Equivalence of the A genome of bread wheat and that of Triticum urartu

Identification of Novel Broad-Spectrum Leaf Rust Resistance Sources from Khapli Wheat Landraces

Wheat leaf rust caused by Puccinia triticina Eriks is an important disease that causes yield losses of up to 40% in susceptible varieties. Tetraploid emmer wheat (T. turgidum ssp. Dicoccum), commonly called Khapli wheat in India, is known to have evolved from wild emmer (Triticum turgidum var. dicoccoides), and harbors a good number of leaf rust resistance genes. In the present study, we are reporting on the screening of one hundred and twenty-three dicoccum wheat germplasm accessions against the leaf rust pathotype 77-5. Among these, an average of 45.50% of the germplasms were resistant, 46.74% were susceptible, and 8.53% had mesothetic reactions. Further, selected germplasm lines with accession numbers IC138898, IC47022, IC535116, IC535133, IC535139, IC551396, and IC534144 showed high level of resistance against the eighteen prevalent pathotypes. The infection type varied from “;”, “;N”, “;N1” to “;NC”. PCR-based analysis of the resistant dicoccum lines with SSR marker gwm508 linked to the Lr53 gene, a leaf rust resistance gene effective against all the prevalent pathotypes of leaf rust in India and identified from a T. turgidum var. dicoccoides germplasm, indicated that Lr53 is not present in the selected accessions. Moreover, we have also generated 35K SNP genotyping data of seven lines and the susceptible control, Mandsaur Local, to study their relationships. The GDIRT tool based on homozygous genotypic differences revealed that the seven genotypes are unique to each other and may carry different resistance genes for leaf rust.

Generation of Doubled Haploid Wheat-Triticum urartu Introgression Lines and Their Characterisation Using Chromosome-Specific KASP Markers

Wheat is one of the most important food and protein sources in the world and although, in recent years wheat breeders have achieved yield gains, they are not sufficient to meet the demands of an ever-growing population. Development of high yielding wheat varieties, resilient to abiotic and biotic stress resulting from climate change, has been limited by wheat's narrow genetic base. In contrast to wheat, the wild relatives of wheat provide a vast reservoir of genetic variation for most, if not all, agronomic traits. Previous studies by the authors have shown the transfer of genetic variation from T. urartu into bread wheat. However, before the introgression lines can be exploited for trait analysis, they are required to have stable transmission of the introgressions to the next generation. In this work, we describe the generation of 86 doubled haploid (DH) wheat-T. urartu introgression lines that carry homozygous introgressions which are stably inherited. The DH lines were characterised using the Axiom^® Wheat Relative Genotyping Array and 151 KASP markers to identify 65 unique T. urartu introgressions in a bread wheat background. DH production has helped accelerate the breeding process and facilitated the early release of homozygous wheat-T. urartu introgression lines. Together with the KASP markers, this valuable resource could greatly advance identification of beneficial alleles that can be used in wheat improvement.

Variation in phosphorus and sulfur content shapes the genetic architecture and phenotypic associations within the wheat grain ionome

Dissection of the genetic basis of wheat ionome is crucial for understanding the physiological and biochemical processes underlying mineral accumulation in seeds, as well as for efficient crop breeding. Most of the elements essential for plants are metals stored in seeds as chelate complexes with phytic acid or sulfur-containing compounds. We assume that the involvement of phosphorus and sulfur in metal chelation is the reason for strong phenotypic correlations within ionome. Adjustment of element concentrations for the effect of variation in phosphorus and sulfur seed content resulted in drastic change of phenotypic correlations between the elements. The genetic architecture of wheat grain ionome was characterized by quantitative trait loci (QTL) analysis using a cross between durum and wild emmer wheat. QTL analysis of the adjusted traits and two-trait analysis of the initial traits paired with either P or S considerably improved QTL detection power and accuracy, resulting in the identification of 105 QTLs and 617 QTL effects for 11 elements. Candidate gene search revealed some potential functional associations between QTLs and corresponding genes within their intervals. Thus, we have shown that accounting for variation in P and S is crucial for understanding of the physiological and genetic regulation of mineral composition of wheat grain ionome and can be implemented for other plants.

Reassessment of the evolution of wheat chromosomes 4A, 5A, and 7B

Comparison of genome sequences of wild emmer wheat and Aegilops tauschii suggests a novel scenario of the evolution of rearranged wheat chromosomes 4A, 5A, and 7B. Past research suggested that wheat chromosome 4A was subjected to a reciprocal translocation T(4AL;5AL)1 that occurred in the diploid progenitor of the wheat A subgenome and to three major rearrangements that occurred in polyploid wheat: pericentric inversion Inv(4AS;4AL)1, paracentric inversion Inv(4AL;4AL)1, and reciprocal translocation T(4AL;7BS)1. Gene collinearity along the pseudomolecules of tetraploid wild emmer wheat (Triticum turgidum ssp. dicoccoides, subgenomes AABB) and diploid Aegilops tauschii (genomes DD) was employed to confirm these rearrangements and to analyze the breakpoints. The exchange of distal regions of chromosome arms 4AS and 4AL due to pericentric inversion Inv(4AS;4AL)1 was detected, and breakpoints were validated with an optical Bionano genome map. Both breakpoints contained satellite DNA. The breakpoints of reciprocal translocation T(4AL;7BS)1 were also found. However, the breakpoints that generated paracentric inversion Inv(4AL;4AL)1 appeared to be collocated with the 4AL breakpoints that had produced Inv(4AS;4AL)1 and T(4AL;7BS)1. Inv(4AS;4AL)1, Inv(4AL;4AL)1, and T(4AL;7BS)1 either originated sequentially, and Inv(4AL;4AL)1 was produced by recurrent chromosome breaks at the same breakpoints that generated Inv(4AS;4AL)1 and T(4AL;7BS)1, or Inv(4AS;4AL)1, Inv(4AL;4AL)1, and T(4AL;7BS)1 originated simultaneously. We prefer the latter hypothesis since it makes fewer assumptions about the sequence of events that produced these chromosome rearrangements.

