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Russian wheat aphid (RWA) [Diuraphis noxia (Kurdjumov)] is an important pest of wheat (Triticure aestivum L.) in several production areas of the world. The most effective and economical approach for controlling RWA is to use resistant cultivars. A wheat line, ST-ARS 02RWA2414-11 (2414-11), showed a high level of resistance to RWA biotype 2. Our objectives were to map the resistance gene and develop polymerase chain reaction (PCR)-based markers for marker-assisted selection (MAS). A mapping population of 212 [F.sub.2] individuals was developed from a cross of 2414-11 and the susceptible cultivar Yuma. The [F.sub.2] individuals and [F.sub.2:3] families were infested using biotype 2 RWA. The RWA resistance of 2414-11 is controlled by a single major gene, provisionally designated as Dn2414. Using standard PCR, 30 marker loci were found to be linked to Dn2414 with recombination frequencies ([theta]) of 0.00 to 0.27 and logarithm of the odds to the base 10 (LOD) scores of 7.6 to 66.1. Of the 30 markers, 26 were tightly linked to Dn2414 with [theta] [less than or equal to] 0.05. A genetic map was constructed consisting of 31 loci spanning a region of 34.7 cM. The close linkage of Dn2414 with several rye chromosome 1R short arm (1RS)-specific simple sequence repeat markers and low 0 values around the Dn2414 gene indicate that Dn2414 is located on chromosome 1RS.1BL (translocation chromosome with IRS and wheat chromosome 1B long arm). Phenotypic and marker profiles of 2414-11 and its relatives are the same as other lines known to carry Dn7. The Dn2414 gene is thus located on 1RS arm, and the large number of PCR markers will be valuable for MAS of this gene.

The Russian wheat aphid (RWA) [Diuraphis noxia (Kurdjumov)] is an economically significant insect pest of wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) in the USA. Since 1986, only one biotype existed in the USA. In 2003, a new biotype appeared which was virulent to all known resistance genes in wheat, with the exception of Dn7. Dn7 is a rye gene transferred into a wheat background via 1RS/1BL translocation. It was previously mapped; however, no PCR-based markers were developed for marker-assisted selection (MAS) and the map was sparse. This study presents a higher density map of Dn7 containing 19 markers. The markers spanning Dn7 are XHor2 and Xscb241 at distances of 1.4 and 1 cM, respectively, from Dn7. PCR markers were developed and tested on 19 wheat cultivars. Two markers which amplified rye-specific fragments proved to be useful for MAS. Xrems1303 amplified a 320-bp band only in cultivars with high-level resistance to biotype 2 and is effective for MAS of Dn7. Xib267 is linked to the susceptible locus and amplified a fragment specific for rye Petkus 1RS and would be a good marker for selecting against the susceptible locus. The value of Dn7 as a source of resistance has increased because of the broad spectrum resistance it provides against several biotypes. The markers developed in this study will be useful for facilitating the transfer of Dn7 into wheat cultivars with or without translocation chromosomes containing 1RS.

Loci detected by Southern blot hybridization of 3,977 expressed sequence tag unigenes were mapped into 159 chromosome bins delineated by breakpoints of a series of overlapping deletions. These data were used to assess synteny levels along homoeologous chromosomes of the wheat A, B, and D genomes, in relation to both bin position on the centromere-telomere axis and the gradient of recombination rates along chromosome arms. Synteny level decreased with the distance of a chromosome region from the centromere. It also decreased with an increase in recombination rates along the average chromosome arm. There were twice as many unique loci in the B genome than in the A and D genomes, and synteny levels between the B genome chromosomes and the A and D genome homoeotogues were lower than those between the A and D genome homoeologues. These differences among the wheat genomes were attributed to differences in the mating systems of wheat diploid ancestors. Synteny perturbations were characterized in 31 paralogous sets of loci with perturbed synteny. Both insertions and deletions of loci were detected and both preferentially occurred in high recombination regions of chromosomes.

The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), is a serious pest in wheat (Triticum aestivum L.). Four Russian triticale (X Triticosecale Wittmack) lines, PI 386148, PI 386149, PI 386150, and PI 386156, have been the most resistant triticales to the RWA in several screening tests. Resistance is controlled by a single dominant gene at a common locus in the three triticale lines PI 386148, PI 386149, and PI 386156. However, expression of these resistance genes in a wheat background and the inheritance of resistance in PI 386150 have not been determined. The objectives of this study were (i) to assess the transmission and the expression of RWA resistance in triticale x wheat hybrids, (ii) to determine the inheritance of resistance in PI 386150, and (iii) to identify the rye chromosome(s) from Secale montanum Guss. associated with RWA resistance. Advanced generations from crosses between PI 386148 triticale x 'Lamar' wheat and PI 386150 triticale x Lamar were produced and the progenies screened to determine their reaction to the RWA. All [F.sub.1] plants derived from both crosses were resistant indicating that resistance is dominant in both triticale lines. C-banding and in situ hybridization with the pSc119.1 probe, a rye-specific DNA probe, showed that chromosome [4R.sup.mon] was associated with resistance in PI 386150. The results of this study should facilitate the transfer of the resistance gene(s) from these triticale lines into wheat.

