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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., 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 [...]

The greenbug, Schizaphis graminum ( Rondani) is an extremely damaging aphid pest of barley ( Hordeum vulgare L.) particularly in the southern Great Plains of the USA. The simply inherited, dominant resistance gene Rsg1 is in all greenbug-resistant US barley cultivars. In this study, we conducted molecular mapping of Rsg1 using an F2:3 population derived from a cross between the greenbug-resistant Post 90*4/ R015 and susceptible CI2260 inbred lines. Segregation of host responses to greenbug biotype E infestation confirmed that a single dominant gene is responsible for greenbug resistance in Post 90*4/ R015. Simple sequence repeat ( SSR) markers evenly distributed along the seven barley chromosomes were employed for the construction of a framework genetic map. Linkage analysis placed the Rsg1 locus in the long arm of chromosome 3 H (3 HL) flanked by SSR markers Bmag0877 and GBM1420 that were 35 c M apart. Polymorphic single-nucleotide polymorphism ( SNP) markers in 3 HL were identified from an Illumina Golden Gate SNP assay and used for targeted mapping to locate Rsg1 to an 8.4-cM interval. Comparative analysis identified syntenic genomic regions in Brachypodium distachyon chromosome 2, in which 37 putative genes were annotated including a NB- LRR-type resistance gene homologue that may be a potential candidate gene for the Rsg1 locus of barley. Results from this study offer a starting point for fine mapping and cloning of this aphid resistance gene in barley. [ABSTRACT FROM AUTHOR]

Nineteen isolates of the cereal aphid pest greenbug, Schizaphis graminum (Rondani) (Hemiptera: Aphididae), were collected from wheat, Triticum aestiuum L.; barley, Hordeum vulgare L.; or noncultivated grass hosts in five locations from Colorado and Wyoming. Parthenogenetic colonies were established. Biotypic profiles of the 19 isolates were determined based on their abilities to damage a set of host plant differentials, and 13 new biotypes were identified. Genetic diversity among the 19 isolates and five previously designated greenbug biotypes (E, C, H, I, and K) was examined with 31 cross-species transferable microsatellite (simple sequence repeat) markers. Neighbor-joining clustering analysis of marker data revealed host-adapted genetic divergence as well as regional differentiation of greenbug populations. Host associated biotypic variation seems to be more obvious in "agricultural biotypes," whereas isolates collected from noncultivated grasses tend to show more geographic divergence. It seems that the biotype sharing the most similar biotypic profiles and the same geographic region with current prevailing one may have the greatest potential to become the new prevailing biotype. Close monitoring of greenbug population dynamics especially biotypic variation on both crop plants and noncultivated grasses in small grain production areas may be a useful strategy for detecting potentially new prevailing virulent biotypes of the greenbug. [ABSTRACT FROM AUTHOR]

False brome grass, Brachypodium distachyon (L.) Beauv, has been proposed as a new model species to bridge rice and temperate cereal crops for genomics research. However, much basic information for this species is still lacking. In this study, six diploid B. distachyon (2n = 2x = 10) accessions (Bd1-1, Bd2-3, Bd3-1, Bd18-1, Bd21 and BD29) were evaluated for their response to infestation by two cereal aphid pests of common wheat (Triticum aestivum L.): the greenbug, Schizaphis graminum Rondani, and the Russian wheat aphid (RWA), Diuraphis noxia Mordvilko. Through database mining of B. distachyon expressed sequence tag (EST) and genomic DNA sequences, 160 EST- and 21 genomic microsatellite markers were developed and used to evaluate genetic diversity among the B. distachyon accessions. All six accessions were resistant to RWA biotype RWA1 but showed distinct responses to feeding by greenbug biotypes C and E, as well as RWA2 RWAs. Although microsatellite-based genetic diversity among different accessions was generally low, Bd1-1 and BD29 were the most diverged from the other four lines. The genetic divergence was correlated with geographical distances between the Brachypodium accessions. Comparison of simple sequence repeat polymorphisms in three inbred lines (Bd2-3, Bd3-1 and Bd18-1) with their respective original parental lines revealed no effect of inbreeding on genetic diversity. Phylogenetic analysis suggested that Aegilops tauschii (Coss.) Schmal., the D genome donor of common wheat, was closer to B. distachyon than to rice. The greenbug - B. distachyon system seems to be a model of choice for plant-aphid interaction studies in the grass genome. [ABSTRACT FROM AUTHOR]

Abstract: Brittle rachis is an important trait to elucidate the domestication process in barley. Brittle rachis in wild barley (Hordeum vulgare ssp. spontaneum (C. Koch.) Thell) is controlled by two dominant complementary genes, Btr1 and Btr2. Cultivated barley (H. vulgare L. ssp. vulgare) lost the brittle rachis character during domestication as a result of mutation at the Btr1 or Btr2 locus. In this study, a high-resolution map of the btr1 locus was constructed using an F2 population of cultivar (cv. ‘Kanto Nakate Gold’)×wild barley (line OUH602). We cloned and sequenced 26 AFLP markers linked with the btr1 and btr2 loci. Ten converted STS markers were located on the short arm of chromosome 3H only, and at least 9 of the 10 STS markers were allelic with their original AFLPs. Efficient conversion of co-dominant STS markers using BAC clones was successful. No suppression of recombination was observed in the btr1 region even though wild barley was used as one of the parents. Initial results of BAC screening confirmed the resolution power of the developed high resolution map. [Copyright &y& Elsevier]