Detection of T. urartu Introgressions in Wheat and Development of a Panel of Interspecific Introgression Lines

Tritcum urartu (2n = 2x = 14, AuAu), the A genome donor of wheat, is an important source for new genetic variation for wheat improvement due to its high photosynthetic rate and disease resistance. By facilitating the generation of genome-wide introgressions leading to a variety of different wheat-T. urartu translocation lines, T. urartu can be practically utilized in wheat improvement. Previous studies that have generated such introgression lines have been unable to successfully use cytological methods to detect the presence of T. urartu in these lines. Many have, thus, used a variety of molecular markers with limited success due to the low-density coverage of these markers and time-consuming nature of the techniques rendering them unsuitable for large-scale breeding programs. In this study, we report the generation of a resource of single nucleotide polymorphic (SNP) markers, present on a high-throughput SNP genotyping array, that can detect the presence of T. urartu in a hexaploid wheat background making it a potentially valuable tool in wheat pre-breeding programs. A whole genome introgression approach has resulted in the transfer of different chromosome segments from T. urartu into wheat which have then been detected and characterized using these SNP markers. The molecular analysis of these wheat-T. urartu recombinant lines has resulted in the generation of a genetic map of T. urartu containing 368 SNP markers, spread across all seven chromosomes of T. urartu. Comparative analysis of the genetic map of T. urartu and the physical map of the hexaploid wheat genome showed that synteny between the two species is highly conserved at the macro-level and confirmed the presence of the 4/5 translocation in T. urartu also present in the A genome of wheat. A panel of 17 wheat-T. urartu recombinant lines, which consisted of introgressed segments that covered the whole genome of T. urartu, were also selected for self-fertilization to provide a germplasm resource for future trait analysis. This valuable resource of high-density molecular markers specifically designed for detecting wild relative chromosomes and a panel of stable interspecific introgression lines will greatly enhance the efficiency of wheat improvement through wild relative introgressions.

Impact of transgene genome location on gene migration from herbicide-resistant wheat (Triticum aestivum L.) to jointed goatgrass (Aegilops cylindrica Host)

BACKGROUND

Wheat (Triticum aestivum) (ABD) and jointed goatgrass (Aegilops cylindrica) (CD) can cross and produce hybrids that can backcross to either parent. Such backcrosses can result in progeny with chromosomes and/or chromosome segments retained from wheat. Thus, a herbicide resistance gene could migrate from wheat to jointed goatgrass. In theory, the risk of gene migration from herbicide-resistant wheat to jointed goatgrass is more likely if the gene is located on the D genome and less likely if the gene is located on the A or B genome of wheat.

RESULTS

BC₁ populations (jointed goatgrass as a recurrent parent) were analyzed for chromosome numbers and transgene transmission rates under sprayed and non-sprayed conditions. Transgene retention in the non-sprayed BC₁ generation for the A, B and D genomes was 84, 60 and 64% respectively. In the sprayed populations, the retention was 81, 59 and 74% respectively.

CONCLUSION

The gene transmission rates were higher than the expected 50% or less under sprayed and non-sprayed conditions, possibly owing to meiotic chromosome restitution and/or chromosome non-disjunction. Such high transmission rates in the BC₁ generation negates the benefits of gene placement for reducing the potential of gene migration from wheat to jointed goatgrass. © 2016 Society of Chemical Industry.

Chromosomal structural changes and microsatellite variations in newly synthesized hexaploid wheat mediated by unreduced gametes

Allohexaploid wheat was derived from interspecific hybridization, followed by spontaneous chromosome doubling. Newly synthesized hexaploid wheat by crossing Triticum turgidum and Aegilops tauschii provides a classical model to understand the mechanisms of allohexaploidization in wheat. However, immediate chromosome level variation and microsatellite level variation of newly synthesized hexaploid wheat have been rarely reported. Here, unreduced gametes were applied to develop synthesized hexaploid wheat, NA0928, population by crossing T. turgidum ssp. dicoccum MY3478 and Ae. tauschii SY41, and further S0-S3 generations of NA0928 were assayed by sequential cytological and microsatellite techniques. We demonstrated that plentiful chromosomal structural changes and microsatellite variations emerged in the early generations of newly synthesized hexaploid wheat population NA0928, including aneuploidy with whole-chromosome loss or gain, aneuploidy with telosome formation, chromosome-specific repeated sequence elimination (indicated by fluorescence in situ hybridization) and microsatellite sequence elimination (indicated by sequencing), and many kinds of variations have not been previously reported. Additionally, we reported a new germplasm, T. turgidum accession MY3478 with excellent unreduced gametes trait, and then succeeded to transfer powdery mildew resistance from Ae. tauschii SY41 to synthesized allohexaploid wheat population NA0928, which would be valuable resistance resources for wheat improvement.

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.

Structure, transcription and post-transcriptional regulation of the bread wheat orthologs of the barley cleistogamy gene Cly1

The majority of genes present in the hexaploid bread wheat genome are present as three homoeologs. Here, we describe the three homoeologous orthologs of the barley cleistogamy gene Cly1, a member of the AP2 gene family. As in barley, the wheat genes (designated TaAP2-A, -B and -D) map to the sub-telomeric region of the long arms of the group 2 chromosomes. The structure and pattern of transcription of the TaAP2 homoeologs were similar to those of Cly1. Transcript abundance was high in the florets, and particularly in the lodicule. The TaAP2 message was cleaved at its miR172 target sites. The set of homoeolog-specific PCR assays developed will be informative for identifying either naturally occurring or induced cleistogamous alleles at each of the three wheat homoeologs. By combining such alleles via conventional crossing, it should be possible to generate a cleistogamous form of bread wheat, which would be advantageous both with respect to improving the level of the crop's resistance against the causative pathogen of fusarium head blight, and for controlling pollen-mediated gene flow to and from genetically modified cultivars.

