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.     

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.     

Fine genetic mapping localizes cucumber scab resistance gene <Emphasis Type="Italic">Ccu</Emphasis> into an <Emphasis Type="Italic">R</Emphasis> gene cluster

Scab, caused by Cladosporium cucumerinum, is an important disease of cucumber, Cucumis sativus. In this study, we conducted fine genetic mapping of the single dominant scab resistance gene, Ccu, with 148 F₉ recombinant inbred lines (RILs) and 1,944 F₂ plants derived from the resistant cucumber inbred line 9110Gt and the susceptible line 9930, whose draft genome sequence is now available. A framework linkage map was first constructed with simple sequence repeat markers placing Ccu into the terminal 670 kb region of cucumber Chromosome 2. The 9110Gt genome was sequenced at 5× genome coverage with the Solexa next-generation sequencing technology. Sequence analysis of the assembled 9110Gt contigs and the Ccu region of the 9930 genome identified three insertion/deletion (Indel) markers, Indel01, Indel02, and Indel03 that were closely linked with the Ccu locus. On the high-resolution map developed with the F₂ population, the two closest flanking markers, Indel01 and Indel02, were 0.14 and 0.15 cM away from the target gene Ccu, respectively, and the physical distance between the two markers was approximately 140 kb. Detailed annotation of the 180 kb region harboring the Ccu locus identified a cluster of six resistance gene analogs (RGAs) that belong to the nucleotide binding site (NBS) type R genes. Four RGAs were in the region delimited by markers Indel01 and Indel02, and thus were possible candidates of Ccu. Comparative DNA analysis of this cucumber Ccu gene region with a melon (C. melo) bacterial artificial chromosome (BAC) clone revealed a high degree of micro-synteny and conservation of the RGA tandem repeats in this region.     

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.     

<Emphasis Type="Italic">Pl</Emphasis><Subscript><Emphasis Type="Italic">Arg</Emphasis></Subscript> from <Emphasis Type="Italic">Helianthus argophyllus</Emphasis> is unlinked to other known downy mildew resistance genes in sunflower

The Pl _Arg locus in the sunflower (Helianthus annuus L.) inbred line Arg1575-2 conferring resistance to at least four tested races (300, 700, 730, 770) of downy mildew (Plasmopara halstedii) was localized by the use of simple sequence repeat (SSR) markers. Bulked segregant analysis (BSA) was conducted on 126 individuals of an F₂ progeny from a cross between a downy mildew susceptible line, CmsHA342, and Arg1575-2. Twelve SSR markers linked to the Pl _Arg locus were identified. All markers were located proximal to Pl _Arg on linkage group LG1 based on the map of Yu et al. (2003) in a window of 9.3 cM. Since Pl _Arg was mapped to a linkage group different from all other Pl genes previously mapped with SSRs, it can be concluded that Pl _Arg provides a new source of resistance against P. halstedii in sunflower.     

Genetic linkage maps of barfin flounder (<Emphasis Type="Italic">Verasper moseri</Emphasis>) and spotted halibut (<Emphasis Type="Italic">Verasper variegatus</Emphasis>) based on AFLP and microsatellite markers

Barfin flounder (Verasper moseri) and spotted halibut (Verasper variegatus) are two economically important marine fish species for aquaculture in China, Korea and Japan. Construction of genetic linkage maps is an interesting issue for molecular marker-assisted selection (MAS) and for better understanding the genome structure. In the present study, we constructed genetic linkage maps for both fish species using AFLP and microsatellite markers based on an interspecific F₁ hybrid family (female V. moseri and male V. variegatus). The female genetic map comprised 98 markers (58 AFLP markers and 40 microsatellite markers), distributing in 27 linkage groups, and spanning 637 cM with an average resolution of 8.9 cM. Whereas the male genetic map consisted of 86 markers (48 AFLP and 38 microsatellite markers) in 24 linkage groups, covering a length of 625 cM with an average marker spacing of 10 cM. The expected genome length was 1,128 cM in female and 1,115 cM in male, and the estimated coverage of genome was 56% for both genetic maps. Moreover, five microsatellite markers were observed to be common to both genetic maps. This is the first time to report the genetic linkage maps of V. moseri and V. variegatus that could serve as the basis for genetic improvement and selective breeding, candidate genes cloning, and genome structure research.     

<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.     

