nevertheless, validation of polymorphic SSR markers of linkage group 1 along with six RFLP-based STS markers of linkage group 12 of the RFLP map of Jan et al. (Theor Appl Genet 96:15–22, 1998) corresponding to linkage group 1 of the SSR map, mapped seven SSR markers (ORS 965-1, ORS 965-2, ORS 959, ORS 371, ORS 716, and ORS 605) including ORS 1008 and one STS marker (STS10D6) to linkage group 1 covering a genetic distance of 65.0 cM. The Pl ₁₃ gene, as a different source with its location on linkage group 1, was flanked by ORS 1008 on one side at a distance of 0.9 cM and ORS 965-1 on another side at a distance of 5.8 cM. These closely linked markers to the Pl ₁₃ gene provide a valuable basis for marker-assisted selection in sunflower breeding programs.     

Molecular mapping of a rust resistance gene <Emphasis Type="Italic">R</Emphasis><Subscript><Emphasis Type="Italic">14</Emphasis></Subscript> in cultivated sunflower line PH 3

Sunflower, the fifth largest oilseed crop in the world, plays an important role in human diets. Recently, sunflower production in North America has suffered serious yield losses from newly evolved races of sunflower rust (Puccinia helianthi Schwein.). The rust resistance gene, designated R ₁₄ , in a germplasm line PH 3 originated from a wild Helianthus annuus L. population resistant to 11 rust races. PH 3 has seedling with an extraordinary purple hypocotyl color. The objectives of this study were to map both the R ₁₄ rust resistance gene and the purple hypocotyl gene-designated PHC in PH 3, and to identify molecular markers for marker-assisted breeding for sunflower rust resistance. A set of 517 mapped SSR/InDel and four SNP markers was used to detect polymorphisms between the parents. Fourteen markers covering a genetic distance of 17.0 cM on linkage group (LG) 11 were linked to R ₁₄ . R ₁₄ was mapped to the middle of the LG, with a dominant SNP marker NSA_000064 as the closest marker at a distance of 0.7 cM, and another codominant marker ORS542 linked at 3.5 cM proximally. One dominant marker ZVG53 was linked on the distal side at 6.9 cM. The PHC gene was also linked to R ₁₄ with a distance of 6.2 cM. Chi-squared analysis of the segregation ratios of R ₁₄ , PHC, and ten linked markers indicated a deviation from an expected 1:2:1 or 3:1 ratio. The closely linked molecular or morphological markers could facilitate sunflower rust-resistant breeding and accelerate the development of rust-resistant hybrids.     

A consensus map for <Emphasis Type="Italic">Cucurbita pepo</Emphasis>

Using random amplified polymorphic DNA (RAPD), amplified fragment length polymorphism (AFLP), simple sequence repeats (SSR), and morphological traits, the first genetic maps for Cucurbita pepo (2n=2x=40) were constructed and compared. The two mapping populations consisted of 92 F₂ individuals each. One map was developed from a cross between an oil-seed pumpkin breeding line and a zucchini accession, into which genes for resistance to Zucchini Yellow Mosaic Virus (ZYMV) from a related species, C. moschata, had been introgressed. The other map was developed from a cross between an oil-seed pumpkin and a crookneck variety. A total of 332 and 323 markers were mapped in the two populations. Markers were distributed in each map over 21 linkage groups and covered an average of 2,200 cM of the C. pepo genome. The two maps had 62 loci in common, which enabled identification of 14 homologous linkage groups. Polyacrylamide gel analyses allowed detection of a high number of markers suitable for mapping, 10% of which were co-dominant RAPD loci. In the Pumpkin-Zucchini population, bulked segregant analysis (BSA) identified seven markers less than 7 cM distant from the locus n, affecting lignification of the seed coat. One of these markers, linked to the recessive hull-less allele (AW11-420), was also found in the Pumpkin-Crookneck population, 4 cM from n. In the Pumpkin-Zucchini population, 24 RAPD markers, previously introduced into C. pepo from C. moschata, were mapped in two linkage groups (13 and 11 markers in LGpz1 and LGpz2, respectively), together with two sequence characterized amplified region (SCAR) markers linked to genes for resistance to ZYMV.     

