Analysis of relationships between Aegilops tauschii and the D genome of wheat utilizing microsatellites.

Sixty Aegilops tauschii accessions and 60 European hexaploid wheat varieties were analyzed with 14 wheat microsatellite (WMS) primer sets to (i) study the phylogeny of Ae. tauschii, (ii) search for a specific genotype of Ae. tauschii most closely related to the D genome of hexaploid wheat, and (iii) narrow down the presumed birthplace of the latter. An average of 6.5 and 4.0 alleles per locus was detected in Ae. tauschii and in wheat, respectively. The highest genetic diversity of Ae. tauschii was found in Transcaucasia and southeast of the Caspian Sea. Distribution of the 87 alleles (without null alleles) found in Aegilops did not allow differentiation of the species into the two subspecies strangulata and tauschii. Excluding null alleles, 41 alleles occurred parallel in wheat and in Aegilops. Data obtained in this study supports the view of the D genome of hexaploid wheat being a composite of several sources but does not support subsp. strangulata as the possible major source of the D genome. The highest number of region-specific alleles (three) in Ae. tauschii occurring also in the D genome of wheat, and therefore most indicative for its evolution was found in present-day Georgia, where subsp. strangulata is not endemic.     

Phylogenetic relationships of Triticum tauschii the D genome donor to hexaploid wheat

Summary In crosses between T. tauschii (D ^t) accesions, their polymorphic gliadin forms were inherited as blocks of gliadin components -Gli-D ^t1, Gli-D ^t2 — as single Mendelian characters. From the progeny of four tri-parental crosses (test-crosses), HMW glutenin subunits derived from T. tauschii (Glu-D ^t1) segregated as alleles of the Glu-D1 locus in bread wheat. In three of the tri-parental crosses, a small proportion (2.5%) of the progeny with atypical segregation patterns, were identified through somatic chromosome counts, to be aneuploids (1.9% hypoploids and 0.6% hyperploids). Chromosomal mapping studies revealed that the synteny of genes for HMW glutenin subunits and gliadins in T. tauschii are conserved in the D genome homologue (chromosome 1D) of T. aestivum. The map distance between the Glu-D1/-D ^t1 and Gli-D1/-D ^t1 loci was calculated to be 63.5 cM, while a linkage to the centromere of 7.7–9.7 cM was estimated for the Glu-D1/-D ^t1 locus.     

Phylogenetic relationships of Triticum tauschii,the D genome donor to hexaploid wheat

Summary Isoelectric focusing of seed esterase (Est-5) isozymes in 79 T. tauschii accessions from diverse sources revealed the presence of six different seed esterase phenotypes. In one of these phenotypes, exclusive to a var. meyeri accession (AUS 18989), no detectable enzymatic activity was observed. Segregation in crosses between T. tauschii (D^t) accessions confirmed three of the seed esterase phenotypes to be alleles of the designated Est-D ^t5 gene locus; the inheritance pattern of these isozymes was not affected by the subspecies differences between the parents. On the bases of variation in Est-5 and their Glu-1 and Gli-1 gene loci (in a previous study in this series), only three strangulata accessions showed consistent homology with their prevalent gene expression in the D genome of hexaploid wheat. The implications of these observations for further interpreting the phyletic nature of the D genome donor in natural hexaploid wheat synthesis are also reported.     

The organization of genes tightly linked to the Ha locus in Aegilops tauschii, the D-genome donor to wheat.

The grain hardness locus, Ha, is located at the distal end of the short arm of chromosome 5D in wheat. Three polypeptides, puroindoline-a, puroindoline-b, and grain softness protein (GSP-1), have been identified as components of friabilin, a biochemical marker for grain softness, and the genes for these polypeptides are known to be tightly linked to the Ha locus. However, this region of the chromosome 5D has not been well characterized and the physical distance between the markers is not known. Separate lambda clones containing the puroindoline-a gene and the puroindoline-b gene have been isolated from an Aegilops tauschii (the donor of the D genome to wheat) genomic lambda library and investigated. Considerable variation appears to exist in the organization of the region upstream of the gene for puroindoline-b among species closely related to wheat. Using in situ hybridization the genes for puroindoline-a, -b, and GSP-1 were demonstrated to be physically located at the tip of the short arm of chromosome 5 of A. tauschii. Four overlapping clones were isolated from a large-insert BAC library constructed from A. tauschii and of these one contained genes for all of puroindoline-a, puroindoline-b, and GSP-1. The gene for puroindoline-a is located between the other two genes at a distance no greater than approximately 30 kb from either gene. The BAC clone containing all three known genes was used to screen a cDNA library constructed from hexaploid wheat and cDNAs that could encode novel polypeptides were isolated.     

