Polyploid Wheat Species Research Articles

The Plastome Sequences of Triticum sphaerococcum (ABD) and Triticum turgidum subsp. durum (AB) Exhibit Evolutionary Changes, Structural Characterization, Comparative Analysis, Phylogenomics and Time Divergence

The mechanism and course of Triticum plastome evolution is currently unknown; thus, it remains unclear how Triticum plastomes evolved during recent polyploidization. Here, we report the complete plastomes of two polyploid wheat species, Triticum sphaerococcum (AABBDD) and Triticum turgidum subsp. durum (AABB), and compare them with 19 available and complete Triticum plastomes to create the first map of genomic structural variation. Both T. sphaerococcum and T. turgidum subsp. durum plastomes were found to have a quadripartite structure, with plastome lengths of 134,531 bp and 134,015 bp, respectively. Furthermore, diploid (AA), tetraploid (AB, AG) and hexaploid (ABD, AGAm) Triticum species plastomes displayed a conserved gene content and commonly harbored an identical set of annotated unique genes. Overall, there was a positive correlation between the number of repeats and plastome size. In all plastomes, the number of tandem repeats was higher than the number of palindromic and forward repeats. We constructed a Triticum phylogeny based on the complete plastomes and 42 shared genes from 71 plastomes. We estimated the divergence of Hordeum vulgare from wheat around 11.04–11.9 million years ago (mya) using a well-resolved plastome tree. Similarly, Sitopsis species diverged 2.8–2.9 mya before Triticum urartu (AA) and Triticum monococcum (AA). Aegilops speltoides was shown to be the maternal donor of polyploid wheat genomes and diverged ~0.2–0.9 mya. The phylogeny and divergence time estimates presented here can act as a reference framework for future studies of Triticum evolution.

Genome-wide identification and characterization of the Lateral Organ Boundaries Domain (LBD) gene family in polyploid wheat and related species.

BackgroundWheat (Triticum aestivum) originated from three different diploid ancestral grass species and experienced two rounds of polyploidization. Exploring how certain wheat gene subfamilies have expanded during the evolutionary process is of great importance. The Lateral Organ Boundaries Domain (LBD) gene family encodes plant-specific transcription factors that share a highly conserved LOB domain and are prime candidates for this, as they are involved in plant growth, development, secondary metabolism and stress in various species.MethodsUsing a genome-wide analysis of high-quality polyploid wheat and related species genome sequences, a total of 228 LBD members from five Triticeae species were identified, and phylogenetic relationship analysis of LBD members classified them into two main classes (classes I and II) and seven subgroups (classes I a–e, II a and II b).ResultsThe gene structure and motif composition analyses revealed that genes that had a closer phylogenetic relationship in the same subgroup also had a similar gene structure. Macrocollinearity and microcollinearity analyses of Triticeae species suggested that some LBD genes from wheat produced gene pairs across subgenomes of chromosomes 4A and 5A and that the complex evolutionary history of TaLBD4B-9 homologs was a combined result of chromosome translocation, polyploidization, gene loss and duplication events. Public RNA-seq data were used to analyze the expression patterns of wheat LBD genes in various tissues, different developmental stages and following abiotic and biotic stresses. Furthermore, qRT-PCR results suggested that some TaLBDs in class II responded to powdery mildew, regulated reproductive growth and were involved in embryo sac development in common wheat.

Geographical distribution and adaptive variation of VRN-A3 alleles in worldwide polyploid wheat (Triticum spp.) species collection.

