Identification of variation in adaptively important traits and genome-wide analysis of trait-marker associations in Triticum monococcum

Integration of genetic and genomics resources in einkorn wheat enables precision mapping of important traits

Einkorn wheat (Triticum monococcum) is an ancient grain crop and a close relative of the diploid progenitor (T. urartu) of polyploid wheat. It is the only diploid wheat species having both domesticated and wild forms and therefore provides an excellent system to identify domestication genes and genes for traits of interest to utilize in wheat improvement. Here, we leverage genomic advancements for einkorn wheat using an einkorn reference genome assembly combined with skim-sequencing of a large genetic population of 812 recombinant inbred lines (RILs) developed from a cross between a wild and a domesticated T. monococcum accession. We identify 15,919 crossover breakpoints delimited to a median and average interval of 114 Kbp and 219 Kbp, respectively. This high-resolution mapping resource enables us to perform fine-scale mapping of one qualitative (red coleoptile) and one quantitative (spikelet number per spike) trait, resulting in the identification of small physical intervals (400 Kb to 700 Kb) with a limited number of candidate genes. Furthermore, an important domestication locus for brittle rachis is also identified on chromosome 7A. This resource presents an exciting route to perform trait discovery in diploid wheat for agronomically important traits and their further deployment in einkorn as well as tetraploid pasta wheat and hexaploid bread wheat cultivars.

GWAS for main effects and epistatic interactions for grain morphology traits in wheat

In the present study in wheat, GWAS was conducted for identification of marker trait associations (MTAs) for the following six grain morphology traits: (1) grain cross-sectional area (GCSA), (2) grain perimeter (GP), (3) grain length (GL), (4) grain width (GWid), (5) grain length-width ratio (GLWR) and (6) grain form-density (GFD). The data were recorded on a subset of spring wheat reference set (SWRS) comprising 225 diverse genotypes, which were genotyped using 10,904 SNPs and phenotyped for two consecutive years (2017-2018, 2018-2019). GWAS was conducted using five different models including two single-locus models (CMLM, SUPER), one multi-locus model (FarmCPU), one multi-trait model (mvLMM) and a model for Q x Q epistatic interactions. False discovery rate (FDR) [P value -log10(p) ≥ 5] and Bonferroni correction [P value -log10(p) ≥ 6] (corrected p value < 0.05) were applied to eliminate false positives due to multiple testing. This exercise gave 88 main effect and 29 epistatic MTAs after FDR and 13 main effect and 6 epistatic MTAs after Bonferroni corrections. MTAs obtained after Bonferroni corrections were further utilized for identification of 55 candidate genes (CGs). In silico expression analysis of CGs in different tissues at different parts of the seed at different developmental stages was also carried out. MTAs and CGs identified during the present study are useful addition to available resources for MAS to supplement wheat breeding programmes after due validation and also for future strategic basic research.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-022-01164-w.

Exploring the diversity of promoter and 5'UTR sequences in ancestral, historic and modern wheat

A data set of promoter and 5'UTR sequences of homoeo-alleles of 459 wheat genes that contribute to agriculturally important traits in 95 ancestral and commercial wheat cultivars is presented here. The high-stringency myBaits technology used made individual capture of homoeo-allele promoters possible, which is reported here for the first time. Promoters of most genes are remarkably conserved across the 83 hexaploid cultivars used with <7 haplotypes per promoter and 21% being identical to the reference Chinese Spring. InDels and many high-confidence SNPs are located within predicted plant transcription factor binding sites, potentially changing gene expression. Most haplotypes found in the Watkins landraces and a few haplotypes found in Triticum monococcum, germplasms hitherto not thought to have been used in modern wheat breeding, are already found in many commercial hexaploid wheats. The full data set which is useful for genomic and gene function studies and wheat breeding is available at https://rrescloud.rothamsted.ac.uk/index.php/s/DMCFDu5iAGTl50u/authenticate.