Comparative genetic linkage maps of <Emphasis Type="Italic">Eucalyptus grandis</Emphasis>, <Emphasis Type="Italic">Eucalyptus globulus</Emphasis> and their F<Subscript>1</Subscript> hybrid based on a double pseudo-backcross mapping approach

Comparative genetic mapping in interspecific pedigrees presents a powerful approach to study genetic differentiation, genome evolution and reproductive isolation in diverging species. We used this approach for genetic analysis of an F₁ hybrid of two Eucalyptus tree species, Eucalyptus grandis (W. Hill ex Maiden.) and Eucalyptus globulus (Labill.). This wide interspecific cross is characterized by hybrid inviability and hybrid abnormality. Approximately 20% of loci in the genome of the F₁ hybrid are expected to be hemizygous due to a difference in genome size between E. grandis (640 Mbp) and E. globulus (530 Mbp). We investigated the extent of colinearity between the two genomes and the distribution of hemizygous loci in the F₁ hybrid using high-throughput, semi-automated AFLP marker analysis. Two pseudo-backcross families (backcrosses of an F₁ individual to non-parental individuals of the parental species) were each genotyped with more than 800 AFLP markers. This allowed construction of de novo comparative genetic linkage maps of the F₁ hybrid and the two backcross parents. All shared AFLP marker loci in the three single-tree parental maps were found to be colinear and little evidence was found for gross chromosomal rearrangements. Our results suggest that hemizygous AFLP loci are dispersed throughout the E. grandis chromosomes of the F₁ hybrid.Communicated by O. Savolainen     

High-resolution genetic mapping of bacterial blight resistance gene <Emphasis Type="Italic">Xa10</Emphasis>

Bacterial blight of rice, caused by Xanthomonas oryzae pv. oryzae (Xoo), is the most devastating disease of rice (Oryza sativa L). Rice lines that carry resistance (R) gene Xa10 confer race-specific resistance to Xoo strains harboring avirulence (Avr) gene avrXa10. Here we report on genetic study, disease evaluation and fine genetic mapping of the Xa10 gene. The inheritance of Xa10-mediated resistance to PXO99^A(pHM1avrXa10) did not follow typical Mendelian inheritance for single dominant gene in F₂ population derived from IR24 × IRBB10. A locus might be present in IRBB10 that caused distorted segregation in F₂ population. To eliminate this locus, an F₃ population (F₃-65) was identified, which showed normal Mendelian segregation ratio of 3:1 for resistance and susceptibility. A new near-isogenic line (F₃-65-1743) of Xa10 in IR24 genetic background was developed and designated as IRBB10A. IRBB10A retained similar resistance specificity as that of IRBB10 and provided complete resistance to PXO99^A(pHM1avrXa10) from seedling to adult stages. Linkage analysis using existing RFLP markers and F₂ mapping population mapped the Xa10 locus to the proximal side of E1981S with genetic distance at 0.93 cM. With five new RFLP markers developed from the genomic sequence of Nipponbare, Xa10 was finely mapped at genetic distance of 0.28 cM between proximal marker M491 and distal marker M419 and co-segregated with markers S723 and M604. The physical distance between M491 and M419 on Nipponbare genome is 74 kb. Seven genes have been annotated from this 74-kb region and six of them are possible Xa10 candidates. The results of this study will be useful in Xa10 cloning and marker-assisted breeding.     

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.     

Interspecies linkage analysis of <Emphasis Type="Italic">mo,</Emphasis> a <Emphasis Type="Italic">Bombyx mori</Emphasis> locus associated with mosaicism and gynandromorphism

The mo (hereditary mosaic) mutation is one of the most famous and interesting mutations of the silkworm, Bombyx mori. Females homozygous for mo generate mosaic and gynandromorphic offspring due to non-elimination of polar bodies and subsequent double fertilization events, irrespective of the genotype of the mated males. Although mo was first reported in 1927, the locus has not been mapped to linkage groups, as the mutation is unstable and appears to be sensitive to genetic background. In this study, linkage analysis of mo was performed using PCR-based markers on single nucleotide polymorphism linkage maps. Bombyx mandarina was used as the mating partner for the B. mori mo strain, as it is easier to identify polymorphic markers between B. mori and B. mandarina than within B. mori strains. Surprisingly, we identified two homozygous linkage groups (LGs) in all of the 12 B₁ (first backcross generation) moths that had deposited mosaic eggs. It was revealed that + ^mo is located on the M chromosome of B. mandarina, which corresponds to two linkage groups of B. mori, LG 14 and 27. Based on further linkage analysis using B. mori as a mating partner, mo was mapped to LG 14. Additionally, we found that mo activity could be modified by a gene(s) on LG 17.     