The distribution of early flowering alleles of VRN-A3 was found to be biased to low latitudes, and these alleles may contribute to environmental adaptability to low latitudes in cultivated emmer wheat. In wheat (Triticum spp.), the flowering time is an important trait for successful seed production and yield by adapting to the regional environment. An early flowering allele of VRN-A3 with 7- and 25-bp insertions in the promoter region (Vrn-A3a-h1) has recently been reported from the analysis of an emmer wheat (Triticum turgidum L. ssp. dicoccum) accession, TN26. This early flowering allele of VRN-A3 might be associated with the regional adaptation of wheat. In this study, we elucidated its geographic distribution to assess the importance of the early flowering allele of VRN-A3 in worldwide wheat collection. From sequence analysis, we identified six VRN-A3 alleles with the 7- and 25-bp insertions, namely, Vrn-A3a-h2, Vrn-A3a-h3, Vrn-A3a-h4, Vrn-A3a-h5, Vrn-A3a-h6, and Vrn-A3c-h2 from wild emmer wheat, while we identified two VRN-A3 alleles with these insertions, Vrn-A3a-h2 and Vrn-A3c-h1 from cultivated tetraploid and hexaploid wheat species in addition to Vrn-A3a-h1. Among VRN-A3 alleles distributed in cultivated wheat, we found that Vrn-A3a-h2 promoted early heading, whereas Vrn-A3c-h1 did not affect heading time. Our analysis showed that the distribution of early flowering alleles of VRN-A3 dominated in cultivated emmer wheat in Ethiopia and India, which actually showed an early flowering phenotype. This implied that the early flowering alleles of VRN-A3 contribute to adaptability to a low-latitude environment in cultivated emmer wheat. We could not find durum (T. turgidum L. ssp. durum) and bread wheat (T. aestivum L. ssp. aestivum) accessions with these early flowering alleles. Our findings indicated that Vrn-A3a-h1 and Vrn-A3a-h2 were useful for breeding of early flowering cultivars in durum and bread wheat varieties.

Genotyping-by-Sequencing Derived Single Nucleotide Polymorphisms Provide the First Well-Resolved Phylogeny for the Genus Triticum (Poaceae).

Wheat (Triticum spp.) has been an important staple food crop for mankind since the beginning of agriculture. The genus Triticum L. is composed of diploid, tetraploid, and hexaploid species, majority of which have not yet been discriminated clearly, and hence their phylogeny and classification remain unresolved. Genotyping-by-sequencing (GBS) is an easy and affordable method that allows us to generate genome-wide single nucleotide polymorphism (SNP) markers. In this study, we used GBS to obtain SNPs covering all seven chromosomes from 283 accessions of Triticum-related genera. After filtering low-quality and redundant SNPs based on haplotype information, the GBS assay provided 14,188 high-quality SNPs that were distributed across the A (71%), B (26%), and D (2.4%) genomes. Cluster analysis and discriminant analysis of principal components (DAPC) allowed us to distinguish six distinct groups that matched well with Triticum species complexity. We constructed a Bayesian phylogenetic tree using 14,188 SNPs, in which 17 Triticum species and subspecies were discriminated. Dendrogram analysis revealed that the polyploid wheat species could be divided into groups according to the presence of A, B, D, and G genomes with strong nodal support and provided new insight into the evolution of spelt wheat. A total of 2,692 species-specific SNPs were identified to discriminate the common (T. aestivum) and durum (T. turgidum) wheat cultivar and landraces. In principal component analysis grouping, the two wheat species formed individual clusters and the SNPs were able to distinguish up to nine groups of 10 subspecies. This study demonstrated that GBS-derived SNPs could be used efficiently in genebank management to classify Triticum species and subspecies that are very difficult to distinguish by their morphological characters.

Evolution of Btr1-А Gene in Diploid Wheat Species of the Genus Triticum L.

During domestication of cereals, the threshability (hulled vs. naked grains) and brittle vs. non-brittle rachis were the most important traits, and man produced a breeding for naked and non-brittle rachis. In wheat, three gene systems that control the non-brittle rachis were identified. One of them is caused by the Btr1 (Non-brittle rachis) genes located in the chromosomes of the 3rd homeologic group. In the framework of this study, the sequences of the 5'- and 3'-untranslated regions and the coding region of the Btr1-A gene were studied for 30 accessions of wild and domesticated diploid wheat species Triticum boeoticum, T. monococcum, T. sinskajae, and T. urartu from Southern Europe, Transcaucasia, and Minor Asia. The Btr1-A gene sequence of T. sinskajae was obtained for the first time. In total, 11 various haplotypes of this gene were identified in the diploid wheat accessions studied, and five of them, namely, Hap12–Hap16, were described for the first time. The variability of T. boeoticum and T. urartu haplotypes, which were not previously described, does not affect the critical substitution at position 355 of the Btr1-A gene (G to A), specific to brittle rachis diploid wheat. The sequence of this gene in T. sinskajae and all T. monococcum accessions was identical to the haplotype Hap9 detected earlier. At position 355 of the Btr1-A gene, this haplotype replaces G with A, which leads to the occurrence of the non-brittle rachis trait. The involvement of diploid species haplotypes, which causes their lack of brittle rachis, in the naked polyploid wheat species is discussed.

