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Wheat expressed sequence tags (wESTs) were identified in a genomic interval predicted to span the Lr34/Yr18 slow rusting region on chromosome 7DS and that corresponded to genes located in the syntenic region of rice chromosome 6 (between 2.02 and 2.38 Mb). A subset of the wESTs was also used to identify corresponding bacterial artificial chromosome (BAC) clones from the diploid D genome of wheat ( Aegilops tauschii). Conservation and deviation of micro-colinearity within blocks of genes were found in the D genome BACs relative to the orthologous sequences in rice. Extensive RFLP analysis using the wEST derived clones as probes on a panel of wheat genetic stocks with or without Lr34/Yr18 revealed monomorphic patterns as the norm in this region of the wheat genome. A similar pattern was observed with single nucleotide polymorphism analysis on a subset of the wEST derived clones and subclones from corresponding D genome BACs. One exception was a wEST derived clone that produced a consistent RFLP pattern that distinguished the Lr34/Yr18 genetic stocks and well-established cultivars known either to possess or lack Lr34/Yr18. Conversion of the RFLP to a codominant sequence tagged site (csLV34) revealed a bi-allelic locus, where a variant size of 79 bp insertion in an intron sequence was associated with lines or cultivars that lacked Lr34/Yr18. This association with Lr34/Yr18 was validated in wheat cultivars from diverse backgrounds. Genetic linkage between csLV34 and Lr34/Yr18 was estimated at 0.4 cM [ABSTRACT FROM AUTHOR]

The incorporation of effective and durable disease resistance is an important breeding objective for wheat improvement. The leaf rust resistance gene Lr34 and stripe rust resistance gene Yr18 are effective at the adult plant stage and have provided moderate levels of durable resistance to leaf rust caused by Puccinia triticina Eriks. and to stripe rust caused by Puccinia striiformis Westend. f. sp. tritici. These genes have not been separated by recombination and map to chromosome 7DS in wheat. In a population of 110 F7 lines derived from a Thatcher × Thatcher isogenic line with Lr34/Yr18, field resistance to leaf rust conferred by Lr34 and to stripe rust resistance conferred by Yr18 cosegregated with adult plant resistance to powdery mildew caused by Blumeria graminis (DC) EO Speer f. sp. tritici. Lr34 and Yr18 were previously shown to be associated with enhanced stem rust resistance and tolerance to barley yellow dwarf virus infection. This chromosomal region in wheat has now been linked with resistance to five different pathogens. The Lr34/Yr18 phenotypes and associated powdery mildew resistance were mapped to a single locus flanked by microsatellite loci Xgwm1220 and Xgwm295 on chromosome 7DS. [ABSTRACT FROM AUTHOR]

Opportunities exist for replacing reduced height ( Rht) genes Rht-B1b and Rht-D1b with alternative dwarfing genes for bread wheat improvement. In this study, the chromosomal locations of several height-reducing genes were determined by screening populations of recombinant inbred lines or doubled haploid lines varying for plant height with microsatellite markers. Linked markers were found for Rht5 (on chromosome 3BS), Rht12 (5AL) and Rht13 (7BS), which accounted for most of the phenotypic variance in height in the respective populations. Large height differences between genotypes (up to 43 cm) indicated linkage to major height-reducing genes. Rht4 was associated with molecular markers on chromosome 2BL, accounting for up to 30% of the variance in height. Confirming previous studies, Rht8 was linked to markers on chromosome 2DS, whereas a population varying for Rht9 revealed a region with a small but significant height effect on chromosome 5AL. The height-reducing effect of these dwarfing genes was repeatable across a range of environments. The molecular markers developed in this study will be useful for marker-assisted selection of alternative height-reducing genes, and to better understand the effects of different Rht genes on wheat growth and agronomic performance. [ABSTRACT FROM AUTHOR]

