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.

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.

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.

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 (CY12-1) 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 Glu-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.

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 coefficient 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.

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.

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

小麦的条锈病(由 Puccinia striiformis Westend 引起了。f。sp。tritici Eriks ) 是在中国的周期性地流行、严重地损坏的小麦生产。抵抗栽培变种的发展能是减少条锈病引起的小麦的收获损失的一个有效方法。锈反应测试和基因分析显示了那 M08，合成六倍体小麦源于在 Triticum 硬质小麦之间的杂交(2n = 6X = 28；染色体 AABB ) 并且 Aegilops tauschii (2n = 2X = 14；染色体 DD ) ，到在苗期的当前的占优势条锈病赛跑的显示出的电阻，被单个显性基因控制，指定了为 YrAm。Bulked segregant 分析被用来鉴别微卫星标记在从生气 M08 导出的一张 F2 人口连接了到基因 YrAm (抵抗) × Jinan 17 (易受影响) 。位于染色体 3B 的三微卫星标记 loci Xgwm77， Xgwm285，和 Xgwm131 被印射到 YrAm 地点。Xgwm131 是最靠近的标记地点并且显示出 7.8 厘米的连接距离到电阻地点。因此，为到条锈病的电阻的 YrAm 可以从 Triticum 硬质小麦被导出并且位于染色体 3B 的长手臂，这被假定。

Chromosome Based Strategies to Decipher the Structure and Evolution of the Hexaploid Wheat Genome: Chromosome 3B,a Case Study

With 17% of all crop area, wheat is the staple food for 40% of the world’s population. Improvement in bread wheat quality and yield in the context of sustainable agriculture is needed in the next decades to meet human

Cloning and Diversity Analysis of TaHKT2;2 in Hexaploid Wheat

In order to study the roles of members of HKT gene family in wheat,TaH KT2; 2 was isolated by using homologous cloning strategy and screening genomic BAC library. TaHKT2; 2 genes were mapped on chromosomes 7 A,7 B and 7 D,named as TaHKT2; 2-7 A,TaHKT2; 2-7 B,and TaHKT2; 2-7 D,respectively.TaHKT2; 2 and TaHKT2; 1 had the same genetic structure,composed of three exons and two introns,and formed a cluster with TaHKT2; 1 on the phylogenetic tree of plant HKT transporters. The coding sequences of TaHKT2; 2-7 A,TaHKT2; 2-7 B,and TaHKT2; 2-7 D were 1 602,1 602 and 1 596 bp long,respectively,but TaHKT2; 2-7 D cDNA sequence was not isolated by RT-PCR in eight wheat varieties. The natural diversity of TaHKT2; 2 genes was analyzed by cloning and sequencing from 12 wheat varieties. The results showed that TaHKT2; 2-7 A was found to be more diverse than TaHKT2; 2-7 B and TaHKT2; 2-7 D. Only a few bases changed in the alleles of TaH KT2; 2 genes in wheat. No amino-acid natural variation lay in the P-loops of deduced protein sequences of all alleles of TaHKT2;2 in 12 wheat varieties. The identity of coding sequences was much higher than that of 5' flanking regions of TaHKT2; 2 genes. TaHKT2; 2 might be selected over the course of wheat domestication and belonged to domestication gene.

Rapid changes of microsatellite flanking sequence in the allopolyploidization of new synthesized hexaploid wheat

It was suggested that the rapid changes of DNA sequence and gene expression oc- curred at the early stages of allopolyploid formation. In this study, we revealed the microsatellite (SSR) differences between newly formed allopolyploids and their donor parents by using 21 primer sets specific for D genome of wheat. It was indicated that rapid changes had occurred in the “shock” process of the allopolyploid formation between tetraploid wheat and Aegilops tauschii. The changes of SSR flanking sequence resulted in appearance of novel bands or disappearance of parental bands. The disappearance of the parental bands showed much higher frequencies in comparison with that of appearance of novel bands. Disappearance of the parental bands was not random. The frequency of disappearance in tetraploid wheat was much higher than in Ae. tauschii, i. e. the disappearance frequency in AABB genome was much higher than in D genome. Changes of SSR flanking sequence occurred at the early stage of F1 hybrid or just after chro- mosome doubling. From the above results, it can be inferred that SSR flanking sequence region was very active and was amenable to change in the process of polyploidization. This suggested that SSR flanking sequence probably had special biological function at the early stage of ploy- ploidization. The rapid and directional changes at the early stage of polyploidization might con- tribute to the rapid evolution of the newly formed allopolyploid and allow the divergent genomes to act in harmony.