Identification and characterization of large-scale genomic rearrangements during wheat evolution.

Following allopolyploidization, nascent polyploid wheat species react with massive genomic rearrangements, including deletion of transposable element-containing sequences. While such massive rearrangements are considered to be a prominent process in wheat genome evolution and speciation, their structure, extent, and underlying mechanisms remain poorly understood. In this study, we retrieved ~3500 insertions of a specific variant of Fatima, one of the most dynamic gypsy long-terminal repeat retrotransposons in wheat from the recently available high-quality genome drafts of Triticum aestivum (bread wheat) and Triticum turgidum ssp. dicoccoides or wild emmer, the allotetraploid mother of all modern wheats. The dynamic nature of Fatima facilitated the identification of large (i.e., up to ~ 1 million bases) Fatima-containing insertions/deletions (indels) upon comparison of bread wheat and wild emmer genomes. We characterized 11 such indels using computer-assisted analysis followed by PCR validation, and found that they might have occurred via unequal intra-strand recombination or double-strand break (DSB) events. Additionally, we observed one case of introgression of novel DNA fragments from an unknown source into the wheat genome. Our data thus indicate that massive large-scale DNA rearrangements might play a prominent role in wheat speciation.

Divergence of VRN-B3 alleles during the evolution of domesticated wheat.

Genetic changes accrued during the domestication of wheat have been crucial in improving the cultivation and yield of this strategic crop. Allelic variation at the VRN3 gene makes a significant contribution to the adaptability of wheat to a wide range of environmental conditions. In the present study, the origin and distribution of the Vrn-B3a and Vrn-B3b alleles during the evolution of wheat were investigated. Analysis of 214 accessions of 11 polyploid wheat species from different eco-geographical areas found the Vrn-B3a and Vrn-B3b alleles in accessions of tetraploid wheat T. dicoccum from Russia and hexaploid wheat of T. spelta from Iran, respectively. DNA sequence analysis of an insertion in the Vrn-B3b promoter region identified a new family of non-autonomous transposable hAT elements that originated in T. urartu lineage. Publicly available whole genome sequence assemblies of 11 T. aestivum and T. durum varieties, as well as WGS of T. dicoccoides were used to investigate the phylogeny and distribution of the TEs inserted in the Vrn-B3a and Vrn-B3b promoter regions, to determine the origin of these alleles. Results showed that both Vrn-B3a and Vrn-B3b diverged during the domestication of wheat, in the T. dicoccum lineage. However, while Vrn-B3a is common in T. dicoccum and T. durum from Ukraine and Russia the Vrn-B3b allele likely has a more recent origin in hexaploid wheat from the Near East.

Genetic mapping of a novel recessive allele for non-glaucousness in wild diploid wheat Aegilops tauschii: implications for the evolution of common wheat.

