Because of polyploidy and large genome size, deletion stocks of bread wheat are an ideal material for physically allocating ESTs and genes to small chromosomal regions for targeted mapping. To enhance the utility of deletion stocks for chromosome bin mapping, we characterized a set of 84 deletion lines covering the 21 chromosomes of wheat using 725 microsatellites. We localized these microsatellite loci to 94 breakpoints in a homozygous state (88 distal deletions, 6 interstitial), and 5 in a heterozygous state representing 159 deletion bins. Chromosomes from homoeologous groups 2 and 5 were the best covered (126 and 125 microsatellites, respectively) while the coverage for group 4 was lower (80 microsatellites). We assigned at least one microsatellite in up to 92% of the bins (mean 4.97 SSR/bin). Only a few discrepancies concerning marker order were observed. The cytogenetic maps revealed small genetic distances over large physical regions around the centromeres and large genetic to physical map ratios close to the telomeres. As SSRs are the markers of choice for many genetic and breeding studies, the mapped microsatellite loci will be useful not only for deletion stock verifications but also for allocating associated QTLs to deletion bins where numerous ESTs that could be potential candidate genes are currently assigned.

Because of the huge size of the common wheat (Triticum aestivum L., 2n ϭ 6x ϭ 42, AABBDD) genome of 17,300 Mb, sequencing and mapping of the expressed portion is a logical first step for gene discovery. Here we report mapping of 7104 expressed sequence tag (EST) unigenes by Southern hybridization into a chromosome bin map using a set of wheat aneuploids and deletion stocks. Each EST detected a mean of 4.8 restriction fragments and 2.8 loci. More loci were mapped in the B genome (5774) than in the A (5173) or D (5146) genomes. The EST density was significantly higher for the D genome than for the A or B. In general, EST density increased relative to the physical distance from the centromere. The majority of EST-dense regions are in the distal parts of chromosomes. Most of the agronomically important genes are located in EST-dense regions. The chromosome bin map of ESTs is a unique resource for SNP analysis, comparative mapping, structural and functional analysis, and polyploid evolution, as well as providing a framework for constructing a sequence-ready, BAC-contig map of the wheat genome.

Genes detected by wheat expressed sequence tags (ESTs) were mapped into chromosome bins delineated by breakpoints of 159 overlapping deletions. These data were used to assess the organizational and evolutionary aspects of wheat genomes. Relative gene density and recombination rate increased with the relative distance of a bin from the centromere. Single-gene loci present once in the wheat genomes were found predominantly in the proximal, low-recombination regions, while multigene loci tended to be more frequent in distal, high-recombination regions. One-quarter of all gene motifs within wheat genomes were represented by two or more duplicated loci (paralogous sets). For 40 such sets, ancestral loci and loci derived from them by duplication were identified. Loci derived by duplication were most frequently located in distal, high-recombination chromosome regions whereas ancestral loci were most frequently located proximal to them. It is suggested that recombination has played a central role in the evolution of wheat genome structure and that gradients of recombination rates along chromosome arms promote more rapid rates of genome evolution in distal, high-recombination regions than in proximal, low-recombination regions.

Sears (1956) pioneered plant chromosome engineering 50 years 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.

Several Triticum aestivum L.-Haynaldia villosa disomic 6VS/6AL translocation lines with powdery mildew resistance were developed from the hybridization between common wheat cultivar Yangmai 5 and alien substitution line 6V(6A). Mitotic and meiotic C-banding analysis, aneuploid analysis with double ditelosomic stocks, in situ hybridization, as well as the phenotypic assessment of powdery mildew resistance, were used to characterize these lines. The same translocated chromosome, with breakpoints near the centromere, appears to be present in all the lines, despite variation among the lines in their morphology and agronomic characteristics. The resistance gene, conferred by H. villosa and designated as Pm21, is a new and promising source of powdery mildew resistance in wheat breeding.