Introgression of stem rust resistance genes SrTA10187 and SrTA10171 from Aegilops tauschii to wheat

Aegilops tauschii, the diploid progenitor of the wheat D genome, is a readily accessible germplasm pool for wheat breeding as genes can be transferred to elite wheat cultivars through direct hybridization followed by backcrossing. Gene transfer and genetic mapping can be integrated by developing mapping populations during backcrossing. Using direct crossing, two genes for resistance to the African stem rust fungus race TTKSK (Ug99), were transferred from the Ae. tauschii accessions TA10187 and TA10171 to an elite hard winter wheat line, KS05HW14. BC₂ mapping populations were created concurrently with developing advanced backcross lines carrying rust resistance. Bulked segregant analysis on the BC₂ populations identified marker loci on 6DS and 7DS linked to stem rust resistance genes transferred from TA10187 and TA10171, respectively. Linkage maps were developed for both genes and closely linked markers reported in this study will be useful for selection and pyramiding with other Ug99-effective stem rust resistance genes. The Ae. tauschii-derived resistance genes were temporarily designated SrTA10187 and SrTA10171 and will serve as valuable resources for stem rust resistance breeding.     

Inheritance and molecular mapping of new greenbug resistance genes in wheat germplasms derived from Aegilops tauschii

Molecular mapping of genes for crop resistance to the greenbug, Schizaphis graminum Rondani, will facilitate selection of greenbug resistance in breeding through marker-assisted selection and provide information for map-based gene cloning. In the present study, microsatellite marker and deletion line analyses were used to map greenbug resistance genes in five newly identified wheat germplasms derived from Aegilops tauschii. Our results indicate that the Gb genes in these germplasms are inherited as single dominant traits. Microsatellite markers Xwmc157 and Xgdm150 flank Gbx1 at 2.7 and 3.3 cM, respectively. Xwmc671 is proximately linked to Gba, Gbb, Gbc and Gbd at 34.3, 5.4, 13.7, 7.9 cM, respectively. Xbarc53 is linked distally to Gba and Gbb at 20.7 and 20.2 cM, respectively. Xgdm150 is distal to Gbc at 17.9 cM, and Xwmc157 is distal to Gbd at 1.9 cM. Gbx1, Gba, Gbb, Gbc, Gbd and the previously characterized Gbz are located in the distal 18% region of wheat chromosome 7DL. Gbd appears to be a new greenbug resistance gene different from Gbx1 or Gbz. Gbx1, Gbz Gba, Gbb, Gbc and Gbd are either allelic or linked to Gb3.     

Maize starch-branching enzyme isoforms and amylopectin structure. In the absence of starch-branching enzyme IIb, the further absence of starch-branching enzyme Ia leads to increased branching

Previous studies indicated that the deficiency of starch-branching enzyme (SBE) Ia in the single mutant sbe1a::Mu (sbe1a) has no impact on endosperm starch structure, whereas the deficiency of SBEIIb in the ae mutant is well known to reduce the branching of starch. We hypothesized that in maize (Zea mays) endosperm, the function of SBEIIb is predominant to that of SBEIa, and SBEIa would have an observable effect only on amylopectin structure in the absence of SBEIIb. To test this hypothesis, the mutant sbe1a was introgressed into lines containing either wx (lacking the granule-bound starch synthase GBSSI) or ae wx (lacking both SBEIIb and GBSSI) in the W64A background. Both western blotting and zymogram analysis confirmed the SBEIa deficiency in sbe1a wx and sbe1a ae wx, and the SBEIIb deficiency in ae wx and sbe1a ae wx. Using zymogram analysis, no pleiotropic effects of sbe1a genes on SBEIIa, starch synthase, or starch-debranching enzyme isoforms were observed. High-performance size exclusion chromatography analysis shows that the chain-length profiles of amylopectin as well as beta-limit dextrin were indistinguishable between wx and sbe1a wx, whereas significant differences for both were observed between ae wx and sbe1a ae wx, suggesting an effect of SBEIa on amylopectin biosynthesis that is observable only in the absence of SBEIIb. The amylopectin branch density and the average number of branches per cluster were both higher in endosperm starch from sbe1a ae wx than from ae wx. These results indicate possible functional interactions between SBE isoforms that may involve enzymatic inhibition. Both the cluster repeat distance and the distance between branch points on the short intracluster chains were similar for all genotypes however, suggesting a similar pattern of individual SBE isoforms in cluster initiation and the determination of branch point location.     