Cuticular wax on the aerial surface of plants has a protective function against many environmental stresses. The bluish-whitish appearance of wheat leaves and stems is called glaucousness. Most modern cultivars of polyploid wheat species exhibit the glaucous phenotype, while in a wild wheat progenitor, Ae. tauschii, both glaucous and non-glaucous accessions exist. Iw2, a wax inhibitor locus on the short arm of chromosome 2D, is the main contributor to this phenotypic variation in Ae. tauschii, and the glaucous/non-glaucous phenotype of Ae. tauschii is usually inherited by synthetic hexaploid wheat. However, a few synthetic lines show the glaucous phenotype although the parental Ae. tauschii accessions are non-glaucous. Molecular marker genotypes indicate that the exceptional non-glaucous Ae. tauschii accessions share the same genotype in the Iw2 chromosomal region as glaucous accessions, suggesting that these accessions have a different causal locus for their phenotype. This locus was assigned to the long arm of chromosome 3D using an F2 mapping population and designated W4, a novel glaucous locus in Ae. tauschii. The dominant W4 allele confers glaucousness, consistent with phenotypic observation of Ae. tauschii accessions and the derived synthetic lines. These results implied that glaucous accessions of Ae. tauschii with the W2W2iw2iw2W4W4 genotype could have been the D-genome donor of common wheat.

Role of antioxidant activity in the longevity of seeds of wheat species and minor forms

The aim and tasks of the study. To evaluate the role of antioxidant activity in the longevity of seeds of representativesof minor species and intraspeciesdiversity of wheat basing on experimental simulation of natural aging of seeds.Material and methods. The longevity of seeds was experimentally evaluated bysimulation of accelerated aging using two methods: Hampton, TeKrony (2005) and B.S. Likhachev (1980).In addition, seeds were stored hermetically sealed glass containers in a freezer at -20°C for 30 days, which corresponds to the long-term regimen of seed storage in the National Plant Genebank of Ukraine. Antiradical activity was determined usingthe stable DPPH radical, as described by S.Arabshahi (2007).Results and discussion. Correlation between the longevity of seeds and the level of antioxidant activity was found.The antioxidant activity (AOA) of einkorns T.sinskajae and T.monococcum was 32.54%–35.08% and 38.51%–39.10%, respectively, and was lower in all the experimental variants than that in polyploidwheat species (39.81%–53.83%). It was lower in T.sinskajae than in T.monococcum.The ranking of the test accessions by the AOA level established in the control remained unchanged after accelerated aging by the both methods and after freezing.Accelerated aging carried out byHampton andTeKrony’smethod reduced the viability and AOA of seeds of the test accessions in comparison with the control. The level of AOA in the control closely correlated with this parameter after accelerated aging by this method: positively with the AOA value (r = 0.91) and negatively with the AOA index (r = -0.86).There is a clear significant positive correlation between the AOA level in the control and the indices of germination power, germinability, sprout length and primary root length after accelerated aging: r = 0.52–0.85. This makes it possible to predictthe response of seeds of wheat accessions to accelerated aging, and consequently, the suitability of seeds for long-term storage, from the initial AOA index. In particular, a higher AOA level means a greater longevity of seeds.The results of the accelerated aging experiment by B.S. Likhachev’s method showed thatthe moisture level of 6% was closer to the optimal one for long-term storage as compared to the moisture content of 5% and 7%.Freezing either had a positive effect on seed parameters or did not change them. The AOA had a positive significant correlation with the germination power: r = 0.98. The indices of the freezing effect on the germination power, germinability and sprout length closely and positively correlated with the AOA of seeds after freezing: r = 0.79 - 0.87.Conclusions. The close significant positive correlation between the AOA level in the control and the indices of germination power, germinability and sprout length after freezing (r = 0.79–0.87) allows us to predict (to some extent)the response of seeds of wheat accessions to freezing from the initial AOA index. More specifically,the higher AOA level is, the greater the seed parameters increase under the influence of freezing

Fine organization of genomic regions tagged to the 5S rDNA locus of the bread wheat 5B chromosome