1. Introduction Bread wheat (Triticum aestivum L.) is one of the most important crop species providing the staple food for 40% of the world's population. As with other crops, the [...], Positional cloning in bread wheat is a tedious task due to its huge genome size and hexaploid character. BAC libraries represent an essential tool for positional cloning. However, wheat BAC libraries comprise more than million clones, which makes their screening very laborious. Here, we present a targeted approach based on chromosome-specific BAC libraries. Such libraries were constructed from flow-sorted arms of wheat chromosome 7D. A library from the short arm (7DS) consisting of 49,152 clones with 113 kb insert size represented 12.1 arm equivalents whereas a library from the long arm (7DL) comprised 50,304 clones of 116kb providing 14.9x arm coverage. The 7DS library was PCR screened with markers linked to Russian wheat aphid resistance gene DnCI2401, the 7DL library was screened by hybridization with a probe linked to greenbug resistance gene Gb3. The small number of clones combined with high coverage made the screening highly efficient and cost effective.

Fluorescence in situ hybridization (FISH) is a powerful means by which single- and low-copy DNA sequences can be localized on chromosomes. Compared to the mitotic metaphase chromosomes that are normally used in FISH, synaptonemal complex (SC) spreads (hypotonically spread pachytene chromosomes) have several advantages. SC spreads (1) are comparatively free of debris that can interfere with probe penetration, (2) have relatively decondensed chromatin that is highly accessible to probes, and (3) are about ten times longer than their metaphase counterparts, which permits FISH mapping at higher resolution. To investigate the use of plant SC spreads as substrates for single-copy FISH, we probed spreads of tomato SCs with two single-copy sequences and one low-copy sequence (ca. 14 kb each) that are associated with restriction fragment length polymorphism (RFLP) markers on SC 11. Individual SCs were identified on the basis of relative length, arm ratio, and differential staining patterns after combined propidium iodide (PI) and 4[prime],6-diamidino-2-phenylindole (DAPI) staining. In this first report of single-copy FISH to SC spreads, the probe sequences were unambiguously mapped on the long arm of tomato SC 11. Coupled with data from earlier studies, we determined the distance in micrometers, the number of base pairs, and the rates of crossing over between these three FISH markers. We also observed that the order of two of the FISH markers is reversed in relation to their order on the molecular linkage map. SC-FISH mapping permits superimposition of markers from molecular linkage maps directly on pachytene chromosomes and thereby contributes to our understanding of the relationship between chromosome structure, gene activity, and recombination.

Russian wheat aphid (RWA) [Diuraphis noxia (Kurdjumov)] is an important pest of wheat (Triticum aestivum L.) in several production areas of the world. The most effective and economical approach for controlling RWA is to use resistant cultivars. A wheat line, ST-ARS 02RWA2414-11 (2414-11), showed a high level of resistance to RWA biotype 2. Our objectives were to map the resistance gene and develop polymerase chain reaction (PCR)-based markers for marker-assisted selection (MAS). A mapping population of 212 F2 individuals was developed from a cross of 2414-11 and the susceptible cultivar Yuma. The F2 individuals and F2:3 families were infested using biotype 2 RWA. The RWA resistance of 2414-11 is controlled by a single major gene, provisionally designated as Dn2414. Using standard PCR, 30 marker loci were found to be linked to Dn2414 with recombination frequencies (θ) of 0.00 to 0.27 and logarithm of the odds to the base 10 (LOD) scores of 7.6 to 66.1. Of the 30 markers, 26 were tightly linked to Dn2414 with θ≤0.05. A genetic map was constructed consisting of 31 loci spanning a region of 34.7 cM. The close linkage of Dn2414 with several rye chromosome 1R short arm (1RS)-specific simple sequence repeat markers and low θvalues around the Dn2414 gene indicate that Dn2414 is located on chromosome 1RS.1BL (translocation chromosome with IRS and wheat chromosome 1B long arm). Phenotypic and marker profiles of 2414-11 and its relatives are the same as other lines known to carry Dn7. The Dn2414 gene is thus located on 1RS arm, and the large number of PCR markers will be valuable for MAS of this gene. [ABSTRACT FROM AUTHOR]

Develops a microsatellite marker fort tagging Dn2, a wheat gene conferring resistance to the Russian wheat aphid. Types of banding patterns; Criteria for evaluating the effectiveness of a molecular marker; Limitations of the phenotype-based selection technique.

Assesses the organizational and evolutionary aspects of wheat genomes. Increase of relative gene density with distance of a bin from the centromere; Representation of two or more duplicated loci; Significant role of recombination in the evolution of wheat genome structure.