Bikram S. Gill

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

SummaryWild 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.

Recent advances in alien gene transfer in wheat

SummaryThe recent advances in alien gene transfer from distantly-related species into wheat are reviewed in the present paper. The main achievements during the last ten years include the great expansion of the range of wide hybridization and development of new techniques for production and characterization of wheat-alien chromosome translocations. Updated results of wide hybridization since 1983 and comprehensive characterization of wheat-alien translocation lines in our laboratory are compiled. The future outlook for alien gene transfer in wheat is also discussed.

Megabase Level Sequencing Reveals Contrasted Organization and Evolution Patterns of the Wheat Gene and Transposable Element Spaces

This article describes the molecular analysis of large contiguous sequences produced from the bread wheat genome. It provides novel insights into the number, distribution, and density of genes along chromosome 3B and reveals an unexpectedly high amount of noncollinear genes compared to model grass genomes. To improve our understanding of the organization and evolution of the wheat (Triticum aestivum) genome, we sequenced and annotated 13-Mb contigs (18.2 Mb) originating from different regions of its largest chromosome, 3B (1 Gb), and produced a 2x chromosome survey by shotgun Illumina/Solexa sequencing. All regions carried genes irrespective of their chromosomal location. However, gene distribution was not random, with 75% of them clustered into small islands containing three genes on average. A twofold increase of gene density was observed toward the telomeres likely due to high tandem and interchromosomal duplication events. A total of 3222 transposable elements were identified, including 800 new families. Most of them are complete but showed a highly nested structure spread over distances as large as 200 kb. A succession of amplification waves involving different transposable element families led to contrasted sequence compositions between the proximal and distal regions. Finally, with an estimate of 50,000 genes per diploid genome, our data suggest that wheat may have a higher gene number than other cereals. Indeed, comparisons with rice (Oryza sativa) and Brachypodium revealed that a high number of additional noncollinear genes are interspersed within a highly conserved ancestral grass gene backbone, supporting the idea of an accelerated evolution in the Triticeae lineages.

Increased food and ecosystem security via perennial grains

Perennial grains hold promise, especially for marginal landscapes or with limited resources where annual versions struggle. Despite doubling of yields of major grain crops since the 1950s, more than one in seven people suffer from malnutrition (1). Global population is growing; demand for food, especially meat, is increasing; much land most suitable for annual crops is already in use; and production of nonfood goods (e.g., biofuels) increasingly competes with food production for land (2). The best lands have soils at low or moderate risk of degradation under annual grain production but make up only 12.6% of global land area (16.5 million km2) (3). Supporting more than 50% of world population is another 43.7 million km2 of marginal lands (33.5% of global land area), at high risk of degradation under annual grain production but otherwise capable of producing crops (3). Global food security depends on annual grains—cereals, oilseeds, and legumes—planted on almost 70% of croplands, which combined supply a similar portion of human calories (4, 5). Annual grain production, though, often compromises essential ecosystem services, pushing some beyond sustainable boundaries (5). To ensure food and ecosystem security, farmers need more options to produce grains under different, generally less favorable circumstances than those under which increases in food security were achieved this past century. Development of perennial versions of important grain crops could expand options.

Current status of wide hybridization in wheat

SummaryCurrent status of wide hybridization in wheat is considered in the light of the number of hybrids produced, the number of genes transferred to commercial cultivars and their use in world wide agricuture. Some original results are presented and results of other authors are compiled to provide update information regarding wide crosses in wheat. Barriers to wide hybridization and progress made in overcoming such barriers are discussed. Areas requiring more research are indicated.

Homoeologous recombination, chromosome engineering and crop improvement

Sears (1956) pioneered plant chromosome engineering 50xa0years ago by directed transfer of a leaf rust resistance gene from an alien chromosome to a wheat chromosome using X-ray irradiation and an elegant cytogenetic scheme. Since then many other protocols have been reported, but the one dealing with induced homoeologous pairing and recombination is the most powerful, and has been extensively used in wheat. Here, we briefly review the current status of homoeologous recombination-based chromosome engineering research in plants with a focus on wheat, and demonstrate that integrated use of cytogenetic stocks and molecular resources can enhance the efficiency and precision of homoeologus-based chromosome engineering. We report the results of an experiment on homoeologous recombination-based transfer of virus resistance from an alien chromosome to a wheat chromosome, its characterization, and the prospects for further engineering by a second round of recombination. A proposal is presented for genome-wide, homoeologous recombination-based engineering for efficient mining of gene pools of wild relatives for crop improvement.