BackgroundThe multigene family encoding the 5S rRNA, one of the most important structurally-functional part of the large ribosomal subunit, is an obligate component of all eukaryotic genomes. 5S rDNA has long been a favored target for cytological and phylogenetic studies due to the inherent peculiarities of its structural organization, such as the tandem arrays of repetitive units and their high interspecific divergence. The complex polyploid nature of the genome of bread wheat, Triticum aestivum, and the technically difficult task of sequencing clusters of tandem repeats mean that the detailed organization of extended genomic regions containing 5S rRNA genes remains unclear. This is despite the recent progress made in wheat genomic sequencing. Using pyrosequencing of BAC clones, in this work we studied the organization of two distinct 5S rDNA-tagged regions of the 5BS chromosome of bread wheat.ResultsThree BAC-clones containing 5S rDNA were identified in the 5BS chromosome-specific BAC-library of Triticum aestivum. Using the results of pyrosequencing and assembling, we obtained six 5S rDNA- containing contigs with a total length of 140,417 bp, and two sets (pools) of individual 5S rDNA sequences belonging to separate, but closely located genomic regions on the 5BS chromosome. Both regions are characterized by the presence of approximately 70–80 copies of 5S rDNA, however, they are completely different in their structural organization. The first region contained highly diverged short-type 5S rDNA units that were disrupted by multiple insertions of transposable elements. The second region contained the more conserved long-type 5S rDNA, organized as a single tandem array. FISH using probes specific to both 5S rDNA unit types showed differences in the distribution and intensity of signals on the chromosomes of polyploid wheat species and their diploid progenitors.ConclusionA detailed structural organization of two closely located 5S rDNA-tagged genomic regions on the 5BS chromosome of bread wheat has been established. These two regions differ in the organization of both 5S rDNA and the neighboring sequences comprised of transposable elements, implying different modes of evolution for these regions.

Dated tribe-wide whole chloroplast genome phylogeny indicates recurrent hybridizations within Triticeae

BackgroundTriticeae, the tribe of wheat grasses, harbours the cereals barley, rye and wheat and their wild relatives. Although economically important, relationships within the tribe are still not understood. We analysed the phylogeny of chloroplast lineages among nearly all monogenomic Triticeae taxa and polyploid wheat species aiming at a deeper understanding of the tribe’s evolution. We used on- and off-target reads of a target-enrichment experiment followed by Illumina sequencing.ResultsThe read data was used to assemble the plastid locus ndhF for 194 individuals and the whole chloroplast genome for 183 individuals, representing 53 Triticeae species and 15 genera. We conducted Bayesian and multispecies coalescent analyses to infer relationships and estimate divergence times of the taxa. We present the most comprehensive dated Triticeae chloroplast phylogeny and review previous hypotheses in the framework of our results. Monophyly of Triticeae chloroplasts could not be confirmed, as either Bromus or Psathyrostachys captured a chloroplast from a lineage closely related to a Bromus-Triticeae ancestor. The most recent common ancestor of Triticeae occurred approximately between ten and 19 million years ago.ConclusionsThe comparison of the chloroplast phylogeny with available nuclear data in several cases revealed incongruences indicating past hybridizations. Recent events of chloroplast capture were detected as individuals grouped apart from con-specific accessions in otherwise monopyhletic groups.

Diversification of the Homoeologous Lr34 Sequences in Polyploid Wheat Species and Their Diploid Progenitors.

Allopolyploidization induces a multiple processes of genomic reorganization, including the structurally functional diversification of the homoeologous genes. An example of such diversification is the appearance of the Lr34 gene on chromosome 7D of bread wheat T. aestivum (BAD), the gene conferring durable, race non-specific protection against three fungal pathogens. In this study, we focused on the variability of a functionally critical region between exons 10-12 of Lr34 among diploid progenitors of wheat genomes and their respective polyploids. In the diploid A-genome species, two basic forms of the studied region have been revealed: (1) non-functional forms containing stop codons, or/and frameshifts (T. monococcum/T. urartu) and (2) forms with no such a mutations (T. boeoticum). The Lr34 sequence of T. urartu containing a TGA stop codon was inherited by the first tetraploid T. dicoccoides (BA), and then reorganized in some accessions of this species due to the insertion of an LTR retroelement in exon 10. Besides T. boeoticum, the second form of the Lr34 sequence is also characteristic of A. speltoides, which presumably donated this form to all polyploid descendants bearing B-genome. No differences were found between the D-genome-specific Lr34 sequences studied here and downloaded from databases, implying the highest level of conservation of the Lr34 predecessor throughout evolution. The sequence data were later used to construct phylograms, and apparent peculiarities in the evolution of the studied region of Lr34 genes discussed.

