Homoeologous Wheat Chromosomes Research Articles

Transfer of fusarium head blight resistance from Thinopyrum elongatum to bread wheat cultivar Chinese Spring

The diploid form of tall wheatgrass, Thinopyrum elongatum (Host) D.R. Dewey (2n = 2x = 14, EE genome), has a high level of resistance to fusarium head blight. The symptoms did not spread beyond the inoculated florets following point inoculation. Using a series of E-genome chromosome additions in a bread wheat cultivar Chinese Spring (CS) background, the resistance was found to be localized to the long arm of chromosome 7E. The CS mutant ph1b was used to induce recombination between chromosome 7E, present in the 7E(7D) substitution and homoeologous wheat chromosomes. Multivalent chromosome associations were detected in the BC1 hybrids, confirming the effectiveness of the ph1b mutant. Genetic markers specific for chromosome 7E were used to estimate the size of the 7E introgression in the wheat genome. Using single sequence repeat (SSR) markers specific for homoeologous wheat chromosome 7, introgressions were detected on wheat chromosomes 7A, 7B, and 7D. Some of the introgression lines were resistant to fusarium head blight.

Overcoming polyploidy pitfalls: a user guide for effective SNP conversion into KASP markers in wheat

Key messageConversion of SNP chip assays into locus-specific KASP markers requires adapted strategies in polyploid species with high genome homeology. Procedures are exemplified by QTL-associated SNPs in hexaploid wheat.Kompetitive allele-specific PCR (KASP) markers are commonly used in marker-assisted commercial plant breeding due to their cost-effectiveness and throughput for high sample volumes. However, conversion of trait-linked SNP markers from array-based SNP detection technologies into KASP markers is particularly challenging in polyploid crop species, due to the presence of highly similar homeologous and paralogous genome sequences. We evaluated strategies and identified key requirements for successful conversion of Illumina Infinium assays from the wheat 90 K SNP array into robust locus-specific KASP markers. Numerous examples showed that commonly used software for semiautomated KASP primer design frequently fails to achieve locus-specificity of KASP assays in wheat. Instead, alignment of SNP probes with multiple reference genomes and Sanger sequencing of relevant genotypes, followed by visual KASP primer placement, was critical for locus-specificity. To identify KASP assays resulting in false calling of heterozygous individuals, validation of KASP assays using extended reference genotype sets including heterozygous genotypes is strongly advised for polyploid crop species. Applying this strategy, we developed highly reproducible, stable KASP assays that are predictive for root biomass QTL haplotypes from highly homoeologous wheat chromosome regions. Due to their locus-specificity, these assays predicted root biomass considerably better than the original trait-associated markers from the Illumina array.

Cytological and molecular characterization of Thinopyrum bessarabicum chromosomes and structural rearrangements introgressed in wheat

Thinopyrum bessarabicum is an important genetic resource for wheat improvement by chromosome engineering. However, the present low-resolution karyotype limits identification of its chromosomes. Oligonucleotide probes to identify tandem repeats provide an efficient way to produce high-resolution karyotypes in many species. In this study, putative tandem repeats were identified using unassembled sequence reads of Th. bessarabicum, and 306 repeat clusters were identified. Among them, 17 had conserved motifs that varied in size from 71 to 856 bp and occupied 0.01% to 1.30% of the genome. Thirty-nine oligonucleotides from 17 clusters were developed, and 21 from 8 clusters produced clear and stable signals in Th. bessarabicum chromosomes. Five tandem repeat clusters were distributed only at the telomeric or subtelomeric regions, and the BSCL242 probe produced signals only on chromosome 7JL. The other three were mainly in intercalary and centromeric regions with a few weak signals in telomeric regions. A new multiplex oligonucleotide probe (ONPM#7) containing 13 oligonucloetides distinguished all wheat and Th. bessarabicum chromosomes after one round of FISH. The high-resolution karyotype of Th. bessarabicum in corresponding with the seven homoeologous group chromosomes of wheat has been developed. Three spontaneous translocations and one isochromosome among Th. bessarabicum chromosomes introgressed into wheat thus have been characterized in combined with molecular marker analysis. The ONPM#7 probe and molecular markers provide powerful tools for engineering transfer of chromosomal segments from Th. bessarabicum to wheat.