Genome evolution due to allopolyploidization in wheat

The wheat group has evolved through allopolyploidization, namely, through hybridization among species from the plant genera Aegilops and Triticum followed by genome doubling. This speciation process has been associated with ecogeographical expansion and with domestication. In the past few decades, we have searched for explanations for this impressive success. Our studies attempted to probe the bases for the wide genetic variation characterizing these species, which accounts for their great adaptability and colonizing ability. Central to our work was the investigation of how allopolyploidization alters genome structure and expression. We found in wheat that allopolyploidy accelerated genome evolution in two ways: (1) it triggered rapid genome alterations through the instantaneous generation of a variety of cardinal genetic and epigenetic changes (which we termed "revolutionary" changes), and (2) it facilitated sporadic genomic changes throughout the species' evolution (i.e., evolutionary changes), which are not attainable at the diploid level. Our major findings in natural and synthetic allopolyploid wheat indicate that these alterations have led to the cytological and genetic diploidization of the allopolyploids. These genetic and epigenetic changes reflect the dynamic structural and functional plasticity of the allopolyploid wheat genome. The significance of this plasticity for the successful establishment of wheat allopolyploids, in nature and under domestication, is discussed.

Genomic asymmetry in allopolyploid plants: wheat as a model

The evolvement of duplicated gene loci in allopolyploid plants has become the subject of intensive studies. Most duplicated genes remain active in neoallopolyploids contributing either to a favourable effect of an extra gene dosage or to the build-up of positive inter-genomic interactions when genes or regulation factors on homoeologous chromosomes are divergent. However, in a small number of loci (about 10%), genes of only one genome are active, while the homoeoalleles on the other genome(s) are either eliminated or partially or completely suppressed by genetic or epigenetic means. For several traits, the retention of controlling genes is not random, favouring one genome over the other(s). Such genomic asymmetry is manifested in allopolyploid wheat by the control of various morphological and agronomical traits, in the production of rRNA and storage proteins, and in interaction with pathogens. It is suggested that the process of cytological diploidization leading to exclusive intra-genomic meiotic pairing and, consequently, to complete avoidance of inter-genomic recombination, has two contrasting effects. Firstly, it provides a means for the fixation of positive heterotic inter-genomic interactions and also maintains genomic asymmetry resulting from loss or silencing of genes. The possible mechanisms and evolutionary advantages of genomic asymmetry are discussed.

Microsatellite mapping of Ae. speltoides and map-based comparative analysis of the S, G, and B genomes of Triticeae species

The first microsatellite linkage map of Ae. speltoides Tausch (2n = 2x = 14, SS), which is a wild species with a genome closely related to the B and G genomes of polyploid wheats, was developed based on two F(2) mapping populations using microsatellite (SSR) markers from Ae. speltoides, wheat genomic SSRs (g-SSRs) and EST-derived SSRs. A total of 144 different microsatellite loci were mapped in the Ae. speltoides genome. The transferability of the SSRs markers between the related S, B, and G genomes allowed possible integration of new markers into the T. timopheevii G genome chromosomal maps and map-based comparisons. Thirty-one new microsatellite loci assigned to the genetic framework of the T. timopheevii G genome maps were composed of wheat g-SSR (genomic SSR) markers. Most of the used Ae. speltoides SSRs were mapped onto chromosomes of the G genome supporting a close relationship between the G and S genomes. Comparative microsatellite mapping of the S, B, and G genomes demonstrated colinearity between the chromosomes within homoeologous groups, except for intergenomic T6A(t)S.1G, T4AL.5AL.7BS translocations. A translocation between chromosomes 2 and 6 that is present in the T. aestivum B genome was found in neither Ae. speltoides nor in T. timopheevii. Although the marker order was generally conserved among the B, S, and G genomes, the total length of the Ae. speltoides chromosomal maps and the genetic distances between homoeologous loci located in the proximal regions of the S genome chromosomes were reduced compared with the B, and G genome chromosomes.

Evolution of polyploid triticum wheats under cultivation: the role of domestication, natural hybridization and allopolyploid speciation in their diversification

The evolution of the polyploid Triticum wheats is distinctive in that domestication, natural hybridization and allopolyploid speciation have all had significant impacts on their diversification. In this review, I outline the phylogenetic relationships of cultivated wheats and their wild relatives and provide an overview of the recent progress and remaining issues in understanding the genetic and ecological factors that favored their evolution. An attempt is made to view the evolution of the polyploid Triticum wheats as a continuous process of diversification that was initiated by domestication of tetraploid emmer wheat and driven by various natural events ranging from interploidy introgression via hybridization to allopolyploid speciation of hexaploid common wheat, instead of viewing it as a group of discrete evolutionary processes that separately proceeded at the tetraploid and hexaploid levels. This standpoint underscores the important role of natural hybridization in the reticulate diversification of the tetraploid-hexaploid Triticum wheat complex and highlights critical, but underappreciated, issues that concern the allopolyploid speciation of common wheat.