(GAA)n microsatellite as an indicator of the A genome reorganization during wheat evolution and domestication.

Although the wheat A genomes have been intensively studied over past decades, many questions concerning the mechanisms of their divergence and evolution still remain unsolved. In the present study we performed comparative analysis of the A genome chromosomes in diploid (Triticum urartu Tumanian ex Gandilyan, 1972, Triticum boeoticum Boissier, 1874 and Triticum monococcum Linnaeus, 1753) and polyploid wheat species representing two evolutionary lineages, Timopheevi (Triticum timopheevii (Zhukovsky) Zhukovsky, 1934 and Triticum zhukovskyi Menabde & Ericzjan, 1960) and Emmer (Triticum dicoccoides (Körnicke ex Ascherson & Graebner) Schweinfurth, 1908, Triticum durum Desfontaines, 1798, and Triticum aestivum Linnaeus, 1753) using a new cytogenetic marker – the pTm30 probe cloned from Triticum monococcum genome and containing (GAA)56 microsatellite sequence. Up to four pTm30 sites located on 1AS, 5AS, 2AS, and 4AL chromosomes have been revealed in the wild diploid species, although most accessions contained one–two (GAA)n sites. The domesticated diploid species Triticum monococcum differs from the wild diploid species by almost complete lack of polymorphism in the distribution of (GAA)n site. Only one (GAA)n site in the 4AL chromosome has been found in Triticum monococcum. Among three wild emmer (Triticum dicoccoides) accessions we detected 4 conserved and 9 polymorphic (GAA)n sites in the A genome. The (GAA)n loci on chromosomes 2AS, 4AL, and 5AL found in of Triticum dicoccoides were retained in Triticum durum and Triticum aestivum. In species of the Timopheevi lineage, the only one, large (GAA)n site has been detected in the short arm of 6At chromosome. (GAA)n site observed in Triticum monococcum are undetectable in the Ab genome of Triticum zhukovskyi, this site could be eliminated over the course of amphiploidization, while the species was established. We also demonstrated that changes in the distribution of (GAA)n sequence on the A-genome chromosomes of diploid and polyploid wheats are associated with chromosomal rearrangements/ modifications, involving mainly the NOR (nucleolus organizer region)-bearing chromosomes, that took place during the evolution of wild and domesticated species.

A Set of Cytogenetic Markers Allows the Precise Identification of All A-Genome Chromosomes in Diploid and Polyploid Wheat

Karyotypes of 3 diploid wheat species containing different variants of the A-genome, Triticum boeoticum (A^b), T. monococcum (A^b), and T. urartu (A^u), were examined using C-banding and FISH with DNA probes representing 5S and 45S rDNA families, the microsatellite sequences GAA_n and GTT_n, the already known satellite sequences pSc119.2, Spelt52, Fat, pAs1, and pTa535, and a newly identified repeat called Aesp_SAT86. The C-banding patterns of the 3 species in general were similar; differences were observed in chromosomes 4A and 6A. Besides 2 major 45S rDNA loci on chromosomes 1A and 5A, 2 minor polymorphic NORs were observed in the terminal part of 5AL and in the distal part of 6AS in all species. An additional minor locus was found in the distal part of 7A^bL of T. boeoticum and T. monococcum, but not in T. urartu. Two 5S rDNA loci were observed in 1AS and 5AS. The pTa535 probe displayed species- and chromosome-specific hybridization patterns, allowing complete chromosome identification and species discrimination. The distribution of pTa535 on the A^u-genome chromosomes was more similar to that on the A-genome chromosomes of T. dicoccoides and T. araraticum, thus confirming the origin of these genomes from T. urartu. The probe pAs1 allowed the identification of 4 chromosomes of T. urartu and 2 of T. boeoticum or T. monococcum. The Aesp_SAT86-derived patterns were polymorphic; main clusters were observed on chromosomes 1A^u and 3A^u of T. urartu and chromosomes 3A^b and 6A^b of T. boeoticum. Thus, a set of probes, pTa535, pAs1, GAA_n and GTT_n, pTa71, pTa794, and Aesp_SAT86, proved to be most informative for the analysis of A-genomes in diploid and polyploid wheat species.