Although molecular markers allow more accurate selection in early generations than conventional screens, large numbers can make selection impracticable while screening in later generations may provide little or no advantage over conventional selection techniques. Investigation of different crossing strategies and consideration of when to screen, what proportion to retain and the impacts of dominant vs. codominant marker expression revealed important choices in the design of marker-assisted selection programs that can produce large efficiency gains. Using F2 enrichment increased the frequency of selected alleles allowing large reductions in minimum population size for recovery of target genotypes (commonly around 90%) and/or selection at a greater number of loci. Increasing homozygosity by inbreeding from F2 to F2:3 also reduced population size by around 90% in some crosses with smaller incremental reductions in subsequent generations. Backcrossing was found to be a useful strategy to reduce population size compared with a biparental population where one parent contributed more target alleles than the other and was complementary to F2 enrichment and increasing homozygosity. Codominant markers removed the need for progeny testing reducing the number of individuals that had to be screened to identify a target genotype. However, although codominant markers allow target alleles to be fixed in early generations, minimum population sizes are often so large in F2 that it is not efficient to do so at this stage. Formulae and tables for calculating genotypic frequencies and minimum population sizes are provided to allow extension to different breeding systems, numbers of target loci, and probabilities of failure. Principles outlined are applicable to implementation of markers for both quantitative trait loci (QTL) and major genes. [ABSTRACT FROM AUTHOR]

The capacity to tiller is a key factor that determines plant architecture. Using molecular markers, a single major gene reducing tiller number, formally named the tiller inhibition gene (tin), was mapped to the short arm of chromosome 1A in wheat. We identified a tightly linked microsatellite marker (Xgwm136) that may be useful in future marker-assisted selection. Thetingene was mapped to the distal deletion bin of chromosome 1AS (FLM value 0.86) and wheat ESTs which were previously mapped to the same deletion bin were used to identify 18 closely related sequences in the syntenic region of rice chromosome 5. For a subset of wheat ESTs that detected flanking markers fortin, we identified closely related sequences within the most distal 300 kb of rice chromosome 5S. The synteny between the distal chromosome ends of wheat 1AS and rice 5S appeared to be disrupted at thehairy glumelocus and seed storage protein loci. We compared map position oftinwith other reduced tillering mutants characterised in other cereals to identify possible orthologous genes. [ABSTRACT FROM AUTHOR]

Genetic stocks were developed for the localization and eventual cloning of the stem rust resistance gene SrR that occurs in wheat lines carrying the 1RS translocation from Secale cereale 'Imperial' rye. We have used a mutation-based approach for molecular analysis of the SrR region in rye. Forty-one independent mutants resulting in loss of SrR resistance were isolated: many of these were deletions of various sizes that were used to locate SrR with respect to chromosome group 1S markers. The analysis of the mutants showed that markers about 1 Mb apart flanking the barley Mla locus also flank SrR. Additionally, three of the approximately 20 closely related sequences of Mla in rye are deleted in each of six interstitial deletion mutants of SrR. The results indicate that the SrR region in rye is syntenic to the Mla region in barley or that SrR is possibly orthologous to the Mla locus. [ABSTRACT FROM AUTHOR]

The short arm of rye (Secale cereale) chromosome 1 has been widely used in breeding programs to incorporate new disease resistance genes into wheat. Using wheat-rye translocation and recombinant lines, molecular markers were isolated and mapped within chromosomal regions of 1RS carrying rust resistance genes Lr26, Sr31, Yr9 from 'Petkus' and SrR from 'Imperial' rye. RFLP markers previously mapped to chromosome 1HS of barley – flanking the complex Mla powdery mildew resistance gene locus – and chromosome 1DS of Aegilops tauschii – flanking the Sr33 stem rust resistance gene – were shown to map on either side of rust resistance genes on 1RS. Three non cross-hybridising Resistance Gene Analog markers, one of them being derived from the Mla gene family, were mapped within same region of 1RS. PCR-based markers were developed which were tightly linked to the rust resistance genes in 'Imperial' and 'Petkus' rye and which have potential for use in marker-assisted breeding. [ABSTRACT FROM AUTHOR]