Genome-wide association study for morphological, phenological, quality, and yield traits in einkorn (Triticum monococcum L. subsp. monococcum)

Einkorn (Triticum monococcum L. subsp. monococcum, 2n = 2× = 14, AmAm) is a diploid wheat whose cultivation was widespread in the Mediterranean and European area till the Bronze Age, before it was replaced by the more productive durum and bread wheats. Although scarcely cultivated nowadays, it has gained renewed interest due to its relevant nutritional properties and as source of genetic diversity for crop breeding. However, the molecular basis of many traits of interest in einkorn remain still unknown. A panel of 160 einkorn landraces, from different parts of the distribution area, was characterized for several phenotypic traits related to morphology, phenology, quality, and yield for 4 years in two locations. An approach based on co-linearity with the A genome of bread wheat, supported also by that with Triticum urartu genome, was exploited to perform association mapping, even without an einkorn anchored genome. The association mapping approach uncovered numerous marker-trait associations; for 37 of these, a physical position was inferred by homology with the bread wheat genome. Moreover, numerous associated regions were also assigned to the available T. monococcum contigs. Among the intervals detected in this work, three overlapped with regions previously described as involved in the same trait, while four other regions were localized in proximity of loci previously described and presumably refer to the same gene/QTL. The remaining associated regions identified in this work could represent a novel and useful starting point for breeding approaches to improve the investigated traits in this neglected species.

Unraveling the genetic architecture of grain size in einkorn wheat through linkage and homology mapping and transcriptomic profiling

Understanding the genetic architecture of grain size is a prerequisite to manipulating grain development and improving the potential crop yield. In this study, we conducted a whole genome-wide quantitative trait locus (QTL) mapping of grain-size-related traits by constructing a high-density genetic map using 109 recombinant inbred lines of einkorn wheat. We explored the candidate genes underlying QTLs through homologous analysis and RNA sequencing. The high-density genetic map spanned 1873 cM and contained 9937 single nucleotide polymorphism markers assigned to 1551 bins on seven chromosomes. Strong collinearity and high genome coverage of this map were revealed by comparison with physical maps of wheat and barley. Six grain size-related traits were surveyed in five environments. In total, 42 QTLs were identified; these were assigned to 17 genomic regions on six chromosomes and accounted for 52.3-66.7% of the phenotypic variation. Thirty homologous genes involved in grain development were located in 12 regions. RNA sequencing identified 4959 genes differentially expressed between the two parental lines. Twenty differentially expressed genes involved in grain size development and starch biosynthesis were mapped to nine regions that contained 26 QTLs, indicating that the starch biosynthesis pathway plays a vital role in grain development in einkorn wheat. This study provides new insights into the genetic architecture of grain size in einkorn wheat; identification of the underlying genes enables understanding of grain development and wheat genetic improvement. Furthermore, the map facilitates quantitative trait mapping, map-based cloning, genome assembly, and comparative genomics in wheat taxa.

Unraveling the genetic architecture of grain size in einkorn wheat through linkage and homology mapping and transcriptomic profiling

Understanding the genetic architecture of grain size is a prerequisite to manipulating grain development and improving the potential crop yield. In this study, we conducted a whole genome-wide quantitative trait locus (QTL) mapping of grain-size-related traits by constructing a high-density genetic map using 109 recombinant inbred lines of einkorn wheat. We explored the candidate genes underlying QTLs through homologous analysis and RNA sequencing. The high-density genetic map spanned 1873 cM and contained 9937 single nucleotide polymorphism markers assigned to 1551 bins on seven chromosomes. Strong collinearity and high genome coverage of this map were revealed by comparison with physical maps of wheat and barley. Six grain size-related traits were surveyed in five environments. In total, 42 QTLs were identified; these were assigned to 17 genomic regions on six chromosomes and accounted for 52.3-66.7% of the phenotypic variation. Thirty homologous genes involved in grain development were located in 12 regions. RNA sequencing identified 4959 genes differentially expressed between the two parental lines. Twenty differentially expressed genes involved in grain size development and starch biosynthesis were mapped to nine regions that contained 26 QTLs, indicating that the starch biosynthesis pathway plays a vital role in grain development in einkorn wheat. This study provides new insights into the genetic architecture of grain size in einkorn wheat; identification of the underlying genes enables understanding of grain development and wheat genetic improvement. Furthermore, the map facilitates quantitative trait mapping, map-based cloning, genome assembly, and comparative genomics in wheat taxa.