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

Synthetic hexaploid wheat (Triticum turgidum × 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.

Water and Nutrient Use Efficiency in Diploid, Tetraploid and Hexaploid Wheats

三双(Triticum boeotlcum， AA；Aegilops speltoides， BB 和 Ae.tauschii， DD ) ，二四倍体(T。dicoccoides， AABB 和 T。dicoccon， AABB ) 并且一六倍体(T.vulgare， AABBDD ) 小麦的变化，它在小麦的进化很重要在这研究被选择。盆栽试验在不同的水和滋养的条件(i)下面在小麦上被执行在 blomass ，收益，水使用效率( WUE )，和营养素理解差别( N ， P 和 K )在在小麦的进化的 ploidies 之中的使用效率(举起和利用效率)；(i i ) 使用效率在水和营养素上澄清水和滋养的条件的效果；并且(病) 在小麦的进化使用效率估计水和营养素的关系。我们的结果然后在小麦的进化期间从双 totetraploid 显示出那到六倍体，两根 blomass 并且未葬生物资源开始增加了然后减少。为蒸发的水消费显著地减少了，，与生长时期的衰落相关谷物产量，产量指数， WUE， N， P 和 K 举起效率，和 N， P andKutilization 效率显著地增加了。谷物产量，产量指数和 WUE 在一样的顺序减少了：T.vulgare 】 T。dicoccon 】 T。dicoccoides 】 Ae。tauschii 】 Ae。speltoides 】 T.boeoticum。水强调显著地减少的根生物资源，未葬为由 47-52% 的蒸发的生物资源，收益，和水消费，而是显著地增加的 WUE。增加滋养的供应增加了小麦未葬生物资源，谷物收益，产量指数，为在不同的水层次下面的蒸发和 WUE 的水消费，而是在干旱条件下面的减少的根生物资源。通常，水应力和低滋养的供应导致了小麦的更低的 nutrientuptakeefficiency。然而，水和滋养的申请没在 nutrientutilizationefficiency 上有重要效果，建议小麦营养素利用效率被遗传型主要控制。比作另外的二双小麦， Ae。squarrosa (DD ) 有重要更高的 WUEand 滋养的利用效率，显示 D 染色体可以带控制水和营养素的高有效的利用的基因。重要关系在 WUE andN， P 和 K 利用效率之间被发现。

六倍体小黑麦×六倍体小麦杂交后代中染色体遗传与结构变异鉴定

六倍体小黑麦是普通小麦品种遗传改良的重要基因资源,可以拓宽小麦的遗传基础。本研究以六倍体小黑麦为供体向普通小麦转移黑麦染色质,以探明六倍体小黑麦×六倍体小麦杂交、回交后代的染色体遗传特性,为小黑麦种质材料的后续研究和利用奠定基础。以六倍体小黑麦16引171为母本,六倍体小麦川麦62为父本配制杂交及回交组合,利用非变性荧光原位杂交技术(non-denaturing florescence in situ hybridization,ND-FISH)对F1、BC1F1和BC1F2植株进行细胞学跟踪鉴定。结果表明,杂种F1回交结实率为2.61%;BC1F1植株2R染色体传递频率最高;BC1F2植株中黑麦染色体在后代的传递率为6R>4R>2R,小麦背景中5B-7B相互易位染色体在BC1F2植株中表现出严重偏分离。在BC1F1和BC1F2植株中观察到24种结构变异染色体,包括染色体片段、等臂易位染色体、易位染色体以及双着丝粒染色体,且部分BC1F2植株的种子表现粒长和千粒重均优于六倍体小麦亲本川麦62。因此,在利用六倍体小黑麦作为桥梁向普通小麦导入黑麦遗传物质时,应尽量采取多次回交的方式,使D组染色体迅速恢复,保证后代育性的恢复,同时关注染色体结构变异材料的潜在应用价值。