Chromosome Rearrangements Caused by Double Monosomy in Wheat-Barley Group-7 Substitution Lines

Interspecific or introgressive hybridization is one of the driving forces in plant speciation, producing allopolyploids or diploids with rearranged genomes. The process of karyotype reshaping following homoploid interspecific hybridization has not been studied experimentally. Interspecific hybridization is widely used in plant breeding to increase genetic diversity and introgress new traits. Numerous introgression stocks were developed for hexaploid wheat Triticum aestivum L. (2n = 6x = 42, genome AABBDD). Double monosomic lines, containing one alien chromosome from the tertiary gene pool of wheat and one homoeologous wheat chromosome, represent a simplified model for studying chromosome rearrangements caused by interspecific hybridization. The pairing of a chromosome from the tertiary gene pool with a wheat homoeologue is restricted by the activity of the wheat Ph1 gene, thus, rearrangements caused by chromosome breakage followed by the fusion of the broken arms can be expected. We analyzed chromosome aberrations in 4 sets of lines that originated from double monosomics of barley (Hordeum vulgare L.) chromosome 7H and wheat group-7 chromosomes with dicentric or ring chromosomes. The dynamics of wheat-barley dicentric chromosomes during plant development was followed and an increased diversity of rearrangements was observed. Besides the targeted group-7 chromosomes, other wheat chromosomes were involved in rearrangements, as chromosomes broken in the centromeric region fused with other broken chromosomes. In some cells, multi-centric chromosomes were observed. The structure and dosage of the introgressed barley chromatin was changed. The transmission of the rearrangements to the progenies was analyzed. The observed aberrations emphasize the importance of cytogenetic screening in gene introgression projects.

A set of Triticum aestivum-Aegilops speltoides Robertsonian translocation lines.

Here we report the production of a set of wheat - Aegilops speltoides Robertsonian translocations covering all Ae. speltoides chromosome arms except the long arm of the homoeologous group 4 chromosome. Aegilops speltoides of the Poaceae family is the most probable donor of the B and G genomes of polyploid Triticum species and also an important source of resistance to diseases and pests of wheat. Previously, we reported the production of a complete set of T aestivum-Ae. speltoides chromosome addition lines and a set of disomic S(B/A)-genome chromosome substitution lines. The isolation of compensating Robertsonian translocations (RobTs) composed of alien chromosome arms translocated to homoeologous wheat chromosome arms is the important next step to exploit the genetic variation of a wild relative of wheat. Here, we report the development of molecular markers specific for the S-genome chromosomes and their use in the isolation of a set of 13 compensating wheat-Ae. speltoides RobTs covering the S genome of Ae. speltoides except for the long arm of chromosome 4S. Most of the RobTs were fully fertile and will facilitate mapping of genes to specific chromosome arms and also will accelerate the introgression of agronomically useful traits from Ae. speltoides into wheat by homologous recombination.

Molecular characterization of α-gliadin gene sequences in Indian wheat cultivars vis-à-vis celiac disease eliciting epitopes

α-Gliadin proteins of the wheat gluten form a multigene family encoded by genomic loci Gli-A2, Gli-B2 and Gli-D2 located on the homoeologous wheat chromosomes 6AS, 6BS, and 6DS, respectively which upon partial digestion elicits celiac disease (CD) in the genetically susceptible individuals. The present investigation was planned to study the variations in the amino acid sequence of the α-gliadin proteins and CD eliciting epitopes in the Indian wheat cultivars. Representative wheat varieties released and cultivated in India during the period 1905–2011 were selected for studying the α-gliadin genes by cloning and sequencing followed by in silico analysis of the gene sequences. A lot of variation for α-gliadin gene sequences especially in T cell stimulatory epitopes glia-α9, glia-α20, glia-α2 and glia-α was observed in different wheat varieties. Modern varieties released during 1971–2011 had higher proportion of intact T-cell stimulatory epitopes. The old wheat varieties released in the period 1905–1970 on the other hand had large proportion of variant epitopes. We identified three wheat varieties namely C591, C273 and K78 having only variant epitopes at Gli-D2 and Gli-B2 and both intact and variant epitopes at Gli-A2. Identification of lower proportion of T-cell stimulatory epitopes in these three varieties is the first step towards developing a wheat variety less immunogenic for celiac disease patients. The gene sequences of the selected varieties have been submitted at NCBI with accession numbers GenBank KJ410473–KJ410488.