Genome-wide polymorphisms from RNA sequencing assembly of leaf transcripts facilitate phylogenetic analysis and molecular marker development in wild einkorn wheat

A survey of genome-wide polymorphisms between closely related species is required to understand the molecular basis of the evolutionary differentiation of their genomes. Two wild diploid wheat species, namely Triticum monococcum ssp. aegilopoides and T. urartu, are closely related and harbour the A^m and A genomes, respectively. The A-genome donor of tetraploid and common wheat is T. urartu, and T. monococcum ssp. monococcum is the cultivated form derived from the wild einkorn wheat subspecies aegilopoides. Although subspecies aegilopoides has been a useful genetic resource in wheat breeding, genome-wide molecular markers for this subspecies have not been sufficiently developed. Here, we describe the detection of genome-wide polymorphisms such as single-nucleotide polymorphisms (SNPs) and insertions/deletions (indels) from RNA sequencing (RNA-seq) data of leaf transcripts in 15 accessions of the two diploid wheat species. The SNPs and indels, detected using the A genome of common wheat as the reference genome, covered the entire chromosomes of these species. The polymorphism information facilitated a comparison of the genetic diversity of einkorn wheat with that of two related diploid Aegilops species, namely, Ae. tauschii and Ae. umbellulata. Cleaved amplified polymorphic sequence (CAPS) markers converted from the SNP data were efficiently developed to confirm the addition of aegilopoides subspecies chromosomes to tetraploid wheat in nascent allohexaploid lines with AABBA^mA^m genomes. In addition, the CAPS markers permitted linkage map construction in mapping populations of aegilopoides subspecies accessions. Therefore, these RNA-seq data provide information for further breeding of closely related species with no reference genome sequence data.

Cytomolecular discrimination of the Am chromosomes of Triticum monococcum and the A chromosomes of Triticum aestivum using microsatellite DNA repeats

The cytomolecular discrimination of the A^m- and A-genome chromosomes facilitates the selection of wheat-Triticum monococcum introgression lines. Fluorescence in situ hybridisation (FISH) with the commonly used DNA probes Afa family, 18S rDNA and pSc119.2 showed that the more complex hybridisation pattern obtained in T. monococcum relative to bread wheat made it possible to differentiate the A^m and A chromosomes within homoeologous groups 1, 4 and 5. In order to provide additional chromosomal landmarks to discriminate the A^m and A chromosomes, the microsatellite repeats (GAA)_n, (CAG)_n, (CAC)_n, (AAC)_n, (AGG)_n and (ACT)_n were tested as FISH probes. These showed that T. monococcum chromosomes have fewer, generally weaker, simple sequence repeat (SSR) signals than the A-genome chromosomes of hexaploid wheat. A differential hybridisation pattern was observed on 6A^m and 6A chromosomes with all the SSR probes tested except for the (ACT)_n probe. The 2A^m and 2A chromosomes were differentiated by the signals given by the (GAA)_n, (CAG)_n and (AAC)_n repeats, while only (GAA)_n discriminated the chromosomes 3A^m and 3A. Chromosomes 7A^m and 7A could be differentiated by the lack of (GAA)_n and (AGG)_n signals on 7A. As potential landmarks for identifying the A^m chromosomes, SSR repeats will facilitate the introgression of T. monococcum chromatin into wheat.

Cytomolecular discrimination of the Am chromosomes of Triticum monococcum and the A chromosomes of Triticum aestivum using microsatellite DNA repeats

The cytomolecular discrimination of the A^m- and A-genome chromosomes facilitates the selection of wheat-Triticum monococcum introgression lines. Fluorescence in situ hybridisation (FISH) with the commonly used DNA probes Afa family, 18S rDNA and pSc119.2 showed that the more complex hybridisation pattern obtained in T. monococcum relative to bread wheat made it possible to differentiate the A^m and A chromosomes within homoeologous groups 1, 4 and 5. In order to provide additional chromosomal landmarks to discriminate the A^m and A chromosomes, the microsatellite repeats (GAA)_n, (CAG)_n, (CAC)_n, (AAC)_n, (AGG)_n and (ACT)_n were tested as FISH probes. These showed that T. monococcum chromosomes have fewer, generally weaker, simple sequence repeat (SSR) signals than the A-genome chromosomes of hexaploid wheat. A differential hybridisation pattern was observed on 6A^m and 6A chromosomes with all the SSR probes tested except for the (ACT)_n probe. The 2A^m and 2A chromosomes were differentiated by the signals given by the (GAA)_n, (CAG)_n and (AAC)_n repeats, while only (GAA)_n discriminated the chromosomes 3A^m and 3A. Chromosomes 7A^m and 7A could be differentiated by the lack of (GAA)_n and (AGG)_n signals on 7A. As potential landmarks for identifying the A^m chromosomes, SSR repeats will facilitate the introgression of T. monococcum chromatin into wheat.

