Wild relatives of common wheat, Triticum aestivum, and related species are an important source of disease and pest resistance and several useful traits have been transferred from these species to wheat. C-banding and in situ hybridization analyses are powerful cytological techniques allowing the detection of alien chromatin in wheat. C-banding permits identification of the wheat and alien chromosomes involved in wheat-alien translocations, whereas genomic in situ hybridization analysis allows determination of their size and breakpoint positions. The present review summarizes the available data on wheat-alien transfers conferring resistance to diseases and pests. Ten of the 57 spontaneous and induced wheat-alien translocations were identified as whole arm translocations with the breakpoints within the centromeric regions. The majority of transfers (45) were identified as terminal translocations with distal alien segments translocated to wheat chromosome arms. Only two intercalary wheat-alien transloctions were identified, one induced by radiation treatment with a small segment of rye chromosome 6RL (H25) inserted into the long arm of wheat chromosome 4A, and the other probably induced by homoeologous recombination with a segment derived from the long arm of a group 7 Agropyron elongatum chromosome with Lr19 inserted into the long arm of 7D. The presented information should be useful for further directed chromosome engineering aimed at producing superior germplasm.

Characterization of wheat-alien translocations conferring resistance to diseases and pests: current status

A 2275-marker genetic map of rice (Oryza sativa L.) covering 1521.6 cM in the Kosambi function has been constructed using 186 F2 plants from a single cross between the japonica variety Nipponbare and the indica variety Kasalath. The map provides the most detailed and informative genetic map of any plant. Centromere locations on 12 linkage groups were determined by dosage analysis of secondary and telotrisomics using > 130 DNA markers located on respective chromosome arms. A limited influence on meiotic recombination inhibition by the centromere in the genetic map was discussed. The main sources of the markers in this map were expressed sequence tag (EST) clones from Nipponbare callus, root, and shoot libraries. We mapped 1455 loci using ESTs; 615 of these loci showed significant similarities to known genes, including single-copy genes, family genes, and isozyme genes. The high-resolution genetic map permitted us to characterize meiotic recombinations in the whole genome. Positive interference of meiotic recombination was detected both by the distribution of recombination number per each chromosome and by the distribution of double crossover interval lengths.

A High-Density Rice Genetic Linkage Map with 2275 Markers Using a Single F2 Population

The Deletion Stocks of Common Wheat

Microsatellite DNA markers are consistently found to be more informative than other classes of markers in hexaploid wheat. The objectives of this research were to develop new primers flanking wheat microsatellites and to position the associated loci on the wheat genome map by genetic linkage mapping in the ITMI W7984 x Opata85 recombinant inbred line (RIL) population and/or by physical mapping with cytogenetic stocks. We observed that the efficiency of marker development could be increased in wheat by creating libraries from sheared rather than enzyme-digested DNA fragments for microsatellite screening, by focusing on microsatellites with the [ATT/TAA]n motif, and by adding an untemplated G-C clamp to the 5'-end of primers. A total of 540 microsatellite-flanking primer pairs were developed, tested, and annotated from random genomic libraries. Primer pairs and associated loci were assigned identifiers prefixed with BARC (the acronym for the USDA-ARS Beltsville Agricultural Research Center) or Xbarc, respectively. A subset of 315 primer sets was used to map 347 loci. One hundred and twenty-five loci were localized by physical mapping alone. Of the 222 loci mapped with the ITMI population, 126 were also physically mapped. Considering all mapped loci, 126, 125, and 96 mapped to the A, B, and D genomes, respectively. Twenty-three of the new loci were positioned in gaps larger than 10 cM in the map based on pre-existing markers, and 14 mapped to the ends of chromosomes. The length of the linkage map was extended by 80.7 cM. Map positions were consistent for 111 of the 126 loci positioned by both genetic and physical mapping. The majority of the 15 discrepancies between genetic and physical mapping involved chromosome group 5.

Development and mapping of microsatellite (SSR) markers in wheat.

In recent years, considerable emphasis has been placed on the development of molecular markers to be used for a variety of objectives. This review attempts to give an account of different molecular markers—restriction fragment length polymorphisms (RFLPs), random amplified polymorphic DNAs (RAPDs), sequence-tagged sites (STS), DNA amplification fingerprinting (DAF), amplified fragment length polymorphisms (AFLPs) and microsatellites (STMS)—currently available for genome mapping and for tagging different traits in wheat. Other markers, including microsatellite-primed polymerase chain reaction (MP-PCR), expressed sequence tags (ESTs) and single nucleotide polymorphisms (SNPs) are also discussed. Recent information on synteny in cereal genomes, marker-assisted selection, marker validation and their relevance to cereal breeding in general and wheat breeding in particular are also examined.

Molecular markers and their applications in wheat breeding.

Bread wheat (Triticum aestivum L. em Thell) is well suited for cytogenetic analysis because the genome, buffered by polyploidy, can tolerate structurally and numerically engineered chromosomes for analysis over infinite generations. This feature of polyploidy can be used in developing a high-resolution, cytogenetically based physical map of the wheat genome. We show that numerous deletions, observed in the progeny of a monosomic addition of a chromosome from Triticum cylindricum in wheat, result from single breakpoints and a concomitant loss of distal fragments. Breakages occurred in euchromatic and heterochromatic regions. Forty-one deletions for chromosomes 7A, 7B, and 7D, and a set of genetically mapped DNA probes, were used to construct physical maps. Recombination was low in proximal chromosomal regions and very high toward the distal ends. Deletion mapping was more efficient than genetic mapping in resolving the order of proximal loci. Despite variation in size and arm ratio, relative gene position was largely conserved among chromosomes 7A, 7B, and 7D and a consensus group 7 physical map was constructed. Several molecularly tagged chromosome regions (MTCRs) of approximately one to a few million base pairs were identified that may be resolved by long-range mapping of DNA fragments. Thus, a cytogenetically based physical map may be used to integrate chromosome and DNA-based maps. The MTCRs may simplify strategies for cloning of agronomically useful genes despite the genetic complexity and the large genome size of wheat.

https://www.pnas.org/content/pnas/89/23/11307.full.pdf

Toward a cytogenetically based physical map of the wheat genome.

The distribution of the telomeric repeats in common wheat and their role in the healing of broken ends of deleted chromosomes was studied. In situ hybridization to mitotic chromosomes was carried out using a synthetic probe that was derived from the sequence of the telomeric repeats of Arabidopsis thaliana. Sites of hybridization were visualized as double dots at both ends of each wheat chromosome. Variation in the strength of the signal that was detected among chromosome arms might be due to the variable number of telomeric repeats of each chromosome end. While signals were absent on normal chromosomes at the pericentric and intercalary regions, hybridization sites were detected at the broken chromosome ends of all deleted chromosomes included in the study. All telocentric chromosomes of multitelocentric lines of 'Chinese Spring' showed a strong signal at the centromeric region. The results suggest that a de novo chromosome healing mechanism exists in wheat involving the addition of the telomeric sequenc...

Joanna E. Werner

Papers

Toward a cytogenetically based physical map of the wheat genome.

Distribution of telomeric repeats and their role in the healing of broken chromosome ends in wheat