Assessment of genetic diversity in bread wheat genotypes for tolerance to drought using AFLPs and agronomic traits

Moisture stress greatly limits the productivity of wheat in many wheat-growing regions of the world. Knowledge of the degree of genetic diversity among parental materials for key selection traits will facilitate the development of high yielding, stress tolerant wheat cultivars. The objectives of this study were to: (i) use amplified fragment length polymorphisms (AFLPs) to assess genetic diversity among bread wheat lines and cultivars with different responses to drought stress in two distinct environments and, (ii) compare genetic diversity estimated by AFLPs with diversity evaluated on agronomic performance under drought stress. Twenty-eight genotypes, 14 from Iran and 14 developed or obtained by CIMMYT, were evaluated in the study. Phenotypic data on the 14 Iranian lines were obtained in Iran, and data on the 14 CIMMYT lines were collected in Mexico. Ten AFLP primer pairs detected 335 polymorphic bands among the 28 cultivars. At the 5th fusion level of the resulting dendrogram, 6 genotype clusters were identified. Thirteen of the 14 CIMMYT genotypes grouped into one cluster while 4 of the remaining groups were comprised only of Iranian genotypes. When the agronomic performance of the Iranian materials was compared with the AFLP diversity analysis, 5 of the 6 drought susceptible genotypes clustered together in the agronomic dendrogram, and were located in the same cluster in the AFLP dendrogram. However, the drought tolerant Iranian materials did not show the same degree of relationship. The CIMMYT materials did not demonstrate a significant association between agronomic performance and genetic diversity determined using AFLPs. Clearly these data show that there are genotypes with similar agronomic performance and different genetic constitutions in this study that can be combined in a breeding program to potentially improve tolerance to drought stress.     

Applicability of Aegilops tauschii drought tolerance traits to breeding of hexaploid wheat

Few genes are available to develop drought-tolerant bread wheat (Triticum aestivum L.) cultivars. One way to enhance bread wheat’s genetic diversity would be to take advantage of the diversity of wild species by creating synthetic hexaploid wheat (SW) with the genomic constitution of bread wheat. In this study, we compared the expression of traits encoded at different ploidy levels and evaluated the applicability of Aegilops tauschii drought-related traits using 33 Ae. tauschii accessions along with their corresponding SW lines under well-watered and drought conditions. We found wide variation in Ae. tauschii, and even wider variation in the SW lines. Some SW lines were more drought-tolerant than the standard cultivar Cham 6. Aegilops tauschii from some regions gave better performing SW lines. The traits of Ae. tauschii were not significantly correlated with their corresponding SW lines, indicating that the traits expressed in wild diploid relatives of wheat may not predict the traits that will be expressed in SW lines derived from them. We suggest that, regardless of the adaptability and performance of the Ae. tauschii under drought, production of SW could probably result in genotypes with enhanced trait expression due to gene interactions, and that the traits of the synthetic should be evaluated in hexaploid level.     

Characterization of HMW-GS and evaluation of their diversity in morphologically elite synthetic hexaploid wheats

High molecular weight glutenin subunit composition and variation in 95 Elite-1 synthetic hexaploid (SH) wheats (Triticum turgidum/Aegilops tauschii; 2n = 6× = 42; AABBDD) were determined by sodium dodecyl sulphate polyacrylamide gel electrophoresis method (SDS-PAGE). Twenty two different alleles at Glu-1 loci in SHs were observed. Forty four different patterns of HMW-GS in synthetics were found. This higher HMW glutenin composition was due to higher proportion of D-genome encoded subunits in these SHs. 8% urea/SDS-PAGE better discriminated subunit 2* than 12% gels. However 12% urea/SDS-PAGE allowed differentiated mobility of Glu-D^t1 subunits. Genetic variability at Glu-D^t1 locus was greater than Glu-A1 and Glu-B1 loci. The relative high frequency of superior alleles, Glu-B1b and Glu-D^t1d indicated the superior bread making quality attributes embedded in these synthetic hexaploid wheats. Of the 95 Elite-1 SHs 27.1% possessed superior alleles at Glu-A1 and 51% had superior alleles at Glu-B1 locus. At Glu-D^t1 frequency of inferior allele 1Dx2 + 1Dy12 was very low (5.26%) and nine different rare alleles along with the higher frequency (22.1%) of D-genome encoded subunit, 1Dx5 + 1Dy10, were observed. These superior alleles shall form the priority selective sieve for their usage in wheat improvement efforts.     