A comparative map viewer integrating genetic maps for <Emphasis Type="Italic">Brassica</Emphasis> and <Emphasis Type="Italic">Arabidopsis</Emphasis>

Background  

Molecular genetic maps provide a means to link heritable traits with underlying genome sequence variation. Several genetic maps have been constructed for Brassica species, yet to date, there has been no simple means to compare this information or to associate mapped traits with the genome sequence of the related model plant, Arabidopsis.     

Fine genetic mapping of <Emphasis Type="Italic">cp</Emphasis>: a recessive gene for compact (dwarf) plant architecture in cucumber, <Emphasis Type="Italic">Cucumis sativus</Emphasis> L.

The compact (dwarf) plant architecture is an important trait in cucumber (Cucumis sativus L.) breeding that has the potential to be used in once-over mechanical harvest of cucumber production. Compact growth habit is controlled by a simply inherited recessive gene cp. With 150 F_2:3 families derived from two inbred cucumber lines, PI 308915 (compact vining) and PI 249561 (regular vining), we conducted genome-wide molecular mapping with microsatellite (simple sequence repeat, SSR) markers. A framework genetic map was constructed consisting of 187 SSR loci in seven linkage groups (chromosomes) covering 527.5 cM. Linkage analysis placed cp at the distal half of the long arm of cucumber Chromosome 4. Molecular markers cosegregating with the cp locus were identified through whole genome scaffold-based chromosome walking. Fine genetic mapping with 1,269 F₂ plants delimited the cp locus to a 220 kb genomic DNA region. Annotation and function prediction of genes in this region identified a homolog of the cytokinin oxidase (CKX) gene, which may be a potential candidate of compact gene. Alignment of the CKX gene homologs from both parental lines revealed a 3-bp deletion in the first exon of PI 308915, which can serve as a marker for marker-assisted selection of the compact phenotype. This work also provides a solid foundation for map-based cloning of the compact gene and understanding the molecular mechanisms of the dwarfing in cucumber.     

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.     

<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.     

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.     

Exploiting co-linearity among grass species to map the <Emphasis Type="Italic">Aegilops ventricosa-</Emphasis>derived <Emphasis Type="Italic">Pch1</Emphasis> eyespot resistance in wheat and establish its relationship to <Emphasis Type="Italic">Pch2</Emphasis>

Introgressions into wheat from related species have been widely used as a source of agronomically beneficial traits. One such example is the introduction of the potent eyespot resistance gene Pch1 from the wild relative Aegilops ventricosa onto chromosome 7DL of wheat. In common with genes carried on many other such introgressions, the use of Pch1 in commercial wheat varieties has been hindered by linkage drag with yield-limiting traits. Attempts to break this linkage have been frustrated by a lack of co-dominant PCR markers suitable for identifying heterozygotes in F₂ populations. We developed conserved orthologous sequence (COS) markers, utilising the Brachypodium distachyon (Brachypodium) genome sequence, to provide co-dominant markers in the Pch1 region. These were supplemented with previously developed sequence-tagged site (STS) markers and simple sequence repeat (SSR) markers. Markers were applied to a panel of varieties and to a BC₆ F₂ population, segregating between wheat and Ae. ventricosa over the distal portion of 7DL, to identify recombinants in the region of Pch1. By exploiting co-linearity between wheat chromosome 7D, Brachypodium chromosome 1, rice chromosome 6 and sorghum chromosome 10, Pch1 was located to an interval between the flanking markers Xwg7S and Xcos7-9. Furthermore candidate gene regions were identified in Brachypodium (364 Kb), rice (178 Kb) and sorghum (315 Kb) as a prelude to the map-based cloning of the gene. In addition, using homoeologue transferable markers, we obtained evidence that the eyespot resistances Pch1 and Pch2 on chromosomes 7D and 7A, respectively, are potentially homoeoloci. It is anticipated that the COS marker methodology could be used for the identification of recombinants in other introgressions into wheat from wild relatives. This would assist the mapping of genes of interest and the breaking of deleterious linkages to enable greater use of these introgressions in commercial varieties.     