Quantification of genetic relationships among A genomes of wheats

The genetic relationships of A genomes of Triticum urartu (Au) and Triticum monococcum (Am) in polyploid wheats are explored and quantified by AFLP fingerprinting. Forty-one accessions of A-genome diploid wheats, 3 of AG-genome wheats, 19 of AB-genome wheats, 15 of ABD-genome wheats, and 1 of the D-genome donor Ae. tauschii have been analysed. Based on 7 AFLP primer combinations, 423 bands were identified as potentially A genome specific. The bands were reduced to 239 by eliminating those present in autoradiograms of Ae. tauschii, bands interpreted as common to all wheat genomes. Neighbour-joining analysis separates T. urartu from T. monococcum. Triticum urartu has the closest relationship to polyploid wheats. Triticum turgidum subsp. dicoccum and T. turgidum subsp. durum lines are included in tightly linked clusters. The hexaploid spelts occupy positions in the phylogenetic tree intermediate between bread wheats and T. turgidum. The AG-genome accessions cluster in a position quite distant from both diploid and other polyploid wheats. The estimates of similarity between A genomes of diploid and polyploid wheats indicate that, compared with Am, Au has around 20% higher similarity to the genomes of polyploid wheats. Triticum timo pheevii AG genome is molecularly equidistant from those of Au and Am wheats.

Microsatellite mapping of a Triticum urartu Tum. derived powdery mildew resistance gene transferred to common wheat (Triticum aestivum L.)

A powdery mildew resistance gene from Triticum urartu Tum. accession UR206 was successfully transferred into hexaploid wheat (Triticum aestivum L.) through crossing and backcrossing. The F1 plants, which had 28 chromosomes and an average of 5.32 bivalents and 17.36 univalents in meiotic pollen mother cells (PMC), were obtained through embryos rescued owing to shriveling of endosperm in hybrid seed of cross Chinese Spring (CS) x UR206. Hybrid seeds were produced through backcrossing F1 with common wheat parents. The derivative lines had normal chromosome numbers and powdery mildew resistance similar to the donor UR206, indicating that the powdery mildew resistance gene originating from T. urartu accession UR206 was successfully transferred and expressed in a hexaploid wheat background. Genetic analysis indicated that a single dominant gene controlled the powdery mildew resistance at the seedling stage. To map and tag the powdery mildew resistance gene, 143 F2 individuals derived from a cross UR206 x UR203 were used to construct a linkage map. The resistant gene was mapped on the chromosome 7AL based on the mapped microsatellite makers. The map spanned 52.1 cM and the order of these microsatellite loci agreed well with the established microsatellite map of chromosome arm 7AL. The resistance gene was flanked by the microsatellite loci Xwmc273 and Xpsp3003, with the genetic distances of 2.2 cM and 3.8 cM, respectively. On the basis of the origin and chromosomal location of the gene, it was temporarily designated PmU.

Comparative genetic maps reveal extreme crossover localization in the Aegilops speltoides chromosomes

A total of 137 loci were mapped in Aegilops speltoides, the closest extant relative of the wheat B genome, using two F(2) mapping populations and a set of wheat-Ae. speltoides disomic addition (DA) lines. Comparisons of Ae. speltoides genetic maps with those of Triticum monococcum indicated that Ae. speltoides conserved the gross chromosome structure observed across the tribe Triticeae. A putative inversion involving the short arm of chromosome 2 was detected in Ae. speltoides. A translocation between chromosomes 2 and 6, present in the wheat B genome, was absent. The ligustica/aucheri spike dimorphism behaved as allelic variation at a single locus, which was mapped in the centromeric region of chromosome 3. The genetic length of each chromosome arm was about 50 cM, irrespective of its physical length. Compared to T. monococcum genetic maps, recombination was virtually eliminated from the proximal 50-100 cM and was localized in short distal regions, which were often expanded compared to the T. monococcum maps. The wheat B genome and the genome of Ae. longissima, a close relative of Ae. speltoides, do not show the extreme localization of crossovers observed in Ae. speltoides.

Intragenic diversity and functional conservation of the three homoeologous loci of the KN1-type homeobox gene Wknox1 in common wheat

Common wheat represents a typical allohexaploid species and provides a good experimental system for studying genomic alterations associated with allopolyploidization. We studied three homoeologous loci of wheat Wknox1 gene, which is highly homologous to the knotted1 (kn1)-like homeobox (KNOX) genes functioning at shoot apical meristems (SAM). Wknox1 transcripts were detected in SAM, and its overexpression caused abnormal leaf morphology with occasional ectopic leaves in transgenic tobacco plants. A comparative study of the three Wknox1 genomic sequences revealed accumulation of a large number of mutations including insertions and deletions, particularly in the fourth intron and the 5'-upstream region. Some structural mutations including MITE-insertions were allocated in the evolutionary lineage of the wheat genome, suggesting that they occurred at all stages of wheat evolution. A mutation rate was the highest in the Wknox1b locus, which is consistent with the known highest degree of differentiation in the B genome. Despite the structural differentiation, all three Wknox1 homoeologs showed an identical expression pattern in wheat and their promoter regions induced the conserved expression pattern in transgenic tobacco plants. A potential of the intragenic diversity in homoeologs of essential genes as a tool for studying the genome evolution associated with allopolyploidization was discussed.

