Comparison of Newly Synthetic Hexaploid Wheat with Its Donors on SSR Products

Microsatellites or SSRs as powerful genetic markers have widely been used in genetics and evolutionary biology in common wheat. Because of the high polymorphism, newly synthesized hexaploid wheat has been used in the construction of genetic segregation population for SSR markers, However, data on the evolution of microsatellites during the polyploidization event of hexaploid wheat are limited. In this study, 66 pairs of specific to A/B genome SSR patterns among newly synthesized hexaploid wheat, the donor tetraploid wheat and Aegilops tauschii were compared. The results indicated that most SSR markers were conserved during the polyploidization events of newly synthetic hexaploid wheat, from Triticum turgidum and Ae. tauschii. Over 70% A/B genome specific SSR markers could amplify the SSR sequences from the D genome ofAe. tauschii. Most amplified fragments from Ae, tauschii were detected in synthetic hexaploid at corresponding positions with the same sizes and patterns as in its parental Ae. tauschii. This suggested that these SSR markers, specific for A/B genome in common wheat, could amplify SSR products of D genome besides A/B genome in the newly synthesized hexaploid wheat, that is, these SSR primers specific for A/B genome in common wheat were nonspecific for the A/B genome in the synthetic hexaploid wheat. In addition, one amplified Ae. tauschii product was not detected in the newly synthetic hexaploid wheat. An extra-amplified product was found in the newly synthetic hexaploid wheat. These results suggested that caution should be taken when using SSR marker to genotype newly synthetic hexaploid wheat.

Effect of HMW-GS 6 + 8 and 1.5 + 10 from Synthetic Hexaploid Wheat on Wheat Quality Traits

To determine the effect of 6 ＋ 8 and 1.5 ＋ 10 HMW-GS of synthetic hexaploid wheat （SHW） on main quality parameters of wheat, a set of recombinant inbred lines （RILs） derived from the cross between a SHW with N, 6 ＋ 8, 1.5 ＋ 10 HMW-GS and a cultivar Chuanyu 12-1 （CY 12-1） with 1, 7 ＋ 8, 2 ＋ 12 were planted in three environments in 2005 and 2006 and totally 16 quality parameters were tested for each line. Significant differences in all tested quality parameters but flour yield were observed between the two parents. The mean values of the RILs were intermediate to the parents for grain and protein parameters and some farinograph parameters, flour water absorption （FWA）, and farinograph softening （SOF） but beyond parents at dough stability time （DST）, breakdown time （BRT）, quality number （QN）, noodle score （NS）, and loaf volume （LOV）. All of the quality traits, especially in grain hardness （GH）, zeleny sedimentation volume （SED）, and most of farinograph parameters had significant difference between the different HMW-GS components. The effects of different alleles of HMW-GS at same locus （Glu-A1 or Glu-B1 or GIu-D1） on the different quality parameters were also different and affected by the other two loci. For most of parameters tested, 6 ＋ 8 was better than 7 ＋ 8 and there was no difference between 1.5 ＋ 10 and 2 ＋ 12. End-use quality was greatly influenced by components of HMW-GS. The components of 1, 6 ＋ 8, 1.5 ＋ 10 had the highest LOV and bread score （BS） values, whereas the components of 1, 7＋ 8 and 1.5 ＋ 10 had the highest NS values. Noodle score performed a positive linear relationship with falling number （FN） and its relationships to other quality parameters were affected by environments. Loaf volume had a significant negative relationship to SOF and positive associations with most of quality parameters. It could be concluded that HMW-GS 6＋ 8 from SHW had better overall quality characteristics than 7 ＋ 8, whereas the effects of 1.5 ＋ 10 on quality was different in respect to quality parameters and the HMW-GS components. Synthetic hexaploid wheat with subunits 6 ＋ 8 and 1.5 ＋ 10 had the potentials to improve the end-use quality of wheat cultivars.