Genetic characterization and molecular mapping of Hessian fly resistance genes derived from Aegilops tauschii in synthetic wheat

Two synthetic hexaploid wheat lines (×Aegilotriticum spp., 2n = 6x = 42, genomes AABBDD), SW8 and SW34, developed from the crosses of the durum wheat cultivar Langdon (Triticum turgidum L. var. durum, 2n = 4x = 28, genomes AABB) with two Aegilops tauschii Cosson accessions (2n = 2x = 14, genome DD), were determined to carry Hessian fly [Mayetiola destructor (Say)] resistance genes derived from the Ae. tauschii parents. SW8 was resistant to the Hessian fly biotype Great Plains (GP) and strain vH13 (virulent to H13). SW34 was resistant to biotype GP, but susceptible to strain vH13. Allelism tests indicated that resistance genes in SW8 and SW34 may be allelic to H26 and H13 or correspond to paralogs at both loci, respectively. H26 and H13 were localized to chromosome 4D and 6D, respectively, in previous studies. Molecular mapping in the present study, however, assigned the H26 locus to chromosome 3D rather than 4D. On the other hand, mapping of the resistance gene in SW34 verified the previous assignment of the H13 locus to chromosome 6D. Linkage analysis and physical mapping positioned the H26 locus to the chromosomal deletion bin 3DL3-0.81–1.00. A linkage map for each of these two resistance genes was constructed using simple sequence repeat (SSR) and target region amplification polymorphism (TRAP) markers.     

Characterization of low-molecular-weight-glutenin subunit genes from the D-genome of Triticum aestivum,Aegilops crassa,Ae. cylindrica and Ae. tauschii

Twenty low-molecular-weight-glutenin subunit (LMW-GS) gene sequences from the D-genome from Aegilops crassa (2n = 4x = 28), Ae. cylindrica (2n = 4x = 28), Ae. tauschii (2n = 2x = 14) and Triticum aestivum (2n = 6x = 42) were obtained using five sets of specific allele primer pairs. Only the sequences of the first primer pair were complete coding sequences (cds) of LMW-GS, and had 305, 304, 306 and 305 LMW-m amino acid residues in Ae. crassa, Ae. cylindrica, Ae. tauschii and T. aestivum, respectively. The repetitive domain and repeat motif numbers of all LMW glutenin subunits showed eight conserved cysteine residues that lead to the same functional activity in different genome. Based on DNA and predicted protein sequences, phylogenetic trees for all sets of sequences were drawn. At the DNA level, the species closest to T. aestivum for the second, third, fourth and fifth set of sequences were Ae. cylindrica, Ae. tauschii and Ae. crassa, respectively. At the protein level, the species closest to T. aestivum based on the first, second and fifth set of sequences were Ae. cylindrica, Ae. crassa and Ae. crassa, respectively. For other sets of sequences, bread wheat proved to be a distinct species. The LMW-GS gene sequences have been recorded in the GenBank with accession numbers JQ726549–JQ726568JQ726549JQ726550JQ726551JQ726552JQ726553JQ726554JQ726555JQ726556JQ726557JQ726558JQ726559JQ726560JQ726561JQ726562JQ726563JQ726564JQ726565JQ726566JQ726567JQ726568.     

Population structure of wild wheat D‐genome progenitor Aegilops tauschii Coss.: implications for intraspecific lineage diversification and evolution of common wheat

