Durum Wheat Accessions Research Articles

A panel of elite accessions of durum wheat (Triticum durumDesf.) suitable for association mapping studies

The effectiveness of association mapping (AM) based on linkage disequilibrium (LD) is currently being tested in a number of crops. An important prerequisite for the application of AM is the availability of collections of accessions with a suitable level of genetic variation for target traits and with limited spurious LD due to the presence of population structure. Herein, the results of a genome-wide molecular characterization of a collection of elite durum wheat accessions well-adapted to Mediterranean environments are presented. Ninety-seven highly polymorphic simple sequence repeats and 166 amplified fragment length polymorphism markers were used to characterize 189 durum accessions, mainly cultivars and advanced breeding lines. Genome-wide significant and sizeable LD indices at a centimorgan scale were observed, while LD mainly decayed within 10 cM. On the other hand, effects due to spurious LD were notably lower than those previously observed in a durum wheat collection sampling durum gene pools of more diverse origin. These results, coupled with the high level of genetic variability detected for a number of important morpho-physiological traits and their high heritability, indicate the suitability of this collection for AM studies targeting agronomically important traits.

Population structure and long-range linkage disequilibrium in a durum wheat elite collection

A collection of 134 durum wheat accessions, mainly including cultivars (cvs.) representative of the major gene pools, was assembled and characterized with 70 SSRs for genetic diversity and level of long-range linkage disequilibrium (LD). Results of both a distance-based and a model-based (Bayesian) cluster analysis evidenced the presence of a structured diversity. In the model-based analysis, six to eight main distinct subpopulations were identified based on the molecular data. Only a relatively small portion (20%) of the molecular variation was accounted for by the geographical origin of the accessions. Major differences were detected between the North American and the Mediterranean cvs., while a considerable overlap characterized the cvs. from CIMMYT-ICARDA and Italy. The North American cvs. showed the highest within group mean genetic similarity (GSm = 0.68). French cvs. revealed sizeable similarities with both the North American as well as the Italian and CIMMYT-ICARDA pools. Considering the germplasm as a whole, high levels of LD were found both at locus pairs with an intrachromosomal distance <50 cM as well as at those with distances more than 50 cM and independent (86, 52 and 54% of SSR pairs at p < 0.01, respectively). After re-evaluating LD within each of the three main subgroups identified through the analysis of the germplasm structure, the LD level remained high for tightly to moderately linked locus pairs (<20 cM apart), but was greatly reduced in the loosely linked (more than 50 cM apart) and independent locus pairs. The implications of these findings as to the possibility of using association mapping for gene/QTL discovery in durum wheat are discussed.

Search for partial resistance to leaf rust in a collection of ancient Spanish wheats.

A collection of 917 accessions of Spanish durum and bread wheat was screened for resistance to leaf rust (Puccinia triticina) under field conditions at three locations. Resistance levels ranged from very low to very high, high susceptibility being most frequent. Relative disease severity (referred to the most susceptible accession = 100%) was lower than 20% in about 6% of the accessions in each location. In the collection most of the lines (84%) displayed a susceptible infection type. A final selection of seven accessions (one of them durum) displaying low severity level in the field and high infection type in a growth chamber was chosen for further studies. High levels of partial resistant with longer latency period and high percentage of early aborted colonies without necrosis were found. They might be used in breeding programmes.

Inheritance of Awnlessness in Tetraploid Wheat Species

Awn absence was shown to be inherited as a dominant character in the tetraploid wheat species Triticum dicoccum (Schrank) Schuebl. and T. durum Desf. but as a recessive one in T. aethiopicum Jakubz. The monogenic control of the character was demonstrated for all studied species. In accessions of emmer and durum wheat, the character is controlled by the dominant gene B1, located on chromosome 5A, and in Ethiopian wheat, by a recessive gene, which we designated asawn. The recessive awn gene was localized on chromosome 3B ofT. aethiopicum with the use of D-genome disomic substitution lines of cultivar Langdon.

Isolation of EST-derived microsatellite markers for genotyping the A and B genomes of wheat.

Genetic variation present in 64 durum wheat accessions was investigated by using three sources of microsatellite (SSR) markers: EST-derived SSRs (EST-SSRs) and two sources of SSRs isolated from total genomic DNA. Out of 245 SSR primer pairs screened, 22 EST-SSRs and 20 genomic-derived SSRs were polymorphic and used for genotyping. The EST-SSR primers produced high quality markers, but had the lowest level of polymorphism (25%) compared to the other two sources of genomic SSR markers (53%). The 42 SSR markers detected 189 polymorphic alleles with an average number of 4.5 alleles per locus. The coefficient of similarity ranged from 0.28 to 0.70 and the estimates of similarity varied when different sources of SSR markers were used to genotype the accessions. This study showed that EST-derived SSR markers developed in bread wheat are polymorphic in durum wheat when assaying loci of the A and B genomes. A minumum of ten EST-SSRs generated a very low probability of identity (0.36x10(-12)) indicating that these SSRs have a very high discriminatory power. EST-SSR markers directly sample variation in transcribed regions of the genome, which may enhance their value in marker-assisted selection, comparative genetic analysis and for exploiting wheat genetic resources by providing a more-direct estimate of functional diversity.