Identification of QTLs for Yield-Related Traits in the Recombinant Inbred Line Population Derived from the Cross Between a Synthetic Hexaploid Wheat-Derived Variety Chuanmai 42 and a Chinese Elite Variety Chuannong 16

Synthetic hexaploid wheat （SHW） represents a valuable source of new resistances to a range of biotic and abiotic stresses. A recombinant inbred line （RIL） population with 127 recombinant inbred lines derived from a SHW-derived variety Chuanmai 42 crossing with a Chinese spring wheat variety Chuannong 16 was used to map QTLs for agronomic traits including grain yield, grains per square meter, thousand-kernel weight, spikes per square meter, grain number per spike, grains weight per spike, and biomass yield. The population was genotyped using 184 simple-sequence repeat （SSR） markers and 34 sequence-related amplified polymorphism （SRAP） markers. Of 76 QTLs （LOD〉2.5） identified, 42 were found to have a positive effect from Chuanmai 42. The QTL QGy.saas-4D.2 associated with grain yield on chromosome 4D was detected in four of the six environments and the combined analysis, and the mean yield, across six environments, of individuals carrying the Chuanmai 42 allele at this locus was 8.9% higher than that of those lines carrying the Chuannong 16 allele. Seven clusters of the yield-coincident QTLs were detected on 1A, 4A, 3B, 5B, 4D, and 7D.

Quantitative trait loci analysis for root traits in synthetic hexaploid wheat under drought stress conditions

Synthetic hexaploid wheat(SHW),possesses numerous genes for drought that can help breeding for drought-tolerant wheat varieties.We evaluated 10 root traits at seedling stage in 111 F9 recombinant inbred lines derived from a F2 population of a SHW line(SHW-L1)and a common wheat line,under normal(NC)and polyethylene glycol-simulated drought stress conditions(DC).We mapped quantitative trait loci(QTLs)for root traits using an enriched high-density genetic map containing 120370 single nucleotide polymorphisms(SNPs),733 diversity arrays technology markers(DArT)and 119 simple sequence repeats(SSRs).With four replicates per treatment,we identified 19 QTLs for root traits under NC and DC,and 12 of them could be consistently detected with three or four replicates.Two novel QTLs for root fresh weight and root diameter under NC explained 9 and 15.7%of the phenotypic variation respectively,and six novel QTLs for root fresh weight,the ratio of root water loss,total root surface area,number of root tips,and number of root forks under DC explained 8.5–14%of the phenotypic variation.Here seven of eight novel QTLs could be consistently detected with more than three replicates.Results provide essential information for fine-mapping QTLs related to drought tolerance that will facilitate breeding drought-tolerant wheat cultivars.

Detection of Genetic Diversity in Synthetic Hexaploid Wheats Using Microsatellite Markers

Ninety-five synthetic hexaploid wheats （2n = 6x = 42, AABBDD） were analyzed using 45 microsatellite markers to investigate the potential genetic diversity in wheat breeding programs. A total of 326 alleles were detected by these microsatellite primer pairs, with an average of 6.65 alleles per locus. The polymorphic information content （PIC）, Simpson index （SI）, and genetic similarity （GS） coefficient showed that the D genome is of the highest genetic diversity among the A, B, and D genomes in the synthetic hexaploid wheats. The results also indicated that the synthetic hexaploid wheat is an efficient way to enrich wheat genetic backgrounds, especially to use the genetic variations of the D genome from Aegilops squarrosa for wheat improvement. The UPGMA dendogram, based on a similarity matrix by a simple matching coeff＇lcient algorithm, delineated the above accessions into 5 major clusters and was in accordance with the available pedigree information. The results demonstrated the utility of microsatellite markers in detecting DNA polymorphism and estimating genetic diversity.