Markers associated with a QTL for grain yield in wheat under drought

Drought is a major abiotic stress that adversely affects wheat production in many regions of the world. The objective of this study was to identify quantitative trait loci (QTL) controlling grain yield and yield components under reduced moisture. A cross between common wheat cultivars ‘Dharwar Dry’ (drought tolerant) and ‘Sitta’ was the source of one hundred twenty-seven recombinant inbred lines evaluated for two-seasons in a field under differing soil moisture regimes in Ciudad Obregon, Sonora, Mexico. An SSR/EST-STS marker map was constructed and a grain yield QTL on the proximal region of chromosome 4AL was found to have a significant impact on performance under reduced moisture. This region was associated with QTL for grain yield, grain fill rate, spike density, grains m−2, biomass production, biomass production rate, and drought susceptibility index (DSI). Molecular markers associated with these traits explained 20, 33, 15, 23, 30, 26, and 41% of phenotypic variation, respectively on chromosome 4A. Microsatellite locus Xwmc89 was associated with all significant QTL covering a 7.7xa0centiMorgans (cM) region and generally explained the greatest proportion of phenotypic variation. The alleles associated with enhanced performance under drought stress were contributed by Dharwar Dry. Microsatellite marker wmc89 may be useful for marker assisted selection to enhance drought tolerance.

Genome comparisons reveal a dominant mechanism of chromosome number reduction in grasses and accelerated genome evolution in Triticeae

Single-nucleotide polymorphism was used in the construction of an expressed sequence tag map of Aegilops tauschii, the diploid source of the wheat D genome. Comparisons of the map with the rice and sorghum genome sequences revealed 50 inversions and translocations; 2, 8, and 40 were assigned respectively to the rice, sorghum, and Ae. tauschii lineages, showing greatly accelerated genome evolution in the large Triticeae genomes. The reduction of the basic chromosome number from 12 to 7 in the Triticeae has taken place by a process during which an entire chromosome is inserted by its telomeres into a break in the centromeric region of another chromosome. The original centromere–telomere polarity of the chromosome arms is maintained in the new chromosome. An intrachromosomal telomere–telomere fusion resulting in a pericentric translocation of a chromosome segment or an entire arm accompanied or preceded the chromosome insertion in some instances. Insertional dysploidy has been recorded in three grass subfamilies and appears to be the dominant mechanism of basic chromosome number reduction in grasses. A total of 64% and 66% of Ae. tauschii genes were syntenic with sorghum and rice genes, respectively. Synteny was reduced in the vicinity of the termini of modern Ae. tauschii chromosomes but not in the vicinity of the ancient termini embedded in the Ae. tauschii chromosomes, suggesting that the dependence of synteny erosion on gene location along the centromere–telomere axis either evolved recently in the Triticeae phylogenetic lineage or its evolution was recently accelerated.

Complex microcolinearity among wheat, rice, and barley revealed by fine mapping of the genomic region harboring a major QTL for resistance to Fusarium head blight in wheat.

A major quantitative trait locus (QTL), Qfhs.ndsu-3BS, for resistance to Fusarium head blight (FHB) in wheat has been identified and verified by several research groups. The objectives of this study were to construct a fine genetic map of this QTL region and to examine microcolinearity in the QTL region among wheat, rice, and barley. Two simple sequence repeat (SSR) markers (Xgwm533 and Xgwm493) flanking this QTL were used to screen for recombinants in a population of 3,156 plants derived from a single F7 plant heterozygous for the Qfhs.ndsu-3BS region. A total of 382 recombinants were identified, and they were genotyped with two more SSR markers and eight sequence-tagged site (STS) markers. A fine genetic map of the Qfhs.ndsu-3BS region was constructed and spanned 6.3xa0cM. Based on replicated evaluations of homozygous recombinant lines for Type II FHB resistance, Qfhs.ndsu-3BS, redesignated as Fhb1, was placed into a 1.2-cM marker interval flanked by STS3B-189 and STS3B-206. Primers of STS markers were designed from wheat expressed sequence tags homologous to each of six barley genes expected to be located near this QTL region. A comparison of the wheat fine genetic map and physical maps of rice and barley revealed inversions and insertions/deletions. This suggests a complex microcolinearity among wheat, rice, and barley in this QTL region.

Molecular characterization of a set of wheat deletion stocks for use in chromosome bin mapping of ESTs.

Abstract. The objective of this study was molecular characterization of a set of deletion stocks and other aneuploids for use in chromosome bin mapping of ESTs in wheat. Wheat aneuploid stocks including 21 nullisomic-tetrasomic (NT), 24 ditelosomic (Dt), and 101 deletion (del) lines were screened with 526 EST clones. A total of 1,951 loci were detected by 493 informative EST clones and tagged 150 of the 159 deletion intervals or chromosome bins. Previously described deletion lines del1AS-4, del6AL-2, del6BS-6, and del7DS-6 were found to have normal chromosome constitution. The short arm deletion in del3AS-3 may be translocated from an unknown chromosome as this stock is nullisomic for the 3AS arm. Thirty-five new deletions were detected in 26 lines. Most of the new deletions occurred in terminal regions of chromosomes and probably resulted from the loss of very small terminal fragments that were difficult to detect cytologically. Eleven chromosome aberrations were also detected in two NT and five Dt lines. Overall, the chromosome bin map provides a resolution of around 28xa0Mb for an anchor map of a basic set of seven chromosomes of the Triticeae. Any target gene can be allocated to a specific 28-Mb bin and associated ESTs, anchored to the other Triticeae/grass maps including rice and, therefore, amenable to molecular cloning by comparative and wheat-based positional cloning methods.