Stability of Carbon Isotope Discrimination and Grain Yield in Durum Wheat

Carbon isotope discrimination (Δ) has been proposed as an indirect selection criterion for transpiration efficiency and grain yield in bread wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.), with potential value for durum wheat (Triticum turgidum convar. durum Desf. MacKey). We evaluated the genetic variation for Δ, the relationship between Δ and grain yield, and the magnitude of genotype × year (G × Y) interactions in durum wheat. Field experiments were conducted under Mediterranean conditions on 144 durum wheat accessions during three successive years. Grain yield and carbon isotope discrimination of flag leaves (ΔF) and kernels (ΔK) were measured. Large genotypic and year variation was observed for ΔF and ΔK. Flag leaf Δ was correlated with grain yield in 2 yr characterized as having greater water limitation (r = 0.29 − 0.38, P &lt; 0.001). Conversely, ΔK and grain yield were significantly correlated in all 3 yr (r = 0.49 − 0.52, P &lt; 0.001). In addition, G × Y interactions were significant for ΔF, ΔK and grain yield. However, significant correlations (P &lt; 0.001) were noted for ΔK across years. As a result, ΔK may serve as a better predictive criterion for higher grain yield under Mediterranean conditions.

Relationships between flag leaf carbon isotope discrimination and several morpho-physiological traits in durum wheat genotypes under Mediterranean conditions

Relationships between flag leaf carbon isotope discrimination (Δ), water status parameters, residual transpiration (RT) and stomatal density (SD) were examined on a collection of 144 durum wheat accessions. Associations between Δ, grain yield (GY) and harvest index (HI) were also studied. The field trial was conducted under Mediterranean conditions. The crop cycle was characterised by a period of drought from February until maturity. A broad range of values we obtained for Δ (16.5–19.9‰) and other physiological traits. Flag leaf Δ was positively and significantly correlated with both HI and GY. Δ was better correlated with HI than with GY, which suggests that higher Δ values indicate higher efficiency of carbon partitioning to the kernel, leading to higher GY. Δ was found positively related with RT and negatively related with SD. This relationship may indicate a possible SD component of RT due to the association between conductance and SD. Strong positive correlations were found between Δ and water status parameters, suggesting that Δ may provide a good indication of plant water status in durum wheat under rainfed Mediterranean conditions.

Relationships between carbon isotope discrimination, dry matter production, and harvest index in durum wheat

Carbon isotope discrimination has been proposed as a criterion for the indirect selection to improve transpiration efficiency and grain yield in bread wheat and barley. Less attention has been devoted to durumwheat (Triticum durumDesf.) despite its economic importance in the Mediterranean basin. The Δ genetic variation and its relationships to dry matter production and harvest index in durum wheat were investigated in this study. For this purpose, field experiments were conducted on 144 durum wheat accessions under Mediterranean conditions (South of France) during three consecutive years with contrasting climatic conditions. Grain yield, above-ground biomass, harvest index, and carbon isotope discrimination of flag leaf and kernel were measured. Differences between years, noted for both leaf and kernel carbon isotope discrimination, were probably related to the variation in water availability from year to year. A large genotypic variation was also noticed for both leaf and kernel carbon isotope discrimination. The two traits were found to be positively correlated with grain yield within and across years, which confirms the interest in carbon isotope discrimination for selection for grain yield improvement under Mediterranean conditions. Both kernel and leaf carbon isotope discrimination correlated better with harvest index than with grain yield, suggesting that carbon isotope discrimination could reflect the efficiency of carbon partitioning to the kernel. The lack of correlation between leaf carbon isotope discrimination and both harvest index and grain yield in favourable water conditions (1996) was probably due to the difference in water availability between the period until flag leaves sampling (favourable conditions) and the strong water stress which accompanied the grain filling. Kernel carbon isotope discrimination correlated better with both harvest index and grain yield than did leaf carbon isotope discrimination. Moreover, a higher broad-sense heritability was obtained for kernel carbon isotope discrimination than for leaf carbon isotope discrimination. As a result, kernel carbon isotope discrimination appeared to be a better predictive criterion for efficiency of the carbon partitioning to the kernel (harvest index), and hence for grain yield, than did flag leaf carbon isotope discrimination.