Resistance to stripe rust in Triticum turgidum,T. tauschii and their synthetic hexaploids

Resistance to stripe rust (caused by Puccinia striiformis Westend.) of 34 Triticum turgidum L. var.durum, 278 T. tauschii, and 267 synthetic hexaploid wheats (T. turgidum x T. tauschii) was evaluated at the seedling stage in the greenhouse and at the adult-plant stage at two field locations. Mexican pathotype 14E14 was used in all studies. Seedling resistance, expressed as low infection type, was present in all three species. One hundred and twenty-eight (46%) accessions of T. tauschii, 8 (23%) of T. turgidum and 31 (12%) of synthetic hexaploid wheats were highly resistant as seedlings. In the field tests, resistance was evaluated by estimating area under disease progress curve (AUDPC). Synthetic hexaploid wheats showed a wide range of variability for disease responses in both greenhouse and field tests, indicating the presence of a number of genes for resistance. In general, genotypes with seedling resistance were also found to be resistant as adult plants. Genotypes, which were susceptible or intermediate as seedlings but resistant as adult plants, were present in both T. turgidum and the synthetic hexaploids. Resistances from either T. turgidum or T. tauschii or both were identified in the synthetic hexaploids in this study. These new sources of resistance could be incorporated into cultivated hexaploid wheats to increase the existing gene pool of resistance to stripe rust.     

Starch characterisation and variability in GBSS loci of synthetic hexaploid wheats and their durum and <Emphasis Type="Italic">Aegilops tauschii</Emphasis> parents

Greater variability in starch properties is found in lower ploidy wheats than in commercial hexaploid wheats. This paper reports on the starch properties and variability in granule bound starch synthase (GBSS) loci of 17 diploid (Aegilops tauschii) and 12 tetraploid (durums) potential progenitors of wheat, compared with 29 synthetic hexaploid wheats produced from such progenitors. Starch properties examined were granule size distribution, swelling power, amylose content, gelatinisation and amylose-lipid dissociation properties. A PCR screening method was able to detect the presence or absence of each of the three GBSS genes. It also detected polymorphisms in eight diploids and nine hexaploids, all displaying the same 25 bases deletion in the D genome allele of GBSS. Two tetraploids and five hexaploids were null 4A for GBSS. There was little difference in the amylose contents and amylose-lipid dissociation peak temperatures of the synthetic hexaploids and the lower ploidy wheats. The synthetic hexaploids showed intermediate swelling power values with the durums giving the highest swelling powers. The durums also had higher B granule contents than the A. tauschii accessions, but not as high as the synthetics. However, the A. tauschii samples gave the highest gelatinisation peak temperatures. The presence of the null 4A mutation was positively correlated with swelling power, amylose content and DSC measurements. The new smaller D genome allele of GBSS was associated with slightly higher swelling power. These results confirm the value of wheat progenitor lines as sources of new starch properties for hexaploid wheat. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Development of diversity array technology (DArT) markers for assessment of population structure and diversity in Aegilops tauschii

Aegilops tauschii Coss. is the D-genome donor to hexaploid bread wheat (Triticum aestivum) and is the most promising wild species as a genetic resource for wheat breeding. To study the population structure and diversity of 81 Ae. tauschii accessions collected from various regions of its geographical distribution, the genomic representation of these lines were used to develop a diversity array technology (DArT) marker array. This Ae. tauschii array and a previously developed DArT wheat array were used to scan the genomes of the 81 accessions. Out of 7500 markers (5500 wheat and 2000 Ae. tauschii), 4449 were polymorphic (3776 wheat and 673 Ae. tauschii). Phylogenetic and population structure studies revealed that the accessions could be divided into three groups. The two Ae. tauschii subspecies could also be separately clustered, suggesting that the current taxonomy might be valid. DArT markers are effective to detect very small polymorphisms. The information obtained about Ae. tauschii in the current study could be useful for wheat breeding. In addition, the new DArT array from this Ae. tauschii population is expected to be an effective tool for hexaploid wheat studies.     

Allozymes and growth habit of Aegilops tauschii: genetic control and linkage patterns

Aegilops tauschii line of spring type growth habit with theearliest heading among all the VIR world germplasm collection of thisspecies was crossed with three Ae. tauschii lines of winter type growthhabit with low, intermediate and the highest vernalization requirement. 12enzyme loci were involved in genetic analysis. The growth habit was foundto be encoded by single codominant major gene, Vrn-D2. Thefollowing linkages were found: Est5 – Nadhd2 in chromosome 3, Vrn-D2 – Aco2 – Cat2 and Pgm – Nadhd1 in chromosome 4, Est2 – Got2 in chromosome 6.     