Aegilops tauschii Coss. is the D‐genome progenitor of hexaploid wheat. Aegilops tauschii, a wild diploid species, has a wide natural species range in central Eurasia, spreading from Turkey to western China. Amplified fragment length polymorphism (AFLP) analysis using a total of 122 accessions of Ae. tauschii was conducted to clarify the population structure of this widespread wild wheat species. Phylogenetic and principal component analyses revealed two major lineages in Ae. tauschii. Bayesian population structure analyses based on the AFLP data showed that lineages one (L1) and two (L2) were respectively significantly divided into six and three sublineages. Only four out of the six L1 sublineages were diverged from those of western habitats in the Transcaucasia and northern Iran region to eastern habitats such as Pakistan and Afghanistan. Other sublineages including L2 were distributed to a limited extent in the western region. Subspecies strangulata seemed to be differentiated in one sublineage of L2. Among three major haplogroups (HG7, HG9 and HG16) previously identified in the Ae. tauschii population based on chloroplast variation, HG7 accessions were widely distributed to both L1 and L2, HG9 accessions were restricted to L2, and HG16 accessions belonged to L1, suggesting that HG9 and HG16 were formed from HG7 after divergence of the first two lineages of the nuclear genome. These results on the population structure of Ae. tauschii and the genealogical relationship among Ae. tauschii accessions should provide important agricultural and evolutionary knowledge on genetic resources and conservation of natural genetic diversity.     

Pm34: a new powdery mildew resistance gene transferred from Aegilops tauschii Coss. to common wheat (Triticum aestivum L.)

Powdery mildew is a major fungal disease in wheat growing areas worldwide. A novel source of resistance to wheat powdery mildew present in the germplasm line NC97BGTD7 was genetically characterized as a monogenic trait in greenhouse and field trials using F₂ derived lines from a NC97BGTD7 X Saluda cross. Microsatellite markers were used to map and tag this resistance gene, now designated Pm34. Three co-dominant microsatellite markers linked to Pm34 were identified and their most likely order was established as: Xbarc177-5D, 5.4cM, Pm34, 2.6cM, Xbarc144-5D, 14cM, Xgwm272-5D. These microsatellite markers were previously mapped to the long arm of the 5D chromosome and their positions were confirmed using Chinese Spring nullitetrasomic Nulli5D-tetra5A and ditelosomic Dt5DL lines. Pm2, the only other known Pm gene on chromosome 5D, has been mapped to the short arm and its specificity is different from that of Pm34.     

A tourist element in the 5'-flanking region of the catalase gene CatA reveals evolutionary relationships among Oryza species with various genome types

Tourist-OsaCatA, a transposable element, was found in the 5′-flanking region of the rice gene CatA. The characteristics of this element are similar to those of the other Tourist elements so far found in Oryza sativa. PCR and sequence analyses of 37 accessions of 18 species revealed that all the Oryza species examined, except for one accession, have either a full-length or a partial Tourist element at this locus. Unlike the Tourist elements previously reported, this Tourist element is found in all four Oryza species complexes in the Oryzeae tribe. All AA genome Oryza species, except O. longistaminata, contain the full-length Tourist element. O. longistaminata and the species of the O. officinalis, O. meyeriana and O. ridleyi complexes contain the partial element. A phylogenetic tree of Oryza species based on the nucleotide sequences of these Tourist elements was constructed. The O. longistaminata accessions were placed near the neighboring cluster of the officinalis complex. We propose that the ancestor of O. longistaminata and that of other species with the AA genome diverged, and the ancestor(s) of the O. officinalis, O. ridleyi and O. meyeriana complexes then diverged from the ancestor of O. longistaminata in the course of the evolution of the Oryza species. The Tourist elements associated with CatA and its orthologs thus provide useful tools for examining evolutionary relationships among Oryza species. Received: 12 March 1999 / Accepted: 7 July 1999     

The transfer of genes for Brown Rust resistance from Aegilops umbellulata Eig. to wheat (Triticum aestivum L.) genome

The potential of a genome-substituted form Avrolata (AABBUU) as a genetic system in genomic and chromosome manipulations for gene transfer from the wild species Aegilops umbellulata Eig. (UU) to cultivated wheat was studied. It was shown that plants combining resistance to leaf brown rust with high productivity may be produced from this form by classical hybridization procedures. The resistance gene introduced to line R-12 is dominant and probably identical to the Lr9 gene. By N-banding, chromosome staining technique and gliadin electrophoresis, the structural changes in chromosomes 1A, 2A, 4B, 6B, 7B, 1D, and 2D of the resistant line R-12 were revealed.     