Effects of preharvest sprouting on the genetic structure of durum wheat ‘landraces’

AbstractPreharvest sprouting that occurs in wheat might affect seed viability and cause genetic erosion during periodical rejuvenation of durum wheat accessions in a gene bank. Two durum wheat landraces (MG 7713 and MG 7805) that had been rejuvenated for several years and did show a high percentage of presprouted seeds in the lot from the fourth rejuvenation cycle were identified. The frequency of durum and bread wheat genotypes and the distribution of the two species in three seed classes (ungerminated seeds, seeds with swollen embryo and germinated seeds) were studied. The modified phenol test was used to identify durum and bread wheat seeds and the genotypic frequencies within each species were assessed on the basis of acid polyacrylamide gel electrophoretic patterns of gliadin storage proteins. In these two landraces, durum wheat was more susceptible to preharvest sprouting than bread wheat and the frequency of bread wheat seeds significantly increased over the three rejuvenation cycles examined. Despite this, preharvest sprouting did not cause significant changes in the genotypic frequencies observed within species or loss of some genotypes that could not be attributed to susceptibility to sprouting.

Yielding ability, yield stability, and quality of exotic durum wheat germplasm in Sicily

AbstractA set of 20 accessions of durum wheat (Triticum turgidum L., durum group), which mostly included exotic landraces from various Mediterranean countries, and four Italian improved varieties were evaluated for grain yield in five environments and for quality traits in two environments of Sicily. Glutenin composition was also assessed electro‐phoretically on six seeds per entry. The cultivars differed (P ≤ 0.01) for yield, protein content and sodium dodecyl sulphate (SDS‐sed) sedimentation volume, in the presence of significant (P ≥ 0.001) genotype‐environment interaction effects. These effects were large for yield and moderately large for protein content relative to genotype main effects following estimation of variance components. Most exotic cultivars yielded comparably with, and some of them showed greater stability than, the best yielding Italian variety 'Simeto. Some exotic cultivars combined outstanding yield, protein content and SDS‐sedimentation values and represent therefore extremely valuable germplasm sources to broaden the local genetic base. The glutenin composition LMW‐2 and HMW 7 + 8 was positively associated with gluten quality. Six entries showed heterogeneous electrophoretic patterns, suggesting the presence of different genotypes within the cultivar.

Evaluation of five strategies for obtaining a core subset from a large genetic resource collection of durum wheat

The use of plant genetic resources contained in a large collection may be enhanced by specifying subsamples, called core samples. Five strategies for selecting a core sample from a collection of 3000 durum wheat accessions were applied and evaluated using four qualitative and eight quantitative spike characters. Each of the following strategies generated about 500 accessions for the core sample: random, random-systematic according to chronology of entry of the accessions into the collection, stratified by countryof-origin, stratified by log frequency by country-of-origin, and stratified by canonical variables. The first three strategies produced samples representative of the whole collection, but the remaining two produced the desired effect of increasing frequencies from less-represented countries-of-origin for several characters. The stratified canonical sample increased phenotypic variances. The quality of core samples is dependent upon good passport and evaluation data to partition the collection. The multivariate approach is extremely useful, but requires considerable data from the whole collection. Ecogeographic origin may be used in the absence of evaluation data on several characters to select useful core samples.

Rapid and Nondestructive Method for Detecting Composition Changes in Wheat Germplasm Accessions

Gene banks around the world have had difficulty detecting changes in the genetic composition of germplasm accessions after seed storage and regeneration. The large number of accessions requires a rapid and easy test. This study was conducted to develop such a test for wheat (Triticum spp.) accessions. A phenol test modified by soaking seeds in 40 °C water for 30 rain and then exposing them to 130 mM phenol solution vapor on a wire screen can differentiate among genetic types by staining in up to five colors: dark brown, brown, light brown, fawn, and ivory. The test is nondestructive. Phenol test results from gene‐bank accessions indicate that there was no significant change in the population composition after regeneration in homogeneous accessions such as bread wheat (T. aestivum L.) cultivars Sprint Aquileja, and Adria, and durum cultivars (T. turgidum L., durum group) Appulo, Capeiti, and Creso. Several heterogeneous durum wheat accessions showed significant changes, however. In the Italian durum wheat accession MG 8001, the dark brown phenol‐stained seeds increased from 11% to 95% after eight regeneration cycles. Gliadin polyacrylamide gel electrophoresis and root tip chromosome counting, conducted on the same seed, indicated that the dark‐brown phenol‐stained seeds were bread wheat instead of durum wheat. Thus, the modified phenol test could be a useful test for gene banks to detect changes in the composition of heterogeneous germplasm accessions after regeneration and storag.