Inheritance of threshability in synthetic hexaploid (Triticum turgidum×T. tauschii) by T. aestivum crosses

Triticum tauschii provides breeders with a valuable source of resistance and tolerance genes. Elucidation of the inheritance of traits in this species that hinder its use in breeding programmes is therefore of interest to wheat breeders. Inheritance of threshability was investigated in the crosses of four non-free-threshing (NFT) synthetic hexaploids (Triticum turgidum×T. tauschii) and two free-threshing (FT) T. aestivum cultivars during four crop seasons over 3 years at E1 Batan and Ciudad Obregon, Mexico. The parents, their F₁ Hybrids and individual F₂ plant-derived F₃ progenies of the crosses revealed that ‘Altar 84’/T. tauschii (219), ‘Chen’/T. tauschii (205), ‘Chen’/T. tauschii (224), and ‘Duergand’/T. tauschii (214) have independently segregating loci with two dominant alleles controlling threshability. Intercrosses among the synthetics, except ‘Altar 84’/T. tauschii (219), showed the genes to be allelic to each other. The cross between the FT cultivars showed no segregation in the F₃ generation, indicating common recessive genes. Based on these findings, population sizes of the synthetic-derived breeding materials should be increased to improve the chances of selecting FT desirable plants in the programme.     

Effects of moisture stress on leaf appearance,tillering and other aspects of development in Triticum tauschii

Summary A glasshouse study was conducted to describe the dynamics of leaf and tiller appearance of four accessions of T. tauschii (Tt 04, Tt 17, Tt 65 and Tt 74) and to determine the influence of moisture stress (treatments were high and low moisture, imposed seven days after transplanting) on these and other aspects of development in this wild wheat.Under high moisture conditions, accessions differed greatly in flag leaf dimensions, culm length and seed number per spike, the values being lower in Tt 04 than in the other accessions. Low moisture strongly reduced values for these traits, with Tt 04 being least affected, but overall, there was no apparent association between the values obtained for these variables in the high moisture conditions and the effects of moisture stress. For three of the four accessions, final leaf number on the main culm was significantly lower in the low moisture treatment than in the respective control (P<0.05), but the differences between treatments (ca. 0.5 leaves or less) were very small. Maximum tiller number, on the other hand, was strongly reduced by low moisture, and initiation of tillering was inhibited until water was reapplied. There were no apparent after-effects of the moisture regime on the rate of subsequent tiller appearance.The four accessions differed in their leaf appearance rates, giving phyllochron values (117–142° Cd leaf^-1) within the range reported for hexaploid wheat. Low moisture tended to increase phyllochron, but in only one accession was this effect significant. Thus, depending on the accession, low moisture did not affect, or slightly decreased (by ca. 15–20%) the rate of leaf appearance. These effects were similar to those reported for cultivated wheat suggesting that there would be little scope for using these accessions of T. tauschii in breeding for stress tolerance.     

Use of Triticum tauschii to improve yield of wheat in low-yielding environments

Triticum tauschii (Coss.) Schmal. is an ancestor of bread wheat (T. aestivum). This species has been widely used as a source ofsimply-inherited traits, but there are few reports of yield increases due tointrogression of genes from this species. Selections from F₂-derivedlines of backcross derivatives of synthetic hexaploid wheats (T.turgidum / T. tauschii) were evaluated for grain yield in diverseenvironments in southern Australia. Re-selections were made in theF₆ generation and evaluated for grain yield, yield componentsincluding grain weight, and grain growth characters in diverse environmentsin southern Australia and north-western Mexico. Re-selection was effectivein identifying lines which were higher yielding than the recurrent parent,except in full-irrigation environments. Grain yields of the selectedderivatives were highest relative to the recurrent parent in thelowest-yielding environments, which experienced terminal moisture deficitand heat stress during grain filling. The yield advantage of the derivativesin these environments was not due to a change in anthesis date orgrain-filling duration, but was manifest as increased rates of grain-filling andlarger grains, indicating that T. tauschii has outstanding potential forimproving wheat for low-yielding, drought-stressed environments.     

Waxy proteins and amylose content in tetraploid wheats Triticum dicoccum Schulb, Triticum durum L. and Triticum polonicum L.