Genome-, Transcriptome- and Proteome-Wide Analyses of the Gliadin Gene Families in Triticum urartu.

Gliadins are the major components of storage proteins in wheat grains, and they play an essential role in the dough extensibility and nutritional quality of flour. Because of the large number of the gliadin family members, the high level of sequence identity, and the lack of abundant genomic data for Triticum species, identifying the full complement of gliadin family genes in hexaploid wheat remains challenging. Triticum urartu is a wild diploid wheat species and considered the A-genome donor of polyploid wheat species. The accession PI428198 (G1812) was chosen to determine the complete composition of the gliadin gene families in the wheat A-genome using the available draft genome. Using a PCR-based cloning strategy for genomic DNA and mRNA as well as a bioinformatics analysis of genomic sequence data, 28 gliadin genes were characterized. Of these genes, 23 were α-gliadin genes, three were γ-gliadin genes and two were ω-gliadin genes. An RNA sequencing (RNA-Seq) survey of the dynamic expression patterns of gliadin genes revealed that their synthesis in immature grains began prior to 10 days post-anthesis (DPA), peaked at 15 DPA and gradually decreased at 20 DPA. The accumulation of proteins encoded by 16 of the expressed gliadin genes was further verified and quantified using proteomic methods. The phylogenetic analysis demonstrated that the homologs of these α-gliadin genes were present in tetraploid and hexaploid wheat, which was consistent with T. urartu being the A-genome progenitor species. This study presents a systematic investigation of the gliadin gene families in T. urartu that spans the genome, transcriptome and proteome, and it provides new information to better understand the molecular structure, expression profiles and evolution of the gliadin genes in T. urartu and common wheat.

Q gene variability in wheat species with different spike morphology

Q gene is the major domestication gene of wheat and many questions concerning Q gene genetics, including Q gene variability and its functional influence on phenotype, remain unanswered for the majority of wheat species. Here we crossed wheat species with dominant (Q) and recessive (q) alleles and confirmed that Q gene controls threshability, rachis fragility and spike shape traits. In the present study 18 new Q gene sequences were obtained and the Q gene sequences from 42 di- and polyploid wheat species with variable spike morphology were analyzed. We identified correlation between Q gene variability (coding mutation 329Val/Ile, promoter variability, microRNA172 binding site substitution) and threshability, rachis fragility and spike shape traits in polyploid wheat species. The analysis of 3D structures of q and Q proteins indicated that 329Val/Ile mutation does not affect overall protein structure and likely protein activity. We conclude that alterations in all three regions are essential for the formation of free-threshing non fragile normal phenotype in polyploid wheat.

Chromosome evolution in marginal populations of Aegilops speltoides: causes and consequences

Genome restructuring is an ongoing process in natural plant populations. The influence of environmental changes on the genome is crucial, especially during periods of extreme climatic fluctuations. Interactions between the environment and the organism manifest to the greatest extent at the limits of the species' ecological niche. Thus, marginal populations are expected to exhibit lower genetic diversity and higher genetic differentiation than central populations, and some models assume that marginal populations play an important role in the maintenance and generation of biological diversity. In this review, long-term data on the cytogenetic characteristics of diploid Aegilops speltoides Tauch populations are summarized and discussed. This species is distributed in and around the Fertile Crescent and is proposed to be the wild progenitor of a number of diploid and polyploid wheat species. In marginal populations of Ae. speltoides, numerical chromosomal aberrations, spontaneous aneuploidy, B-chromosomes, rDNA cluster repatterning and reduction in the species-specific and tribe-specific tandem repeats have been detected. Significant changes were observed and occurred in parallel with changes in plant morphology and physiology. Considerable genomic variation at the chromosomal level was found in the marginal populations of Ae. speltoides. It is likely that a specific combination of gene mutations and chromosomal repatterning has produced the evolutionary trend in each specific case, i.e. for a particular species or group of related species in a given period of time and in a certain habitat. The appearance of a new chromosomal pattern is considered an important factor in promoting the emergence of interbreeding barriers.