The utility of the nontranscribed spacer of 5S rDNA units grouped into unit classes assigned to haplomes - a test on cultivated wheat and wheat progenitors

Data is presented on the evolutionary dynamics of non-transcribed spacers (NTSs) of 5S rRNA genes in some diploid and polyploid Triticum and Aegilops species. FISH experiments with probes representing different unit classes revealed presence and (or) absence of these sequences in genomes or separate chromosomes of the species. Among the three diploid species only Aegilops speltoides has all of the different unit classes in ribosomal clusters as detected by the probes. Triticum urartu does not have the long D1 signals and Aegilops tauschii does not have the long A1 signals. Both polyploids possess all types of sequences, but because of genome rearrangements after polyploidization there is significant repatterning of single different rDNA unit classes in chromosomal positions when compared with those in diploid progenitors. Additional refined work is needed to ascertain if the sequences in the polyploids are mixed or are located in mini clusters in close proximity to each other. Mantel tests for association between the presence of the FISH signals of the A, B, and D genomes together and separately with the unit class data of the material, i.e., the probes used in FISH, indicated that all signals were associated with their respective probe material, but that there was no association of the unit classes found and the signals to each haplome. All combinations of the partial Mantel tests, e.g., between the A and B haplomes while controlling the effect of the all probes signals, with correlations ranging from 0.48 to 0.79 were all significant. Principal coordinate analysis showed that the signals of most unit class specific probes were more or less equally distant except for the long (S1 and short G1 signals, which were not different, and that the short A1 signals were closely related to the former two, whereas the signals of the long G1 were even less related.

Advanced backcross QTL analysis in progenies derived from a cross between a German elite winter wheat variety and a synthetic wheat (Triticum aestivum L.)

We report here the second advanced backcross quantitative trait locus (AB-QTL) analysis carried out in winter wheat. Seven agronomic traits were studied in a BC2F1 population derived from a cross between the German winter wheat variety Flair and the synthetic wheat line XX86 developed in Japan. We selected 111 BC2F1 lines and genotyped these with 197 microsatellite markers. Field data for seven agronomic traits were collected from corresponding BC2F3 families that were grown at up to six locations in Germany. QTL analyses for yield and yield components were performed using single-marker regression and interval mapping. A total of 57 putative QTLs derived from XX86 were detected, of which 24 (42.1%) were found to have a positive effect from the synthetic wheat XX86. These favourable QQTLs were mainly associated with thousand-grain weight and grain weight per ear. Many QTLs for correlated traits were mapped in similar chromosomal regions. The AB-QTL data obtained in the present study are discussed and compared with results from previous QTL analyses.

Repetitive DNas of wild emmer wheat (Triticum dicoccoides) and their relation to S-genome species: molecular cytogenetic analysis

We have analyzed the chromosomal GISH molecular banding patterns of three populations of the wild allopolyploid wheat Triticum dicoccoides in an attempt to unravel the evolutionary relationships between highly repetitive DNA fractions of T. dicoccoides and proposed diploid progenitors of the B genome. Aegilops speltoides showed almost complete affinity of its repetitive DNA to C-heterochromatin of T. dicoccoides, whereas other S-genome species demonstrated relatedness only to distal heterochromatin. This substantiates the priority of Ae. speltoides as the most similar to the wheat B-genome donor in comparison with other Sitopsis species. Using molecular banding technique with DNA of different Aegilops species as a probe permits tracing of the origin of each heterochromatin cluster. Molecular banding analysis reveals polymorphism between three wild emmer wheat populations. Comparison of molecular banding patterns with chromosomal distribution of the Ty1-copia retrotransposons, which constitute a large share of T. dicoccoides genome, makes it possible to propose that the activity of transposable elements may lie in the background of observed intraspecific polymorphism.

Microsatellite markers reveal chimeric origin of redesignated chromosome 4A of wheat from Triticum urartu and other species

Although a new nomenclature has been adopted for wheat in which chromosome 4A (4AO) has been renamed 4B (4BN) and chromosome 4B (4BO) has been renamed 4A (4AN), their specific origin remains uncertain. The use of wheat microsatellite (WMS) markers mapped to chromosomes 4AN and 4BN in a set of polyploid wheats and diploid genome donors has unequivocally indicated that the entire short arm of 4AN, some part of 4ANL. and a segment of 4BNL were derived from Triticum urartu. The presence of a T. urartu-specific allele at locus gwm368 on 4BNL and of an Aegilops speltoides allele at locus gwm397 on 4ANL suggests the possibility of a reciprocal translocation between 4ANL and 4BNL. The subcentromeric and telomeric regions of 4ANL corresponding to heterochromatic C-bands were derived neither from diploid wheats nor from Ae. speltoides or Aegilops longissima.

The specific isolation of complete 5S rDNA units from chromosome 1A of hexaploid, tetraploid, and diploid wheat species using PCR with head-to-head oriented primers

The presence of 5S rDNA units on chromosome 1A of Triticum aestivum was shown by the development of a specific PCR test, using head-to-head oriented primers. This primer set allowed the amplification of complete 5S DNA units and was used to isolate 5S-Rrna-A1 sequences from polyploid and diploid wheat species. Multiple-alignment and parsimony analyses of the 132 sequences divided the sequences into four types. The isolates from T. aestivum and the tetraploid species (T. dicoccoides, T. dicoccum, T. durum, T. araraticum, and T. timopheevi) were all of one type, which was shown to be closely related to the type mainly characteristic for T. urartu. The other two types were isolated exclusively from the diploid species T. monococcum, T. aegilopoides, T. thaoudar, and T. sinskajae and the hexaploid species T. zhukovski. Triticum monococcum was the only species for which representatives of each of the four sequence types were found to be present. Further, we discuss the possible multicluster arrangement of the 5S-Rrna-A1 array.Key words: Triticum, A genome, PCR markers, 5S ribosomal RNA locus.