Highly recombinogenic regions at seed storage protein loci on chromosome 1DS of Aegilops tauschii, the D-genome donor of wheat

A detailed RFLP map was constructed of the distal end of the short arm of chromosome 1D of Aegilops tauschii, the diploid D-genome donor species of hexaploid wheat. Ae. tauschii was used to overcome some of the limitations commonly associated with molecular studies of wheat such as low levels of DNA polymorphism. Detection of multiple loci by most RFLP probes suggests that gene duplication events have occurred throughout this chromosomal region. Large DNA fragments isolated from a BAC library of Ae. tauschii were used to determine the relationship between physical and genetic distance at seed storage protein loci located at the distal end of chromosome 1DS. Highly recombinogenic regions were identified where the ratio of physical to genetic distance was estimated to be <20 kb/cM. These results are discussed in relation to the genome-wide estimate of the relationship between physical and genetic distance.     

Genetic and physiological architecture of early vigor in Aegilops tauschii, the D-genome donor of hexaploid wheat. A quantitative trait loci analysis

Plant growth can be studied at different organizational levels, varying from cell, leaf, and shoot to the whole plant. The early growth of seedlings is important for the plant's establishment and its eventual success. Wheat (Triticum aestivum, genome AABBDD) seedlings exhibit a low early growth rate or early vigor. The germplasm of wheat is limited. Wild relatives constitute a source of genetic variation. We explored the physiological and genetic relationships among a range of early vigor traits in Aegilops tauschii, the D-genome donor. A genetic map was constructed with amplified fragment-length polymorphism and simple sequence repeat markers, and quantitative trait loci (QTL) analysis was performed on the F(4) population of recombinant inbred lines derived from a cross between contrasting accessions. The genetic map consisted of 10 linkage groups, which were assigned to the seven chromosomes and covered 68% of the D genome. QTL analysis revealed 87 mapped QTLs (log of the odds >2.65) in clusters, 3.1 QTLs per trait, explaining 32% of the phenotypic variance. Chromosomes 1D, 4D, and 7D harbored QTLs for relative growth rate, biomass allocation, specific leaf area, leaf area ratio, and unit leaf rate. Chromosome 2D covered QTLs for rate and duration of leaf elongation, cell production rate, and cell length. Chromosome 5D harbored QTLs for the total leaf mass and area and growth rate of the number of leaves and tillers. The results show that several physiological correlations between growth traits have a genetic basis. Genetic links between traits are not absolute, opening perspectives for identification of favorable alleles in A. tauschii to improve early vigor in wheat.     

Characterization of a dominant mutation for the liguleless trait: Aegilops tauschii liguleless (Lgt)

Background

Leaves of Poaceae have a unique morphological feature: they consist of a proximal sheath and a distal blade separated by a ligular region. The sheath provides structural support and protects young developing leaves, whereas the main function of the blade is photosynthesis. The auricles allow the blade to tilt back for optimal photosynthesis and determine the angle of a leaf, whereas the ligule protects the stem from the entry of water, microorganisms, and pests. Liguleless variants have an upright leaf blade that wraps around the culm. Research on liguleless mutants of maize and other cereals has led to identification of genes that are involved in leaf patterning and differentiation.

Results

We characterized an induced liguleless mutant (LM) of Aegilops tauschii Coss., a donor of genome D of bread wheat Triticum aestivum L.. The liguleless phenotype of LM is under dominant monogenic control (Lg^t). To determine precise position of Lg^t on the Ae. tauschii genetic map, highly saturated genetic maps were constructed containing 887 single-nucleotide polymorphism (SNP) markers derived via diversity arrays technology (DArT)seq. The Lg^t gene was mapped to chromosome 5DS. Taking into account coordinates of the SNP markers, flanking Lg^t, on the pseudomolecule 5D, a chromosomal region that contains this gene was determined, and a list of candidate genes was identified. Morphological features of the LM phenotype suggest that Lg^t participates in the control of leaf development, mainly, in leaf proximal–distal patterning, and its dominant mutation causes abnormal ligular region but does not affect reproductive development.

Conclusions

Here we report characterization of a liguleless Ae. tauschii mutant, whose phenotype is under control of a dominant mutation of Lg^t. The dominant mode of inheritance of the liguleless trait in a Triticeae species is reported for the first time. The position of the Lg^t locus on chromosome 5DS allowed us to identify a list of candidate genes. This list does not contain Ae. tauschii orthologs of any well-characterized cereal genes whose mutations cause liguleless phenotypes. Thus, the characterized Lg^t mutant represents a new model for further investigation of plant leaf patterning and differentiation.