Mapping of a locus for adult plant resistance to downy mildew in broccoli (<Emphasis Type="Italic">Brassica oleracea</Emphasis> convar. <Emphasis Type="Italic">italica</Emphasis>)

The identification of the gene Pp523, conferring downy mildew resistance to adult plants of broccoli (Brassica oleracea convar. italica), led to the construction of a genetic map that included this resistance locus, 301 amplified fragment length polymorphisms, 55 random amplified polymorphic DNAs, 46 inter-simple sequence repeats, three simple sequence repeats, four other PCR markers and a flower colour locus, all gathered into nine major linkage groups. Nineteen additional molecular markers were clustered into one group of four markers, one group of three markers and six pairs of markers. The map spans over 731.9 cM, corresponding to 89.5% of the 818 cM estimated to be the total genome length. A significant number of the mapped markers, 19.3%, showed distorted segregation. The average distance between mapped adjacent markers is 1.64 cM, which places this map among the densest published to date for this species. Using bulked segregant analysis, we identified a group of molecular markers flanking and closely linked in coupling to the resistance gene and included these in the map. Two markers linked in coupling, OPK17_980 and AT.CTA_133/134, are located at 3.1 cM and 3.6 cM, respectively, at each side from the resistance gene. These markers can be used for marker-assisted selection in breeding programs aiming at the introgression of this gene in susceptible B. oleracea genotypes. The fine mapping of the genomic region surrounding the Pp523 resistance gene is currently being carried out, a basic condition for its isolation via positional cloning.     

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.     

Identification and characterization of a novel powdery mildew resistance gene <Emphasis Type="Italic">PmG3M</Emphasis> derived from wild emmer wheat, <Emphasis Type="Italic">Triticum dicoccoides</Emphasis>

Powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt) is one of the most important wheat diseases worldwide. Wild emmer wheat, Triticum turgidum ssp. dicoccoides, the tetraploid ancestor (AABB) of domesticated bread and durum wheat, harbors many important alleles for resistance to various diseases, including powdery mildew. In the current study, two tetraploid wheat mapping populations, derived from a cross between durum wheat (cv. Langdon) and wild emmer wheat (accession G-305-3M), were used to identify and map a novel powdery mildew resistance gene. Wild emmer accession G-305-3M was resistant to all 47 Bgt isolates tested, from Israel and Switzerland. Segregation ratios of F₂ progenies and F₆ recombinant inbred line (RIL) mapping populations, in their reactions to inoculation with Bgt, revealed a Mendelian pattern (3:1 and 1:1, respectively), indicating the role of a single dominant gene derived from T. dicoccoides accession G-305-3M. This gene, temporarily designated PmG3M, was mapped on chromosome 6BL and physically assigned to chromosome deletion bin 6BL-0.70-1.00. The F₂ mapping population was used to construct a genetic map of the PmG3M gene region consisted of six simple sequence repeats (SSR), 11 resistance gene analog (RGA), and two target region amplification polymorphism (TRAP) markers. A second map, constructed based on the F₆ RIL population, using a set of skeleton SSR markers, confirmed the order of loci and distances obtained for the F₂ population. The discovery and mapping of this novel powdery mildew resistance gene emphasize the importance of the wild emmer wheat gene pool as a source for crop improvement.     

Identification and fine mapping of <Emphasis Type="Italic">S-d</Emphasis>, a new locus conferring the partial pollen sterility of intersubspecific F<Subscript>1</Subscript> hybrids in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