This paper reports the waxy proteins and apparent amylose contents of the tetraploid species Triticum dicoccum, Triticum polonicum and Triticum durum. Three waxy proteins were found in the three species; two showed the same electrophoretic mobility as the alleles Wx-A1a and Wx-B1a of the hexaploid variety ‘Chinese Spring’, while the third showed the same mobility as the allele Wx-B1e belonging to the hexaploid wheat variety ‘Bai Huo’. In apparent amylose content no significant differences between the alleles Wx-B1a and the Wx-B1e were found for each species. However, the mean amylose contents of T. durum and T. polonicum were significantly greater than that of T. dicoccum, regardless of which allele was present. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hybrid necrosis and hybrid chlorosis in Indian varieties of Triticum dicoccum Schubl.

Summary Forty-four varieties of Triticum dicoccum ex India were crossed to two T. aestivum testers, namely, C 306 (Ne ₁ ne ₂ ch ₁ Ch ₂) and Sonalika (ne ₁ Ne ₂ ch ₁ Ch ₂), to determine their necrosis and chlorosis genes. Thirty-six (81%) varieties were found to be carriers of the Ch ₁gene; the genotype of their necrosis gene (Ne ₁or ne ₁) could not be determined for want of suitable testers with the authors. Six varieties were found to be noncarriers for necrosis and chlorosis genes and two varieties were found to be carriers of the Ne ₁gene and non-carriers for the chlorosis gene. Allelic variation at the Ch ₂locus was observed; two alleles, Ch ₂ ^s and Ch ₂ ^m, are suggested.     

Genome discrimination and chromosome pairing in the Hordeum chilense × Aegilops tauschii amphiploid

Tritordeum (X Tritordeum Ascherson et Graebner) is a synthetic amphiploid belonging to the Triticeae tribe, which resulted from crosses between Hordeum chilense and wheat. It presents useful agronomic traits that could be transferred to wheat, widening its genetic basis. In situ hybridisation with total genomic DNA from H. chilense and cloned, repetitive DNA sequences (pTa71 and pAs1) probes were used to discriminate the parental origin of all chromosomes, to analyse the chromosome pairing and to identify the chromosomes in pollen mother cells (PMCs) at metaphase I of the tritordeum line HT251 (H^chH^chDD, 2n = 4x = 28). The H. chilense total genomic DNA and the ribosomal sequence pTa71 probes, allowed the unequivocal discrimination of the 14 chromosomes of H^ch genome-origin and the 14 chromosomes of D genome-origin. Chromosome pairing analysis revealed meiotic irregularities such as reduced percentage of PMCs with complete homologous pairing, high frequency of univalents, most of H. chilense-origin and a reduced frequency of intragenomic multivalents from both genomes. The H. chilense genome revealed high meiotic instability. After individual chromosome identification at metaphase I with the pAs1 probe, we found the occurrence of pairing between chromosomes of different homoeology groups. The possible interest of the tetraploid tritordeum in the improvement of other Triticeae species is also discussed.     

新疆冬小麦品种农艺及产量性状遗传多样性分析

Grain yield is one of the most important goals in wheat breeding, and agronomic or yield-related traits can directly reflect the characteristics of varieties. In order to determine the evolution of genetic diversity in agronomic traits of Xinjiang winter wheat varieties and their adaptabilities to different ecological environments, 134 winter wheat landraces and 54 moderns bred varieties from Xinjiang were selected for agronomic trait investigation. They were planted in three different ecological environments (Urumqi and Yining in Xinjiang province, and Tai’an in Shandong province) for two consecutive growth seasons, and nine agronomic and yield-related traits were surveyed and analyzed. The estimated broad sense heritability of nine agronomic and yield traits was in descending order: plant height > grain width > grain length/width ratio > spike length > spikelet number > thousand- kernel weight > grain number per spike > grain length > fertile spikelet number. Correlation analyses of nine agronomic and yield traits showed that these traits were correlated with each other. It was found that the plant height, spike length and grain length/width ratio of landraces were higher than that in modern bred varieties, but the grain number per spike, thousand kernel weight, grain length and grain width in landraces were less than that in modern bred varieties. However, the correlation coefficient of these nine traits was higher in modern bred varieties than that in landraces. These variations reflected the evolution of Xinjiang winter wheat varieties in agronomic traits in recent years. This study may provide important information for breeders to select the breeding parents in different winter wheat regions.     