ORIGIN AND TAXONOMY OF WHEAT IN THE LIGHT OF RECENT RESEARCH

There is still disagreement among scientists on the exact origin of common wheat (Triticum aestivum ssp. aestivum), one of the most important crops in the world. The first step in the development of the hexaploid aestivum group (ABD) may have been hybridisation between T. urartu (A), as pollinator, and a species related to the Sitopsis section of the Aegilops genus (S) as cytoplasm donor, leading to the creation of the tetraploid species T. turgidum ssp. dicoccoides (AB). The following step may have involved hybridisation between T. turgidum ssp. dicoccon (AB genome, cytoplasm donor), a descendant of T. turgidum ssp. dicoccoides, and Ae. tauschii (D genome, pollinator), resulting in the hexaploid species T. aestivum ssp. spelta (ABD) or some other hulled type. This form may have given rise to naked types, including T. aestivum ssp. aestivum (ABD). The ancestors of the tetraploid T. timopheevii (AG) may have been the diploid T. urartu (A genome, pollinator) and Ae. speltoides (S genome, cytoplasm donor). Species in the timopheevii group developed later than those in the turgidum group, as confirmed by the fact that the G genome is practically identical to the S genome of Ae. speltoides, while the more ancient B genome has undergone divergent evolution. Hybridisation between T. timopheevii (AG, cytoplasm donor) and T. monococcum (A m, pollinator) may have resulted in the species T. zhukovskyi (AGA m). Research into the relationships between the various species is of assistance in compiling the taxonomy of wheat and in avoiding misunderstandings arising from the fact that some species are known by two or more synonymous names.

Structural evolution of wheat chromosomes 4A, 5A, and 7B and its impact on recombination

The construction of comparative genetic maps of chromosomes 4A(m) and 5A(m) of Triticum monococcum and chromosomes of homoeologous groups 4, 5 and 7 of T. aestivum has provided insight into the evolution of these chromosomes. The structures of chromosomes 4A, 5A and 7B of modern-day hexaploid bread wheat can be explained by a 4AL/5AL translocation that occurred at the diploid level and is present both in T. monococcum and T. aestivum. Three further rearrangements, a 4AL/7BS translocation, a pericentric inversion and a paracentric inversion, have taken place in the tetraploid progenitor of hexaploid wheat. These structural rearrangements and the evolution of chromosomes 4A, 5A and 7B of bread wheat are discussed. The presence of the 4AL/5AL translocation in several Triticeae genomes raises two questions - which state is the more primitive, and is the translocation of mono- or poly-phylogenetic origin?The rearrangements that have occurred in chromosome 4A resulted in segments of both arms having different positions relative to the telomere, compared to 4A(m) and to 4B and 4D. Comparisons of map length in these regions indicate that genetic length is a function of distance from the telomere, with the distal regions showing the highest recombination.

Polymorphism and inheritance of gliadin polypeptides in T. monococcum L

More than 80 different gliadin electrophoretic patterns (spectra) have been found in 109 accessions of the diploid wheat Triticum monococcum. Each pattern consists of 15-20 gliadin bands. Some patterns are clearly related and might arise from one another through single mutations in the gliadin-coding loci. From the analysis of 15 grains of each, only 61 accessions were found to be uniform; others consisted of two or more grain variants differing in their gliadin spectrum. An analysis of F2 grains from three crosses between different accessions showed that groups (blocks) of components are jointly and codominantly inherited. Two independent major Gli loci were established. The close resemblance of the composition of some blocks of T. monococcum to some of those in polyploid wheats indicates that one locus in each T. monococcum genotype is located on chromosome 1A (Gli-A1) and the other on 6A (Gli-A2). However, the blocks of T. monococcum include more bands than corresponding (equivalent) blocks of polyploid wheats. Two out of 275 F2 grains of the cross k-14244 x k-20409 were found to have gliadin spectra which can be explained as a result of intralocus recombination. Also, a second gliadin-coding locus on chromosome 1A was found in the cross k-46140 x k-46753. This locus recombines with the main Gli-A1 locus with a frequency of about 22% and was clearly analogous to the additional Gli locus found earlier on chromosome 1A of certain polyploid wheats.

Reconstruction of the phylogeny of the genus Triticum from variation in repeated nucleotide sequences

The potential of variation in repeated nucleotide sequences as a tool for phylogenetic studies was examined by investigating the phylogeny of 13 diploid species of the genus Triticum L. sensu Bowden. Low intraspecific variation in repeated nucleotide sequence families in Triticum indicated that restriction fragment profiles of repeated nucleotide sequences in Southern blots are reliable and uniform characteristics of each species. Cloned repeated nucleotide sequences were hybridized with Southern blots of DNAs of the Triticum species and the outgroup, Lophopyrum elongatum (Host) Á. Löve. The presence or absence of bands in the Southern blot autoradiograms was considered to be a character for phylogenetic analysis. A most parsimonious tree was resolved with the PAUP version 3.0L computer package. The tree was consistent with cytotaxonomic and evolutionary data available on the species.

Cytological analysis on the distribution and origin of the alien chromosome pair conferring blue aleurone color in several European common wheat (Triticum aestivum L.) strains

Meiotic chromosome pairing and Giemsa C-banding analyses in crosses of several European blue-grained wheat strains with Chinese Spring double ditelosomic and other aneuploid lines showed that Triticum aestivum Blaukorn strains "Berlin," "Probstdorf," "Tschermak," and "Weihenstephan" are chromosome substitutions, in which the complete wheat chromosome 4A pair is replaced, whereas the strains "Brünn" and "Moskau" are 4B substitutions. The alien chromosome pair in all of these strains is an A genome chromosome (4A) from diploid Triticum monococcum or T. boeoticum not present in common tetraploid and hexaploid cultivated wheats. The Blaukorn strain Weihenstephan "W 70a86" possesses, in addition to a rye chromosome pair 5R compensating for the loss of part of chromosome 5D, a 4A/5DL translocation replacing chromosome pair 4B of wheat.