Fast‐forward genetics by radiation hybrids to saturate the locus regulating nuclear–cytoplasmic compatibility in Triticum

SummaryThe nuclear‐encoded species cytoplasm specific (scs) genes control nuclear–cytoplasmic compatibility in wheat (genus Triticum). Alloplasmic cells, which have nucleus and cytoplasm derived from different species, produce vigorous and vital organisms only when the correct version of scs is present in their nucleus. In this study, bulks of in vivo radiation hybrids segregating for the scs phenotype have been genotyped by sequencing with over 1.9 million markers. The high marker saturation obtained for a critical region of chromosome 1D allowed identification of 3318 reads that mapped in close proximity of the scs. A novel in silico approach was deployed to extend these short reads to sequences of up to 70 Kb in length and identify candidate open reading frames (ORFs). Markers were developed to anchor the short contigs containing ORFs to a radiation hybrid map of 650 individuals with resolution of 288 Kb. The region containing the scs locus was narrowed to a single Bacterial Artificial Chromosome (BAC) contig of Aegilops tauschii. Its sequencing and assembly by nano‐mapping allowed rapid identification of a rhomboid gene as the only ORF existing within the refined scs locus. Resequencing of this gene from multiple germplasm sources identified a single nucleotide mutation, which gives rise to a functional amino acid change. Gene expression characterization revealed that an active copy of this rhomboid exists on all homoeologous chromosomes of wheat, and depending on the specific cytoplasm each copy is preferentially expressed. Therefore, a new methodology was applied to unique genetic stocks to rapidly identify a strong candidate gene for the control of nuclear–cytoplasmic compatibility in wheat.

Chromosome Engineering and Physical Mapping of the Thinopyrum ponticum Translocation in Wheat Carrying the Rust Resistance Gene Sr26

ABSTRACTStudies were initiated to induce homoeologous recombination between the 6Ae#1L chromosome segment from Thinopyrum ponticum (Podp.) Barkw. & D. R. Dewey carrying the stem rust resistance gene Sr26 and homoeologous wheat (Triticum aestivum L.) chromosomes. The research was undertaken in an attempt to restore the yielding potential of wheat lines carrying Sr26. DNA probes mapping to chromosome arm 6L of Triticeae species were screened for chromosome 6Ae#1‐specific markers. Thirty out of 32 probes showed useful polymorphisms. A total of 1406 seedlings in a ph1bph1b background were screened for the presence of homoeologous recombinants with 24 of the polymorphic probes, as well as the 6Ae#1 isozyme markers for aconitase and glutamate oxaloacetate transaminase, and stem rust resistance. Eleven genotypes were found to have reduced amounts of 6Ae#1 chromatin and seven of these were resistant to stem rust (+Sr26). Six lines were homoeologous recombinants involving wheat chromosome 6A and four lines involved crossovers with 6D. The physical location of Sr26 was determined to be on the extreme distal segment of the 6Ae#1 chromosome. Rust resistant lines carrying shortened 6Ae#1 segments with improved agronomic performance are now being utilized by wheat breeding programs in several countries.

Potential new sources of wheat curl mite resistance in wheat to prevent the spread of yield-reducing pathogens