Use of the gene pools of Triticum turgidum ssp. dicoccoides and Aegilops squarrosa for the breeding of common wheat (T. aestivum), through chromosome-doubled hybrids

Summary Triticum turgidum ssp. dicoccoides (wild emmer wheat, AABB, 2n=28) and Aegilops squarrosa (goat grass, DD, 2n=14) comprise a rich reservoir of valuable genetic material, which could be useful for the breeding of common wheat (T. aestivum, AABBDD, 2n=42). Many accessions of both wild species, most of them selected for resistance to stripe rust, were used to make amphiploids. Two strategies were applied: (1) the production of autopolyploid cytotypes of the wild species, followed by hybridisation, and (2) the production of allotriploid interspecific hybrids, followed by doubling of the number of chromosomes. The first route was unsuccessful because of failure of the crosses between the autopolyploid cytotypes, possibly due to incongruity between the two species and to reduced fertility in the autopolyploid cytotypes. The second route yielded the desired synthetic hexaploids. However, the rate of success of the crosses was low and there were great differences between years, and within years between crosses. Embryo rescue was applied to obtain the primary hybrids (2n=21), which were highly sterile and had on average 0.3 bivalents and 20.4 univalents per pollen mother cell. Various abnormalities were recorded. Doubling of the number of chromosomes sometimes occurred spontaneously or was brought about by colchicine treatment. The large scale of the interspecific hybridisation programme ensured that one-third of the female and one-sixth of the male accessions were represented in the synthetic hexaploids.     

Investigation of possible associations between partial fertility restoration and agronomic traits in Triticum aestivum L.

Summary Many conventional hard red spring wheat (Triticum aestivum L. em Thell) lines, including several North Dakota cultivars, carry a gene (or genes) which restore partial male fertility to male sterile plants with Triticum timopheevi Zhuk. cytoplasm. Since this gene has no fertility restoration function in T. aestivum cytoplasm, the postulation can be made that it is being retained in conventional lines because of pleiotropic effects, favorable linkages or chance. The research reported in this paper examined these possibilities. Forty F₆ lines, derived from a single F₂ plant which was heterozygous for a gene (or genes) for partial fertility restoration, were evaluated for two years in a yield trial planted at Fargo, North Dakota. The 40 lines were testcrossed to a male sterile line having T. timopheevi cytoplasm, and the mean seed set of testcrosses was used as a measure of a line's fertility restoration potential. Twenty-seven lines had the gene for partial fertility, and 13 lines apparently lacked this gene. The 40 lines differed for heading date, anther extrusion, plant height, grain yield, 200-kernel weight, test weight, and grain protein percentage. However, comparisons of lines having the restorer gene with those lacking the gene did not provide any obvious explanation for the retention of the partial fertility restorer gene in the breeding stocks of the North Dakota conventional hard red spring wheat breeding program. The possibility that the restorer gene was linked with genes for resistance to stem rust or leaf rust also was evaluated by testing lines for their reaction to several races of rust. No conclusive association was found.Contribution from the Agric. Exp. Sta., North Dakota State University, Fargo, ND 58105, Journal Article no.     

Comparative mapping and its use for the genetic analysis of agronomic characters in wheat

Summary The advent of molecular marker systems has made it possible to develop comparative genetic maps of the genomes of related species in the Triticeae. These maps are being applied to locate and evaluate allelic and homoeoallelic variation for major genes and quantitative trait loci within wheat, and to establish the pleiotropic effects of genes. Additionally, the known locations of genes in related species can direct searches for homoeologous variation in wheat and thus facilitate the identification of new genes. Examples of such analyses include the validation of the effects of Vrn1 on chromosome 5A on flowering time in different crosses within wheat; the indication of pleiotropic effects for stress responses by the Fr1 locus on chromosome 5A; the detection of homoeologous variation for protein content on the homoeologous Group 5 chromosomes; and the detection of a new photoperiod response gene Ppd-H1 in barley from homoeology with Ppd2 of wheat.     

Genetics of morphological traits in wild wheat,Triticum turgidum var.dicoccoides

Summary The mode of inheritance, linkage groups, and chromosomal location of 23 morphological and 4 biochemical traits were characterized in the wild tetraploid emmer wheat,Triticum turgidum var.dicoccoides. These traits were described and their mode of inheritance was determined by their segregation in four F₂ populations derived from crosses between four var.dicoccoides accessions and a tetraploiddurum cultivar. Linkage groups among the genes encoding for these traits were determined or postulated, and their chromosomal location was deduced by linkage to previously located genes. The genetic control of the following traits was characterized and is first reported here: black keel; hairy leaf sheath; hairy auricles; hairy rachilla; hairy kernel brush; obtuse flag leaf; and curved neck/peduncle. The linkage data indicated that developmentally-related genes tended to occur in clusters.