Variations in the quality parameters and gluten proteins in synthetic hexaploid wheats solely expressing the Glu-D1 locus

This study evaluated the quality potential of seven synthetic hexaploid wheats(2 n=6 x=42, AABBDD) expressing only allelic variation at Glu-D1 of Aegilops tauschii(SHWSD). Major quality parameters related to dough strength, gluten proteins(including high-molecular-weight glutenin subunits(HMW-GS) and low-molecular-weight glutenin subunits(LMW-GS), gliadins), and their ratios between SHWSD and the weak gluten wheat control Chuannong 16(CN16) were measured in at least three environments(except STD7). The zeleny sedimentation value(ZSV), dough development time(DDT), dough stability time(DST), and farinograph quality number(FQN) of SHWSD were considered stable under different environments, with their respective ranges being 8.00–17.67 mL, 0.57–1.50 min, 0.73–1.80 min, and 9.50–27.00. The ZSV, DDT, DST, and FQN of SHWSD were smaller than those of CN16, suggesting that SHWSD had a weaker dough strength than CN16. Although SHWSD had a lower gluten index than CN16, its wet and dry gluten contents were similar to or even higher than those of CN16 in all environments tested. The protein content of grains(12.81–18.21%) and flours(14.20–20.31%) in SHWSD was higher than that in CN16. The amount of HMW-GS in SHWSD sharply decreased under the expression of fewer HMW-GS genes, and the LMW-GS, gliadins, and total glutenins were simultaneously increased in SHWSD in comparison with CN16. Moreover, SHWSD had higher ratios of LMWGS/glutenin and gliadin/glutenin but a lower ratio of HMW-GS/glutenin than CN16. These results provide necessary information for the utilization of SHWSD in weak-gluten wheat breeding.

Genetic Analysis and Molecular Tagging a Novel Yellow Rust Resistance Gene Derived from Synthetic Hexaploid Wheat Germplasm M08

Yellow rust of wheat （caused by Puccinia striiformis Westend. f. sp. tritici Eriks.） has been periodically epidemic and severely damaged wheat production in China. The development of resistant cultivars could be an effective way to reduce yield losses of wheat caused by yellow rust. Rust reaction tests and genetic analysis indicated that M08, the synthetic hexaploid wheat derived from hybridization between Triticum durum （2n = 6X = 28; genome AABB） and Aegilops tauschii （2n = 2X = 14; genome DD）, showed resistance to current prevailing yellow rust races at seedling stage, which was controlled by a single dominant gene, designated as YrAm. Bulked segregant analysis was used to identify microsatellite markers linked to gene YrAm in an F2 population derived from cross M08 （resistant） × Jinan 17 （susceptible）. Three microsatellite marker loci Xgwm77, Xgwm285, and Xgwml31 located on chromosome 3B were mapped to the YrAm locus. Xgwml31 was the closest marker locus and showed a linkage distance of 7.8 cM to the resistance locus. Thus, it is assumed that YrAm for resistance to yellow rust may be derived from Triticum durum and is located on the long arm of chromosome 3B.

Application of Synthetic Hexaploid Wheat Derived from T.Durum, Ae.taushii in Common Wheat Breeding for FHB Resistance

The F 1 and F 4 plants of 'synthetic hexaploid wheat/common wheat'crosses and part of their parents were inoculated with Fusarium graminearum to evaluate FHB resistance.The results showed tht the scab resistance in the F 1 varied with the synthetic wheat accessions used as crossing parents.In the F 4,some resistant head lines were generated from the crosses,although their parents had different scab resistance levels.It indicated that synthetic hexaploid wheat are useful in wheat breeding for scab resistance.

Synthetic hexaploid wheat and its utilization for wheat genetic improvement in China