High-resolution mapping and gene prediction of <Emphasis Type="Italic">Xanthomonas Oryzae</Emphasis> pv. <Emphasis Type="Italic">Oryzae</Emphasis> resistance gene <Emphasis Type="Italic">Xa7</Emphasis>

Bacterial blight (BB) caused by Xanthomonas oryzae pv. oryzae (Xoo) is a devastating disease in rice worldwide. The resistance gene Xa7, which provides dominant resistance against the pathogen with avirulence (Avr) gene AvrXa7, has proved to be durably resistant to BB. A set of SSR markers were selected from the “gramene” database based on the Xa7 gene initial mapping region on chromosome 6. These markers were used to construct a high-resolution genetic map of the chromosomal region surrounding the Xa7 gene. An F₂ mapping population with 721 highly susceptible individuals derived from a cross between the near isogenic lines (NILs) IRBB7 and IR24 were constructed to localize the Xa7 gene. In a primary analysis with eleven polymorphic SSR markers, Xa7 was located in approximately the 0.28-cM region. To walk closer to the target gene, recombinant F₂ individuals were tested using newly developed STMS (sequence tagged microsatellite) markers. Finally, the Xa7 gene was mapped to a 0.21-cM interval between the markers GDSSR02 and RM20593. The Xa7-linked markers were landed on the reference sequence of cv. Nipponbare through bioinformatics analysis. A contig map corresponding to the Xa7 gene was constructed. The target gene was assumed to span an interval of approximately 118.5-kb which contained a total of fourteen genes released by the TIGR Genome Annotation Version 5.0. Candidate-gene analysis of Xa7 revealed that the fourteen genes encode novel domains that have no amino acid sequence similar to other cloned Xa(xa) genes. Shen Chen and Zhanghui Huang are contributed equally to this work.     

Fine genetic mapping of greenbug aphid-resistance gene <Emphasis Type="Italic">Gb3</Emphasis> in <Emphasis Type="Italic">Aegilops tauschii</Emphasis>

The greenbug, Schizaphis graminum (Rondani), is an important aphid pest of small grain crops especially wheat (Triticum aestivum L., 2n = 6x = 42, genomes AABBDD) in many parts of the world. The greenbug-resistance gene Gb3 originated from Aegilops tauschii Coss. (2n = 2x = 14, genome D^tD^t) has shown consistent and durable resistance against prevailing greenbug biotypes in wheat fields. We previously mapped Gb3 in a recombination-rich, telomeric bin of wheat chromosome arm 7DL. In this study, high-resolution genetic mapping was carried out using an F_2:3 segregating population derived from two Ae. tauschii accessions, the resistant PI 268210 (original donor of Gb3 in the hexaploid wheat germplasm line ‘Largo’) and susceptible AL8/78. Molecular markers were developed by exploring bin-mapped wheat RFLPs, SSRs, ESTs and the Ae. tauschii physical map (BAC contigs). Wheat EST and Ae. tauschii BAC end sequences located in the deletion bin 7DL3-0.82–1.00 were used to design STS (sequence tagged site) or CAPS (Cleaved Amplified Polymorphic Sequence) markers. Forty-five PCR-based markers were developed and mapped to the chromosomal region spanning the Gb3 locus. The greenbug-resistance gene Gb3 now was delimited in an interval of 1.1 cM by two molecular markers (HI067J6-R and HI009B3-R). This localized high-resolution genetic map with markers closely linked to Gb3 lays a solid foundation for map based cloning of Gb3 and marker-assisted selection of this gene in wheat breeding.     

RFLP mapping of loci controlling self-incompatibility in <Emphasis Type="Italic">Brassica campestris</Emphasis> and their comparative mapping with <Emphasis Type="Italic">B. napus</Emphasis> and <Emphasis Type="Italic">B. oleracea</Emphasis>

RFLP analysis of a cDNA probe SLG6, governing self incompatibility (SI) in Brassica oleracea, using a recombinant inbred population of Brassica campestris followed by genetic linkage analysis led to the detection of two marker loci, SLG6a and SLG6b controlling SI. SLG6a was mapped in linkage group (LG) 9 and was flanked by the RFLP markers ec4f10 (6.4 cM) and wg5b9 (4.2 cM). SLG6b positioned in LG 2 and was flanked by the RFLP markers wg2d11 (9.9 cM) and ec4e7 (26.9 cM). These results indicated the scope of marker-aided introgression of these genes into self-compatible genotypes for production of SI lines suitable for hybridization in B. campestris. Comparative mapping of LG 9 containing SLG6b with corresponding linkage groups of B. oleracea (BO 2) and B. napus (BN 16) led to the detection of small homologous regions with SLG6 locus linked with another RFLP locus. This evidenced for homology of the SLG genes across Brassica species and possibility of using any single cloned SLG gene for development of SI lines in any Brassica species.