Chromosome structure of durum wheat

Meiotic pairing was analyzed at metaphase I in ABRR triticale × rye hybrids to identify the arm homoeology of chromosomes ofTriticum turgidum. Wheat chromosomes were identified using a C-banding technique. Results confirmed that the homoeologous relationships between chromosome arms of the A and B genomes inT. turgidum are the same as inT. aestivum, and that a double translocation involving4AL,5AL, and7BS is present inT. turgidum. It is proposed that a pericentric inversion involving a substantial portion of chromosome4A is present inT. turgidum andT. aestivum.

Heterochromatin differentiation and phylogenetic relationship of the A genomes in diploid and polyploid wheats

Heterochromatin differentiation, including band size, sites, and Giemsa staining intensity, was analyzed by the HKG (HCl-KOH-Giemsa) banding technique in the A genomes of 21 diploid (Triticum urartu, T. boeoticum and T. monococcum), 13 tetraploid (T. araraticum, T. timopheevi, T. dicoccoides and T. turgidum var. Dicoccon, Polonicum), and 7 cultivars of hexaploid (T. aestivum) wheats from different germplasm collections. Among wild and cultivated diploid taxa, heterochromatin was located mainly at centromeric regions, but the size and staining intensity were distinct and some accessions' genomes had interstitial and telomeric bands. Among wild and cultivated polyploid wheats, heterochromatin exhibited bifurcated differentiation. Heterochromatinization occurred in chromosomes 4A(t) and 7A(t) and in smaller amounts in 2A(t), 3A(t), 5A(t), and 6A(t) within the genomes of the tetraploid Timopheevi group (T. araraticum, and T. timopheevi) and vice versa within those of the Emmer group (T. dicoccoides and T. turgidum). Similar divergence patterns occurred among chromosome 4A(a) and 7A(a) of cultivars of hexaploid wheat (T. aestivum). These dynamic processes could be related to geographic distribution and to natural and artifical selection. Comparison of the A genomes of diploid wheats with those of polyploid wheats shows that the A genomes in existing diploid wheats could not be the direct donors of those in polyploid wheats, but that the extant taxa of diploids and polyploids probably have a common origin and share a common A-genomelike ancestor.

Elucidation of the B-genome donor to Triticum turgidum by unique- and repeated-sequence DNA hybridizations

In vitro DNA:DNA hybridizations and hydroxyapatite thermal-elution chromatography were employed to identify the diploid Triticum species ancestral to the B genome of T. turgidum. Unique and repeated sequences from the various Triticum species were separated by hybridization and thermal elution on hydroxyapatite. Unique- and repeated-sequence fractions of labeled T. turgidum var. durum DNA were hybridized to the corresponding fractions of unlabeled DNAs of T. searsii, T. speltoides, T. longissimum, T. sharonensis, and T. bicorne. Thermal stability profiles were constructed to evaluate base-sequence complementarity between T. turgidum var. durum and the diploid Triticum species. The heteroduplex thermal stabilities indicated that, of the five species examined, T. searsii was the most closely related to the B genome of T. turgidum var. durum. The thermal stability profiles further indicated that the repeated DNA fractions from the Triticum species are more similar than the unique-sequence fractions. This indicates that all of the Triticum species are very closely related and, in all probability, have diverged from a single progenitor species.

Additional evidence implicating Triticum searsii as the B-genome donor to wheat

In vitro DNA:DNA hybridizations and hydroxyapatite thermal-elution chromatography were employed to identify the diploid wheat species ancestral to the B genome of Triticum turgidum. 3H-T. turgidum DNA was hybridized to the unlabeled DNAs of T. urartu, T. speltoides, T. sharonensis, T. bicorne, T. longissimum, and T. searsii. 3H-Labeled DNAs of T. monococcum and a synthetic tetraploid AADD were hybridized with unlabeled DNAs of T. urartu and T. searsii to determine the relationship of the A genome of polyploid wheat and T. urartu. The heteroduplex thermal stabilities indicated that T. searsii was most closely related to the B genome of T. turgidum (AB) and that the genome of T. urartu and the A genome have a great deal of base-sequence homology. Thus, it appears that T. searsii is the B-genome donor to polyploid wheat or a major chromosome donor if the B genome is polyphyletic in origin.

Implication of Triticum searsii as the B-genome donor to wheat using DNA hybridizations

In vitro DNA:DNA hybridizations and hydroxyapatite thermal chromatography were employed to help identify the species ancestral to the B genome of the polyploid wheats. We hybridized 3H-Triticum aestivum DNA to the unlabeled DNAs of T. urartu, T. speltoides, T. sharonensis, T. bicorne, T. longissimum, and T. searsii, 3H-Labeled DNA of T. urartu was hybridized with the DNA of a synthetic tetraploid. AADD. The heteroduplex thermal stabilities indicated that T. searsii was most closely related to T. aestivum (ABD) and that the genome of T. urartu was more closely related to the A genome than the B genome. The degree of reassociation which may have occurred between the six diploid species and the D genome of T. aestivum was evaluated by hybridizing 3H-T. tauschii DNA with the DNAs of the diploids. The results indicated that T. urartu had the least sequence homology to T. tauschii, the D-genome donor lending additional support to the conclusion that T. urartu is related to the A genome. Thus, it is highly probable that T. searsii is the B-genome donor to the polyploid wheats or a major chromosome donor if the B genome is, in fact, polyphyletic in origin.

The identification of the nucleolus organiser chromosomes of diploid wheat

The two nucleolus organiser chromosomes of diploid wheat are identified as 1A and 5A by the combination of in situ hybridisation and cytological markers.