The wheat curl mite (WCM), Aceria tosichella Keifer (Trombidiformes: Eriophyidae), is a major pest in cropping regions of the world and is recognised as the primary vector of several yield-reducing pathogens, primarily affecting wheat. Management of WCM is complicated due to several aspects of the mite's biology and ecology; however, commercially viable mite resistant wheat varieties may offer practical long-term management options. Unfortunately, mite populations have adapted to previously identified sources of resistance, highlighting the need for further sources of resistance and the value of stacking different resistances to give greater degrees and longevity of control. In this study we assessed the susceptibility of 42 wheat-derived genotypes to mite population growth using a new experimental method that overcomes methodological limitations of previous studies. Experimental wheat lines included a variety of wheat genotypes, related Triticeae species, wheat-alien chromosome amphiploids, and chromosome addition or substitution lines. From these we identify new promising sources of WCM resistance associated with Thinopyrum intermedium, Th. ponticum and Hordeum marinum chromosomes. More specifically we identify group 1J and 5J chromosomes of the L3 and L5 wheat-Th. intermedium addition lines as new sources of resistance that could be exploited to transfer resistance onto homoeologous wheat chromosomes. This study offers new methods for reliable in situ estimations of mite abundance on cereal plants, and new sources of WCM resistance that may assist management of WCM and associated viruses in wheat.

Genetic Rearrangements of Six Wheat–Agropyron cristatum 6P Addition Lines Revealed by Molecular Markers

Agropyron cristatum (L.) Gaertn. (2n = 4x = 28, PPPP) not only is cultivated as pasture fodder but also could provide many desirable genes for wheat improvement. It is critical to obtain common wheat–A. cristatum alien disomic addition lines to locate the desired genes on the P genome chromosomes. Comparative analysis of the homoeologous relationships between the P genome chromosome and wheat genome chromosomes is a key step in transferring different desirable genes into common wheat and producing the desired alien translocation line while compensating for the loss of wheat chromatin. In this study, six common wheat–A. cristatum disomic addition lines were produced and analyzed by phenotypic examination, genomic in situ hybridization (GISH), SSR markers from the ABD genomes and STS markers from the P genome. Comparative maps, six in total, were generated and demonstrated that all six addition lines belonged to homoeologous group 6. However, chromosome 6P had undergone obvious rearrangements in different addition lines compared with the wheat chromosome, indicating that to obtain a genetic compensating alien translocation line, one should recombine alien chromosomal regions with homoeologous wheat chromosomes. Indeed, these addition lines were classified into four types based on the comparative mapping: 6PI, 6PII, 6PIII, and 6PIV. The different types of chromosome 6P possessed different desirable genes. For example, the 6PI type, containing three addition lines, carried genes conferring high numbers of kernels per spike and resistance to powdery mildew, important traits for wheat improvement. These results may prove valuable for promoting the development of conventional chromosome engineering techniques toward molecular chromosome engineering.

Development of a genetic linkage map for Sharon goatgrass (Aegilops sharonensis) and mapping of a leaf rust resistance gene

Aegilops sharonensis (Sharon goatgrass), a diploid wheat relative, is known to be a rich source of disease resistance genes for wheat improvement. To facilitate the transfer of these genes into wheat, information on their chromosomal location is important. A genetic linkage map of Ae. sharonensis was constructed based on 179 F2 plants derived from a cross between accessions resistant (1644) and susceptible (1193) to wheat leaf rust. The linkage map was based on 389 markers (377 Diversity Arrays Technology (DArT) and 12 simple sequence repeat (SSR) loci) and was comprised of 10 linkage groups, ranging from 2.3 to 124.6 cM. The total genetic length of the map was 818.0 cM, with an average interval distance between markers of 3.63 cM. Based on the chromosomal location of 115 markers previously mapped in wheat, the four linkage groups of A, B, C, and E were assigned to Ae. sharonensis (S(sh)) and homoeologous wheat chromosomes 6, 1, 3, and 2. The single dominant gene (designated LrAeSh1644) conferring resistance to leaf rust race THBJ in accession 1644 was positioned on linkage group A (chromosome 6S(sh)) and was flanked by DArT markers wpt-9881 (at 1.9 cM distal from the gene) and wpt-6925 (4.5 cM proximal). This study clearly demonstrates the utility of DArT for genotyping uncharacterized species and tagging resistance genes where pertinent genomic information is lacking.