Single-copy nuclear PolA1 gene sheds light on the origin of S genome with relationships to B and G genomes of polyploid wheat species

PolA1, a single-copy nuclear gene encoding the largest subunit of RNA polymerase I, comprises highly polymorphic intron 19 and nucleotide tag (Ntag) sequences. We analyzed these sequences in 42 accessions, which differed in ploidy, of Triticum–Aegilops and Hordeum species. The lengths of the intron 19 sequences were ca. 110 bp long in Triticum–Aegilops species, except in four Sitopsis species, Ae. longissima, Ae. searsii, Ae. sharonensis, Ae. speltoides, which had introns similar in length to those of Hordeum species, i.e., ca. 240 bp long. Phylogenetic analyses of the Ntag sequences showed that the four Sitopsis and remaining Triticum–Aegilops species were classified into two discrete Hordeum and Triticum clades, respectively. The A and D genome-specific Ntag sequences of polyploid wheats were highly homologous with those of T. urartu and Ae. tauschii, respectively. In Ae. bicornis, another Sitopsis species, two accessions had the short intron 19 and Triticum–type Ntag sequence, which were highly homologous with those of the B genome in polyploid wheats, whereas one accession contained the long intron 19 and Hordeum–type Ntag sequences. In contrast, partial sequence analyses revealed that the three accessions of Ae. bicornis shared highly homology to single-copy DMC1 and EF-G genes. The discrepancy between these results indicates that the Sitopsis species were probably established by hybrid speciation including ancient introgressive hybridization between progenitors of Triticum–Aegilops and Hordeum. Although many researchers have proposed Ae. speltoides as a candidate for the B genome donor, our data suggest the existence of diploid B genome species in the past that were responsible for the origin of both polyploid wheats and Sitopsis species, including Ae. speltoides.

The MRN complex of wheat

The MRN complex is formed by the interaction of the products of the Mre11, Rad50 and Nbs1 genes. This complex plays a central role on repair of double-strand breaks (DSBs) and acts in a great number of cellular processes. In this study we have performed the analysis of the MRN complex in diploid and polyploid species of wheat. The molecular characterization was carried out in the diploid T. monococcum (genome A) and Ae. tauschii (genome D) and in the tetraploid T. turgidum (genomes A and B). The results obtained showed that in all cases the genes presented the main characteristics previously described in other species. A modified FISH protocol was used to locate the Rad50, Mre11 and the Nbs1 genes on the homoeologous chromosomes 5, 2 and 1, respectively. Analysis of expression showed that the hexaploid T. aestivum was the species with the higher level of expression whereas the rest of the species analysed showed no relation with its ploidy. Also, quantification of the expression of each homoeologous gene in the polyploid species evidenced in some cases a process of silencing after polyploidization. The study of the interaction between the proteins demonstrated that the interaction of proteins was not restricted to each genome, detecting interaction between proteins belonging to different genomes.

Characterization of theNbs1Gene and Analysis of the Expression of Homologous and Homoeologous MRN Complex Genes in Meiocytes and Somatic Cells of Different Wheat Species

The products of the Nbs1, Mre11, and Rad50 genes form the MRN complex. This complex is involved mainly in DNA repair, but it also has roles in meiosis, replication, telomere maintenance, and cell cycle checkpoint signaling. Its study is therefore of great importance if our knowledge of these important cell processes is to improve. To date, few studies have examined the MRN complex in plants. The present work describes the molecular characteristics of the Nbs1 gene in diploid and polyploid species of wheat. The expression of MRN complex homologous genes in meiocytes and in somatic cells treated with genotoxic agents is also examined. In addition, the expression of MRN complex homoeologous genes is analyzed in polyploid wheat species to determine the function of the duplicated genes. The characterization of Nbs1 in wheat species revealed its low sequence homology with the Nbs1 gene of other organisms. However, the homologous Nbs1 genes of wheat showed very great similarity. All the Nbs1 wheat genes examined...