利用人工合成小麦基因资源实现产量突破的育种实践

【目的】人工合成小麦是科学家模拟小麦进化过程,利用四倍体小麦与二倍体节节麦杂交、染色体加倍而成的新型六倍体小麦资源,集合了双亲的优良性状,可作为桥梁改良现代小麦。【方法】回顾了四川小麦80多年的育种历程,对利用人工合成小麦基因资源实现产量水平突破的方法技术进行了分析,希望为进一步利用小麦近缘属种育种应用提供参考。【结果】利用大群体有限回交法和重组自交系育种法改良初级人工合成小麦及其衍生系,育成了产量提升近30%的小麦品种川麦42及衍生品种川麦104,维持四川小麦产量台阶20余年。【结论】种质资源发掘、育种方法技术创新,是实现产量新突破有效途径。

人工合成小麦和地方品种穗发芽抗性育种利用效率

【目的】小麦穗发芽是影响小麦产量和品质的重要限制因子。具有抗穗发芽特性的人工合成小麦和地方品种是改良栽培小麦穗发芽抗性的重要基因资源,通过分子标记辅助选择转育人工合成小麦和地方品种穗发芽抗性位点,评价人工合成小麦和地方品种导入系抗穗发芽育种利用效率,筛选抗穗发芽小麦新材料,为小麦穗发芽抗性育种提供数据和材料支撑。【方法】以抗穗发芽的人工合成小麦SYN792和四川地方品种涪陵须须麦为母本,以穗发芽敏感品种川麦45为轮回亲本,构建2个BC1F7群体。2017年,通过整穗发芽鉴定法对2个BC1F7群体的1796个株系进行穗发芽表型初筛,然后利用与人工合成小麦PHS-3D和地方品种PHS-A1穗发芽抗性位点连锁的SSR标记进行分子标记选择,筛选出整穗发芽率(SGR)小于35%且携带PHS-3D和PHS-A1抗性位点的导入系;2018和2019年,连续2年对初筛选出的PHS-3D和PHS-A1导入系进行整穗发芽率、籽粒发芽指数(GI)和产量相关性状鉴定,其中,籽粒发芽鉴定试验设置25℃(18GI)和32℃(19GI)2个发芽温度。通过不同环境下穗发芽抗性和产量数据,分析人工合成小麦和地方品种导入系抗穗发芽育种利用效率,筛选抗穗发芽且综合性状好的优异导入系。【结果】经整穗发芽鉴定初筛,从1796个衍生系中筛选出SGR值小于35%的株系537个;进一步对筛选出的537个株系进行分子标记检测,发现332个株系导入了人工合成小麦PHS-3D和地方品种PHS-A1穗发芽抗性位点,包括人工合成小麦导入系73个、地方品种导入系259个;地方品种PHS-A1导入系的频率显著高于人工合成小麦PHS-3D导入系。2018和2019年通过对332个穗发芽抗性位点导入系穗发芽鉴定发现,不同年份穗发芽指标间均呈极显著正相关,穗发芽指标SGR和GI表现出相对稳定的趋势;人工合成小麦和地方品种导入系的3个穗发芽指标平均值(18GI、18SGR和19SGR)均低于23%,差异不显著。不同发芽温度导入系的GI值差异较大,发芽温度32℃时,人工合成小麦PHS-3D导入系的GI值显著低于地方品种PHS-A1导入系。筛选出的73个人工合成小麦导入系中,红粒系穗发芽指标值均低于白粒系;其中,11个人工合成小麦白粒导入系表现中抗及以上抗性水平,14个红粒导入系在不同发芽温度时GI值均低于35%。2年产量相关性状分析表明,人工合成小麦PHS-3D导入系的千粒重显著高于地方品种PHS-A1导入系,而穗粒数显著小于地方品种PHS-A1导入系。根据产量性状和穗发芽抗性表现,筛选出23个穗发芽抗性和综合性状均较好的优异导入系,包括7个人工合成小麦PHS-3D导入系、16个地方品种PHS-A1导入系;人工合成小麦优异导入系中有2个白粒导入系穗发芽抗性中抗以上,2个红粒导入系不同发芽温度的GI值均低于25%,表现出稳定的穗发芽抗性。【结论】人工合成小麦和地方品种均可用于改良现代栽培小麦穗发芽抗性,利用地方品种进行穗发芽抗性育种改良效率优于人工合成小麦;但人工合成小麦导入系的穗发芽抗性的稳定性优于地方品种导入系。筛选出的23个人工合成小麦和地方品种导入系是小麦穗发芽抗性和产量性状改良的重要基因资源;特别是人工合成小麦白粒导入系(编号5201)和红粒导入系(编号5497和5505)是非常有育种利用价值的穗发芽抗性育种亲本材料。