Next‐generation sequencing and syntenic integration of flow‐sorted arms of wheat chromosome 4A exposes the chromosome structure and gene content

Wheat is the third most important crop for human nutrition in the world. The availability of high-resolution genetic and physical maps and ultimately a complete genome sequence holds great promise for breeding improved varieties to cope with increasing food demand under the conditions of changing global climate. However, the large size of the bread wheat (Triticum aestivum) genome (approximately 17 Gb/1C) and the triplication of genic sequence resulting from its hexaploid status have impeded genome sequencing of this important crop species. Here we describe the use of mitotic chromosome flow sorting to separately purify and then shotgun-sequence a pair of telocentric chromosomes that together form chromosome 4A (856 Mb/1C) of wheat. The isolation of this much reduced template and the consequent avoidance of the problem of sequence duplication, in conjunction with synteny-based comparisons with other grass genomes, have facilitated construction of an ordered gene map of chromosome 4A, embracing ≥85% of its total gene content, and have enabled precise localization of the various translocation and inversion breakpoints on chromosome 4A that differentiate it from its progenitor chromosome in the A genome diploid donor. The gene map of chromosome 4A, together with the emerging sequences of homoeologous wheat chromosome groups 4, 5 and 7, represent unique resources that will allow us to obtain new insights into the evolutionary dynamics between homoeologous chromosomes and syntenic chromosomal regions.

Development of a set of compensating Triticum aestivum – Dasypyrum villosum Robertsonian translocation lines

Dasypyrum villosum (L.) Candargy, a wild relative of bread wheat ( Triticum aestivum L.), is the source of many agronomically important genes for wheat improvement. Production of compensating Robertsonian translocations (cRobTs), consisting of D. villosum chromosome arms translocated to homoeologous wheat chromosome arms, is one of the initial steps in exploiting this variation. The cRobTs for D. villosum chromosomes 1V, 4V, and 6V have been reported previously. Here we report attempted cRobTs for wheat - D. villosum chromosome combinations 2D/2V, 3D/3V, 5D/5V, and 7D/7V. The cRobTs for all D. villosum chromosomes were recovered except for the 2VS and 5VL arms. As was the case with the 6D/6V combination, no cRobTs involving 2D/2V chromosomes were recovered; instead, cRobT T2BS·2VL involving a nontargeted chromosome was recovered. All cRobTs are fertile, although the level of spike fertility and hundred kernel weight (HKW) varied among the lines. The set of cRobTs involving 12 of the 14 D. villosum chromosomes will be useful in wheat improvement programs. In fact, among the already reported cRobTs, T6AL·6VS carrying the Pm21 gene is deployed in agriculture and many useful genes have been reported on other cRobTs including resistance to stem rust race UG99 on T6AS·6VL.

Tetraploid and hexaploid wheat varieties reveal large differences in expression of alpha-gliadins from homoeologous Gli-2 loci.

BackgroundΑ-gliadins form a multigene protein family encoded by multiple α-gliadin (Gli-2) genes at three genomic loci, Gli-A2, Gli-B2 and Gli-D2, respectively located on the homoeologous wheat chromosomes 6AS, 6BS, and 6DS. These proteins contain a number of important celiac disease (CD)-immunogenic domains. The α-gliadins expressed from the Gli-B2 locus harbour fewer conserved CD-epitopes than those from Gli-A2, whereas the Gli-D2 gliadins have the highest CD-immunogenic potential. In order to detect differences in the highly CD-immunogenic α-gliadin fraction we determined the relative expression level from the homoeologous Gli-2 loci in various tetraploid and hexaploid wheat genotypes by using a quantitative pyrosequencing method and by analyzing expressed sequence tag (EST) sequences.ResultsWe detected large differences in relative expression levels of α-gliadin genes from the three homoeologous loci among wheat genotypes, both as relative numbers of expressed sequence tag (EST) sequences from specific varieties and when using a quantitative pyrosequencing assay specific for Gli-A2 genes. The relative Gli-A2 expression level in a tetraploid durum wheat cultivar ('Probstdorfer Pandur') was 41%. In genotypes derived from landraces, the Gli-A2 frequency varied between 12% and 58%. In some advanced hexaploid bread wheat cultivars the genes from locus Gli-B2 were hardly expressed (e.g., less than 5% in 'Lavett') but in others they made up more than 40% (e.g., in 'Baldus').ConclusionHere, we have shown that large differences exist in relative expression levels of α-gliadins from the homoeologous Gli-2 loci among wheat genotypes. Since the homoelogous genes differ in the amount of conserved CD-epitopes, screening for differential expression from the homoeologous Gli-2 loci can be employed for the pre-selection of wheat varieties in the search for varieties with very low CD-immunogenic potential. Pyrosequencing is a method that can be employed for such a 'gene family-specific quantitative transcriptome profiling'.