Biodiversity in a germplasm collection of durum wheat

Over 7,600 durum wheat accessions belonging to 22 country gene pools were evaluated in Syria, during the seasons 1985–86 through 1987–88 under semi-arid Mediterranean climatic conditions. Data on seven agronomic traits are presented to assess the distinctiveness and the phenotypic diversity of these pools. Univariate statistical analysis revealed differences among materials of diverse origins for all traits. Mean phenotypic diversity within countries was highest in the germplasm from India, lowest in that from Bulgaria. In a canonical variate analysis, the first three canonical variables explained 77.7% of the total variance. A cluster analysis was performed to supplement the generated information by the canonical analysis. The multivariate analyses evidenced the distinctiveness of the Ethiopian germplasm. The gene pools from Syria and Jordan, closely resembling each other, appeared separate from all others. A certain peculiarity was also shown by the germplasms from Greece, Morocco and France, while the remaining countries clustered into four groups. The results of the present evaluation could provide useful information for breeding activities, germplasm collection, and establishment of ‘core’ collections. Evidence is presented that environment played a major role in creating the overall variation for the considered traits, although germplasm exchange seemed also an important factor.

Geographic variation for spike and grain characteristics in durum wheat germplasm adapted to dryland conditions

A set of 109 durum wheat accessions, selected from a world collection for their adaptation to dry environments, were further evaluated under moisture-limiting conditions. The entries were subdivided into 9 gene pools based on their country of origin. Data are presented to assess the phenotypic variation and distinctiveness of these pools for some spike and grain features. Analysis of variance revealed highly significant differences among materials of diverse origin for all traits except protein content. Within-country variation was generally wide as regards spike length and 1000-kernel weight. The Ethiopian germplasm presented a remarkable variability and distinctiveness. Entries with interesting values for breeding purposes were detected in most country gene pools. In general, selection of germplasm adapted to harsh environments did not result in a loss of variability for the considered traits, with the possible exception of protein content. Ethiopian entries presented the highest number of significant pairwise character correlations. The two most important traits in a discriminant analysis by country of origin were spike density and 1000-kernel weight. Diversity among gene pools was further assessed by a canonical analysis. The first three canonical variables explained 90% of the total variance. The gene pools of Ethiopia, Greece, Egypt and Jordan appeared clearly distinct from the other germplasm subsets studied. Geographically distinct patterns emerging from this study could be the result of the combined effect of natural and human selection.

Null alleles for gliadin blocks in bread and durum wheat cultivars

Wheat gliadin proteins are coded by clusters of genes (complex loci) located on the short arms of chromosomes of homoeologous groups 1 and 6 in bread (6x) and durum (4x) wheats. The proteins expressed by the various complex loci have been designated gliadin blocks. In a survey of accessions from the Germplasm Institute (C.N.R., Bari, Italy) collection, several different accessions have been found that lack particular blocks of proteins (null alleles). In some bread wheat accessions, seeds do not express gliadins that are coded by chromosomes 1D and 6A in normal cultivars. Similarly, some durum wheat accessions lack ω-gliadin components coded for by genes on chromosomes 1A and 1B. The missing proteins do not result from the absence of whole chromosomes, but may be the consequence of partial deletion of these genes at a complex locus or result from their silencing.

Genetic redundancy among durum wheat accessions as assessed by SSRs and endosperm proteins

Reducing duplication in ex-situ collections is complicated and requires good quality genetic markers. This study was conducted to assess the value of endosperm proteins and SSRs for validation of potential duplicates and monitoring intra-accession variability. Fifty durum wheat (Triticum turgidum ssp. durum) accessions grouped in 23 potential duplicates, and previously characterised for 30 agro-morphological traits, were analysed for gliadin and high molecular weight glutenin (HMWG) subunit alleles, total protein, and 24 SSRs, covering a wide genome area. Similarity and dissimilarity matrices were generated based on protein and SSRs alleles. For heterogeneous accessions at gliadins the percent pattern homology (PH) between gliadin patterns and the Nei’s coefficient of genetic identity (I) were computed. Eighteen duplicates identical for proteins showed none or less than 3 unshared SSRs alleles. For heterogeneous accessions PH and I values lower than 80 identified clearly off-types with more than 3 SSRs unshared. Only those biotypes differing in no more than one protein-coding locus were confirmed with SSRs. A good concordance among proteins, morphological traits, and SSR were detected. However, the discrepancy in similarity detected in some cases showed that it is advisable to evaluate redundancy through distinct approaches. The analysis in proteins together with SSRs data are very useful to identify duplicates, biotypes, close related genotypes, and contaminations.