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.

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 five micronutrient concentrations were identified 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 significant 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 efficiency of cultivated wheat by using molecular marker-assisted selection.

Wheat breeding in the hometown of Chinese Spring

The common wheat landrace Chinese Spring(CS) was made famous by the work of Ernie Sears, a great cytogenetist, who developed a number of CS-based aneuploid series that were used to identify individual wheat chromosomes. Based on this, a standard karyotype and nomenclature system was developed for wheat chromosomes that allowed wheat researchers to analyze and manipulate the wheat genome with unprecedented precision and efficiency. Nevertheless, not much is known about the utilization of CS at its hometown, Chengdu in Sichuan province, during early wheat breeding activity. In this review, we follow the speculation that CS is a selection from the Cheng-du-guang-tou(CDGT) landrace. We provide a description of how CDGT became a founder landrace for wheat breeding activities in early times. We show that CDGT-derived varieties were reinforced genetically by crosses to six more exotic parents. These varieties remained the major elite cultivar for several decades. Later, synthetic hexaploid wheats were introduced into the breeding program, firstly using those from CIMMYT and later using materials produced with local tetraploid wheat and goat grass. Finally, we discuss the strategies and future directions to improve wheat yield and resistance through an expanded genetic basis,especially by recapturing lost genetic variations from landraces and related wild species, a process that may set an example for wheat breeders in China and elsewhere.

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.

Comparing two approaches for introgression of germplasm from Aegilops tauschii into common wheat

Allelic diversity in the wild grass Aegilops tauschii is vastly greater than that in the D genome of common wheat(Triticum aestivum), of which Ae. tauschii is the source. Since the 1980 s,there have been numerous efforts to harness a much larger share of Ae. tauschii^ extensive and highly variable gene pool for wheat improvement. Those efforts have followed two distinct approaches: production of amphiploids, known as "synthetic hexaploids," between T. turgidum and Ae. tauschii,and direct hybridization between 丁. aestiuum and Ae. tauschii;both approaches then involve backcrossing to 丁. aestiuum. Both synthetic hexaploid production and direct hybridization have led to the transfer of numerous new genes into common wheat that confer improvements in many traits. This work has led to release of improved cultivars in China, the United States, and many other countries. Each approach to D-genome improvement has advantages and disadvantages. For example, production of synthetic hexaploids can incorporate useful germplasm from both T. turgidum and Ae.tauschii, thereby enhancing the A, B, and D genomes; on the other hand, direct hybridization rapidly restores the recurrent parent's A and B genomes and avoids incorporation of genes with adverse effects on threshability, hybrid necrosis, vernalization response, milling and baking quality, and other traits, which are often transferred when T. turgidum is used as a parent. Choice of method will depend in part on the type of wheat being developed and the target environment. However, more extensive use of the so-far underexploited direct hybridization approach is especially warranted.