Molecular genetic linkage maps for allotetraploid Leymus wildryes (Gramineae: Triticeae).

Molecular genetic maps were constructed for two full-sib populations, TTC1 and TTC2, derived from two Leymus triticoides x Leymus cinereus hybrids and one common Leymus triticoides tester. Informative DNA markers were detected using 21 EcoRI-MseI and 17 PstI-MseI AFLP primer combinations, 36 anchored SSR or STS primer pairs, and 9 anchored RFLP probes. The 164-sib TTC1 map includes 1069 AFLP markers and 38 anchor loci in 14 linkage groups spanning 2001 cM. The 170-sib TTC2 map contains 1002 AFLP markers and 36 anchor loci in 14 linkage groups spanning 2066 cM. Some 488 homologous AFLP loci and 24 anchor markers detected in both populations showed similar map order. Thus, 1583 AFLP markers and 50 anchor loci were mapped into 14 linkage groups, which evidently correspond to the 14 chromosomes of allotetraploid Leymus (2n = 4x = 28). Synteny of two or more anchor markers from each of the seven homoeologous wheat and barley chromosomes was detected for 12 of the 14 Leymus linkage groups. Moreover, two distinct sets of genome-specific STS markers were identified in these allotetraploid Leymus species. These Leymus genetic maps and populations will provide a useful system to evaluate the inheritance of functionally important traits of two divergent perennial grass species.

Compensating ability in pollen fertilization between group-6 and -7 homoeologous chromosomes of barley and wheat.

By using alpha-amylase isozymes as markers for chromosomes of homoeologous groups 6 and 7, we analyzed the segregation of chromosome constitution in the progenies from crosses between double-ditelosomic or ditelosomic lines of hexaploid wheat cultivar 'Chinese Spring' (CS) as the female parent and double-monosomic F1 hybrids of CS x wheat-barley substitution lines for barley chromosomes 6H or 7H. From this analysis we estimated the transmission rate via pollen of barley chromosomes 6H and 7H in the double-monosomics and evaluated the compensating ability between barley and wheat chromosomes in homoeologous groups 6 and 7. The results indicated that both 6H and 7H showed their highest compensating ability for their respective homoeologous wheat chromosomes 6A (37.5% transmission rate) and 7A (39.4%), intermediate for 6D (34.1%) and 7D (29.6%), and lowest for 6B (26.6%) and 7B (22.6%) chromosomes.

Cytogenetic analysis of hybrids resistant to yellow rust and powdery mildew obtained by crossing common wheat (Triticum aestivum L., AABBDD) with wheat of the Timopheevi group (AtAtGG)

The karyotypes of 47 hybrid lines obtained from crosses of common wheat Triticum aestivum L. (cv. Rodina and line 353) with Triticum timopheevii Zhuk, (AtAtGG) and related species T. militinae Zhuk. et Migusch. (AtAtGG) and T. kiharae Dorof. et Migusch. (AtAtGGDsqDsq) were analyzed by C-banding. Most lines were resistant to yellow rust and powdery mildew. The introgression of alien genetic material to the common wheat genome was realized via substitutions of complete At-, G-, and D-genome chromosomes, chromosome arms, or their fragments. The pattern of chromosome substitutions in resistant lines differed from that in introgressive hybrids selected for other traits. Substitutions of chromosomes 6G, 2At, 2G, and 5G were revealed in 31, 23, 18, and 13 lines, respectively. Substitutions of chromosomes 4G-, 4At, and 6At were not observed. In 15 lines, a 5BS. 5BL-5GL translocation was identified. High frequency of substitutions of chromosomes 2At, 2G, 5G, and 6G indicate that they may carry the resistance genes and that they are closely related to the respective homoeologous chromosomes of common wheat that determines their high compensation ability.