The partial pollen abortion of hybrids between the indica and japonica subspecies of Asian cultivated rice is one of the major barriers in utilizing intersubspecific heterosis in hybrid rice breeding. Although a single hybrid pollen sterility locus may have little impact on spikelet fertility, the cumulative effect of several loci usually leads to a serious decrease in spikelet fertility. Isolating of the genes conferring hybrid pollen sterility is necessary to understand this phenomenon and to overcome the resulting genetic barrier. In this study, a new locus for F₁ pollen sterility, S-d, was identified on the short arm of chromosome 1 by analyzing the genetic effect of substituted segments of the near-isogenic line E11-5 derived from the japonica variety Taichung 65 (recurrent parent) and the indica variety Dee-geo-woo-gen (donor parent). The S-d locus was first mapped to a 0.8 cM interval between SSR markers PSM46 and PSM80 using a F₂ population of 125 individuals. The flanking markers were then used to identify recombinants from a population of 2,160 plants derived from heterozygotes of the primary F₂ population. Simultaneously, additional markers were developed from genomic sequence divergence in this region. Analysis of the recombinants in the region resulted in the successful mapping of the S-d locus to a 67-kb fragment, containing 17 predicted genes. Positional cloning of this gene will contribute to our understanding of the molecular basis for partial pollen sterility of intersubspecific F₁ hybrids in rice.     

Mapping <Emphasis Type="Italic">Ol-4</Emphasis>, a gene conferring resistance to <Emphasis Type="Italic">Oidium neolycopersici</Emphasis> and originating from <Emphasis Type="Italic">Lycopersicon peruvianum</Emphasis> LA2172, requires multi-allelic,single-locus markers

Lycopersicon peruvianum LA2172 is completely resistant to Oidium neolycopersici, the causal agent of tomato powdery mildew. Despite the large genetic distance between the cultivated tomato and L. peruvianum, fertile F₁ hybrids of L. esculentum cv. Moneymaker × L. peruvianum LA2172 were produced, and a pseudo-F₂ population was generated by mating F₁ half-sibs. The disease tests on the pseudo-F₂ population and two BC₁ families showed that the resistance in LA2172 is governed by one dominant gene, designated as Ol-4. In the pseudo-F₂ population, distorted segregation was observed, and multi-allelic, single-locus markers were used to display different marker-allele configurations per locus. Parameters for both distortion and linkage between genetic loci were determined by maximum likelihood estimation, and the necessity of using multi-allelic, single-locus markers was illustrated. Finally, a genetic linkage map of chromosome 6 around the Ol-4 locus was constructed by using the pseudo-F₂ population.     

Mapping 245 SSR markers on the <Emphasis Type="Italic">Vitis vinifera</Emphasis> genome: a tool for grape genetics

The aim of the present work was to develop a microsatellite marker-based map of the Vitis vinifera genome (n=19), useful for genetic studies in this perennial heterozygous species, as SSR markers are highly transferable co-dominant markers. A total of 346 primer pairs were tested on the two parents (Syrah and Grenache) of a full sib population of 96 individuals (S × G population), successfully amplifying 310 markers. Of these, 88.4% markers were heterozygous for at least one of the two parents. A total of 292 primer pairs were then tested on Riesling, the parent of the RS₁ population derived from selfing (96 individuals), successfully amplifying 299 markers among which 207 (62.9%) were heterozygous. Only 6.7% of the markers were homozygous in all three genotypes, stressing the interest of such markers in grape genetics. Four maps were constructed based on the segregation of 245 SSR markers in the two populations. The Syrah map was constructed from the segregations of 177 markers that could be ordered into 19 linkage groups (total length 1,172.2 cM). The Grenache map was constructed with the segregations of 178 markers that could be ordered into 18 linkage groups (total length 1,360.6 cM). The consensus S × G map was constructed with the segregations of 220 markers that were ordered into 19 linkage groups (total length 1,406.1 cM). One hundred and eleven markers were scored on the RS₁ population, among them 27 that were not mapped using the S × G map. Out of these 111 markers, 110 allowed to us to construct a map of a total length of 1,191.7 cM. Using these four maps, the genome length of V. vinifera was estimated to be around 2,200 cM. The present work allowed us to map 123 new SSR markers on the V. vinifera genome that had not been ordered in a previous SSR-based map (Riaz et al. 2004), representing an average of 6.5 new markers per linkage group. Any new SSR marker mapped is of great potential usefulness for many applications such as the transfer of well-scattered markers to other maps for QTL detection, the use of markers in specific regions for the fine mapping of genes/QTL, or for the choice of markers for MAS.