Diploidization and chromosomal pairing affinities in the tetraploid wheats and their putative amphiploid progenitor

The genomes of the diploid wheats Triticum boeoticum and T. urartu are closely related, giving 7II in the f1 hybrid (T(b)T(u)) and 8.4 (0-14) II + 2.5 (0-7) IV in the derived amphiploid (T(b)T(b)T(u)T(u)). The genomes of the tetraploid wheats are also closely related, giving up to 7II at the polyhaploid level (AB) in the absence of the gene Ph but 14II at the tetraploid level (AABB) in the normal presence of Ph. If the amphiploid is the progenitor of the tetraploids, one or the other homoeologue (T(b) or T(u)) in each of the 7 homoeologous groups (the 7 potential IV) must have differentiated with respect to pairing affinity in order to account for 14II in the tetraploid. Consequently, in tetraploid X amphiploid hybrids (T(b)T(u)AB) carrying the Ph gene from the tetraploid, the seven differentiated chromosomes (B) would be expected to give 7I while, on the basis of their observed chiasma frequency, T(b), T(u) and the less differentiated A would be expected to give 4.17I + 3.57II + 3.23III), assuming homoeologous pairing. The expected chromosomal configuration freqencies at MI (11.17I + 3.57II + 3.23III) closely fit the observed values (11.22I + 3.45II + 3.19III + 0.071IV) for such hybrids (X(2) = 0.0046; P>0.99). Thus diploidization of the boeoticum-urartu amphiploid clearly could account for the origin of the tetraploid wheats. Furthermore, T. aestivum X amphiploid hybrids (T(b)T(u)ABD) with and without Ph indicated that B as well as A chomosomes tended to pair with their presumed T(b)T(u) homologues in the absence of Ph. Other tests showed that the tetraploid wheats could not plausibly have originated from any postulated Triticum-Sitopsis (TTSS) parental combinations with or without such chromosomal differentiation.

C-banded wheat chromosomes in wheat and triticale

The C-banding patterns of wheat chromosomes in 7 hexaploid triticale and 7 wheat genotypes are described and compared. All 14 wheat chromosome pairs were individually identified in the triticales and a tetraploid wheat, and all the B and two A genome chromosome pairs in the hexaploid wheat genotypes. Little variation was found between genotypes in the distribution of C-bands but considerable variation was found in their size, total number and total length.

GENETIC REGULATION OF HETEROGENETIC CHROMOSOME PAIRING IN POLYPLOID SPECIES OF THE GENUS TRITICUM sensu lato

Polyploid species of Triticum sensu lato were crossed with Triticum aestivum L. em. Thell. cv. Chinese Spring monotelodisomics or ditelosomics that were monosomic for chromosome 5B. Progeny from these crosses were either euploid, nullisomic for 5B, monotelosomic for a given Chinese Spring chromosome, or nullisomic for 5B and monotelosomic simultaneously. The Chinese Spring telosome in the hybrids permitted the evaluation of autosyndesis of chromosomes of the tested species. In addition, several Chinese Spring eu- and aneuhaploids were produced. Genotypes of T. cylindricum Ces., T. juvenale Thell., T. triunciale (L.) Raspail, T. ovatum (L.) Raspail, T. columnare (Zhuk.) Morris et Sears, T. triaristatum (Willd.) Godr. et Gren., and T. rectum (Zhuk.) comb. nov. were all shown to have suppressive effects on heterogenetic pairing in hybrids lacking 5B or 3AS, whereas T. kotschyi (Boiss.) Bowden had no effect. It was concluded that diploid-like meiosis in these species is due to genetic regulation. A number of these genotypes promoted heterogenetic pairing in the presence of 5B. A model is presented to explain this dichotomous behavior of the tested genotypes. Monotelosomic-3AL haploids had a greater amount of pairing than did euhaploid Chinese Spring, which substantiated the presence of a pairing suppressor(s) on the 3AS arm. Evidence is presented that shows that T. juvenale does not have a genome homologous with the D genome of T. aestivum.

THE ANALYSIS OF MEIOSIS IN HYBRIDS. I. ANEUPLOID HYBRIDS

The use of telocentric chromosomes in the analysis of the genomic relationships of wheat Triticum aestivum L. em Thell. and its relatives is described and 20 examples are given. The connection between this method of analysis and other mathematical theories of chromosome pairing in hybrids is established. A demonstration of the validity of the assumption that all chromosome arms pair at an equal frequency in calculating expected meiotic analyses is presented. This establishes a practical basis for the development of further mathematical models for predicting meiotic chromosome behavior. Numerical values of the genomic relationships of wheat and its immediate relatives are given.

Assessment of genomic and species relationships in Triticum and Aegilops by PAGE and by differential staining of seed albumins and globulins

Endosperm protein components from common bread wheats (Triticum aestivum L.) and related species were extracted with aluminum lactate, pH 3.2, and examined by electrophoresis in the same buffer. Electrophoretic patterns of the albumins and globulins were compared to evaluate the possibility that a particular species might have contributed its genome to tetraploid or hexaploid wheat. Together with protein component mobilities, differential band staining with Coomassie Brilliant Blue R250 was employed to test the identity or non-identity of bands. Eight species and 63 accessions, representative of Triticum and Aegilops were tested. Considerable intraspecific variation was observed for patterns of diploid but not for tetraploid or hexaploid species. Patterns of some accessions of Triticum urartu agreed closely with major parts of the patterns of Triticum dicoccoides and T. aestivum. A fast-moving, green band was found in all accessions of T. urartu and of Triticum boeoticum, however, that was not found in those of T. dicoccoides or T. aestivum. This band was present in all accessions of Triticum araraticum and Triticum zhukovskyi. Patterns of Aegilops longissima, which has been suggested as the donor of the B genome, differed substantially from those of T. dicoccoides and T. aestivum. Finally, two marker proteins of intermediate mobility were also observed and may be used to discriminate between accessions of T. araraticum/T. zhukovskyi and those of T. dicoccoides/T. aestivum.