Synthetic hexaploid wheat （Triticum turgidum x Aegilops tauschii） was created to explore for novel genes from T. turgidum and Ae. tauschii that can be used for common wheat improvement. In the present paper, research advances on the utilization of synthetic hexaploid wheat for wheat genetic improvement in China are reviewed. Over 200 synthetic hexaploid wheat （SHW） accessions from the International Maize and Wheat Improvement Centre （CIMMYT） were introduced into China since 1995. Four cultivars derived from these, Chuanmai 38, Chuanmai 42, Chuanmai 43 and Chuanmai 47, have been released in China. Of these, Chuanmai 42, with large kernels and resistance to stripe rust, had the highest average yield （〉 6 t/ha） among all cultivars over two years in Sichuan provincial yield trials, outyielding the commercial check cultivar Chuanmai 107 by 22,7%. Meanwhile, by either artificial chromosome doubling via colchicine treatment or spontaneous chromosome doubling via a union of unreduced gametes （2n） from T. turgidum-Ae, tauschii hybrids, new SHW lines were produced in China. Mitotic-like meiosis might be the cytological mechanism of spontaneous chromosome doubling. SHW lines with genes for spontaneous chromosome doubling may be useful for producing new SHW-alien amphidiploids and double haploid in wheat genetic improvement.

Quantitative Trait Loci Associated with Micronutrient Concentrations in Two Recombinant Inbred Wheat Lines

Micronutrient malnutrition affects over three billion people worldwide, especially women and children in developing countries. Increasing the bioavailable concentrations of essential elements in the edible portions of crops is an effective resolution to address this issue. To determine the genetic factors controlling micronutrient concentration in wheat, the quantitative trait locus （QTL） analysis for iron, zinc, copper, manganese, and selenium concentrations in two recombinant inbred line populations was performed. In all, 39 QTLs for ifve micronutrient concentrations were identiifed in this study. Of these, 22 alleles from synthetic wheat SHW-L1 and seven alleles from the progeny line of the synthetic wheat Chuanmai 42 showed an increase in micronutrient concentrations. Five QTLs on chromosomes 2A, 3D, 4D, and 5B found in both the populations showed signiifcant phenotypic variation for 2-3 micronutrient concentrations. Our results might help understand the genetic control of micronutrient concentration and allow the utilization of genetic resources of synthetic hexaploid wheat for improving micronutrient efifciency of cultivated wheat by using molecular marker-assisted selection.

Breaking wheat yield barriers requires integrated efforts in developing countries

Most yield progress obtained through the so called ＂Green Revolution＂, particularly in the irrigated areas of Asia, has reached a limit, and major resistance genes are quickly overcome by the appearance of new strains of disease causing organisms.New plant stresses due to a changing environment are difficult to breed for as quickly as the changes occur.There is consequently a continual need for new research programs and breeding strategies aimed at improving yield potential, abiotic stress tolerance and resistance to new, major pests and diseases.Recent advances in plant breeding encompass novel methods of expanding genetic variability and selecting for recombinants, including the development of synthetic hexaploid, hybrid and transgenic wheats.In addition, the use of molecular approaches such as quantitative trait locus（QTL） and association mapping may increase the possibility of directly selecting positive chromosomal regions linked with natural variation for grain yield and stress resistance.The present article reviews the potential contribution of these new approaches and tools to the improvement of wheat yield in farmer＇s fields, with a special emphasis on the Asian countries, which are major wheat producers, and contain the highest concentration of resource-poor wheat farmers.

Integrating the physical and genetic map of bread wheat facilitates the detection of chromosomal rearrangements

The bread wheat genome harbors a high content of repetitive DNA,which is amenable to detection and characterization using fluorescence in situ hybridization(FISH)karyotyping.An integrated genetic map was derived from a recombinant inbred population bred from a cross between a synthetic hexaploid wheat and a commercial Chinese bread wheat cultivar,based on 28 variable FISH sites and>150000 single nucleotide polymorphism(SNP)loci.The majority(20/28)of the variable FISH sites were physically located within a chromosomal region consistent with the genetic location inferred from that of their co-segregating SNP loci.The eight exceptions reflected the presence of either a translocation(1 R/1 B,1 A/7 A)or a presumptive intra-chromosomal inversion(4 A).For eight out of the nine FISH sites detected on the Chinese Spring(CS)karyotype,there was a good match with the reference genome sequence,indicating that the most recent assembly has dealt well with the problem of placing tandem repeats.The integrated genetic map produced for wheat is informative as to the location of blocks of tandemly repeated DNA and can aid in improving the quality of the genome sequence assembly in regions surrounding these blocks.