Structural rearrangement in chromosome 2M of Aegilops comosa has prevented the utilization of the Compair and related wheat-Ae. comosa translocations in wheat improvement

The genetic constitutions of chromosome 2M of Aegilops comosa and the derived wheat-Ae. comosa translocations were analyzed by molecular cytogenetic techniques. Hybridization of 15 RFLP markers covering the entire length of the group-2 chromosomes revealed that chromosome 2M was structurally rearranged compared to the homoeologous chromosomes of wheat by either a pericentric inversion or a terminal intrachromosomal translocation. The breakpoint of the rearrangement was located in a region between the loci Xpsr131 and Xcdo405, resulting in the translocation of 47% of 2MS to 2ML. This aberrant structure of 2M allowed homoeologous recombination between 2M and its wheat counterpart only in the translocated segment on 2ML. C-banding and genomic in situ hybridization analyses confirmed that all translocation chromosomes consisted of the complete 2MS arm, a large part of 2ML, and very small distal segments derived from 2AS or 2DS, as expected from the aberrant structure of chromosome 2M. Thus, the translocation in the line 2A-2M?4/2 can be described as T2AS-2M?1L ⋅ 2M?1S and the translocations in the lines Compair and 2D-2M?3/8 as T2DS-2M?1L ⋅ 2M?1S. RFLP analysis determined the breakpoints in these translocation chromosomes to be within the telomeric 16% of the wheat chromosome arms. The breakpoint of the 2A/2M translocation was between Xbcd348 and Xcdo783, and that of the 2D/2M translocation was between Xcdo783 and Xpsr666. Because the translocation chromosomes retain the structural aberration found in chromosome 2M, further exploitation of the wheat-Ae. comosa translocations for cultivar improvement is questionable.

Variation of the crossability of durum wheat with maize

With the aim of examining crossability of durum wheat with maize, two sets of durum wheat genotypes and a set of D-genome chromosome substitution lines of the durum wheat variety ‘Langdon’ were crossed with maize, and followed by 2,4-dichlorophenoxyacetic acid (2,4-D) treatment in detached-tiller culture. In crosses of 25 durum wheat genotypes (breeding lines) with maize, percent frequencies of embryo formation increased from 1.4% to 2.8% by adding silver nitrate to the detached-tiller culture solution. In crosses of 32 durum wheat genotypes (advanced lines and varieties) with maize using the silver nitrate addition, frequencies of embryo formation ranged from 0.0% to 15.8%; seven genotypes showing more than 6.0% embryo formation frequency were related in their pedigrees. In crosses of a set of chromosome substitution lines with maize, higher frequencies of embryo formation were obtained in substitution lines with chromosomes 1D, 3D, 4D and 7D. These results suggest that 1) adding silver nitrate to the 2,4-D treatment increases overall frequency of embryo formation but is not effective enough to induce the development of seeds and embryos from all durum wheat genotypes, and 2) some D-genome chromosomes substituted in a durum wheat genetic background may enhance crossability with maize in combination with homoeologous chromosomes of durum wheat.

A cytogenetic approach to the improvement of aluminium tolerance in wheat

summaryHigh levels of soil aluminium place serious constraints on wheat production on acidic soils, especially in the tropical areas of Africa and South America. Conventional plant breeding has improved the tolerance of the wheat crop, but available genetic variation is limited. The wild relatives of wheat provide a valuable gene pool for the introduction of further genetic variation. One wild species, Aegilops uniaristata Vis. (2n = 2x = 14,NN), is being utilized as a new source of tolerance. Of the addition lines of individual N genome chromosomes of A. uniaristata to wheat (Triticum aestivum L.) which have been established and characterized, chromosome 3N has been shown to confer tolerance to wheat. The three substitution lines in which 3N replaces the homoeologous wheat chromosomes, 3A, 3B or 3D, have also been produced. Growing plants to maturity in a low pH/high Al hydroponics system confirmed that chromosome 3N conferred tolerance to the substitution lines as well as to the addition line. By manipulating the genetic control of homoeologous chromosome pairing, chromosome 3N is being recombined with its wheat homoeologues in order to introduce a smaller alien segment which carries the gene(s) for tolerance but not the agronomically unacceptable brittle rachis gene also carried on chromosome 3N.