Triticum monococcum lines with distinct metabolic phenotypes and phloem-based partial resistance to the bird cherry-oat aphid Rhopalosiphum padi

Crop protection is an integral part of establishing food security, by protecting the yield potential of crops. Cereal aphids cause yield losses by direct damage and transmission of viruses. Some wild relatives of wheat show resistance to aphids but the mechanisms remain unresolved. In order to elucidate the location of the partial resistance to the bird cherry-oat aphid, Rhopalosiphum padi, in diploid wheat lines of Triticum monococcum, we conducted aphid performance studies using developmental bioassays and electrical penetration graphs, as well as metabolic profiling of partially resistant and susceptible lines. This demonstrated that the partial resistance is related to a delayed effect on the reproduction and development of R. padi. The observed partial resistance is phloem based and is shown by an increase in number of probes before the first phloem ingestion, a higher number and duration of salivation events without subsequent phloem feeding and a shorter time spent phloem feeding on plants with reduced susceptibility. Clear metabolic phenotypes separate partially resistant and susceptible lines, with the former having lower levels of the majority of primary metabolites, including total carbohydrates. A number of compounds were identified as being at different levels in the susceptible and partially resistant lines, with asparagine, octopamine and glycine betaine elevated in less susceptible lines without aphid infestation. In addition, two of those, asparagine and octopamine, as well as threonine, glutamine, succinate, trehalose, glycerol, guanosine and choline increased in response to infestation, accumulating in plant tissue localised close to aphid feeding after 24 h. There was no clear evidence of systemic plant response to aphid infestation.

Identifying variation in resistance to the take-all fungus, Gaeumannomyces graminis var. tritici, between different ancestral and modern wheat species

BACKGROUND

Ancestral wheat relatives are important sources of genetic diversity for the introduction of novel traits for the improvement of modern bread wheat. In this study the aim was to assess the susceptibility of 34 accessions of the diploid wheat Triticum monococcum (A genome) to Gaeumannomyces graminis var. tritici (Ggt), the causal agent of take-all disease. The second aim was to explore the susceptibility of tetraploid wheat (T. durum) and the B genome progenitor species Aegilops speltoides to Ggt.

RESULTS

Field trials, conducted over 5 years, identified seven T. monococcum accessions with a good level of resistance to take-all when exposed to natural inoculum under UK field conditions. All other accessions were highly susceptible or did not exhibit a consistent phenotype across years. DArT marker genotyping revealed that whole genome diversity was not closely related to resistance to take-all within T. monococcum, suggesting that multiple genetic sources of resistance may exist within the species. In contrast the tetraploid wheat cultivars and Ae. speltoides were all highly susceptible to the disease, including those with known elevated levels of benzoxazinoids.

CONCLUSIONS

The diploid wheat species T. monococcum may provide a genetic source of resistance to take-all disease that could be utilised to improve the performance of T. aestivum in high disease risk situations. This represents an extremely valuable resource to achieve economic and sustainable genetic control of this root disease.

De novo transcriptome assembly and analyses of gene expression during photomorphogenesis in diploid wheat Triticum monococcum

BACKGROUND

Triticum monococcum (2n) is a close ancestor of T. urartu, the A-genome progenitor of cultivated hexaploid wheat, and is therefore a useful model for the study of components regulating photomorphogenesis in diploid wheat. In order to develop genetic and genomic resources for such a study, we constructed genome-wide transcriptomes of two Triticum monococcum subspecies, the wild winter wheat T. monococcum ssp. aegilopoides (accession G3116) and the domesticated spring wheat T. monococcum ssp. monococcum (accession DV92) by generating de novo assemblies of RNA-Seq data derived from both etiolated and green seedlings.

PRINCIPAL FINDINGS

The de novo transcriptome assemblies of DV92 and G3116 represent 120,911 and 117,969 transcripts, respectively. We successfully mapped ∼90% of these transcripts from each accession to barley and ∼95% of the transcripts to T. urartu genomes. However, only ∼77% transcripts mapped to the annotated barley genes and ∼85% transcripts mapped to the annotated T. urartu genes. Differential gene expression analyses revealed 22% more light up-regulated and 35% more light down-regulated transcripts in the G3116 transcriptome compared to DV92. The DV92 and G3116 mRNA sequence reads aligned against the reference barley genome led to the identification of ∼500,000 single nucleotide polymorphism (SNP) and ∼22,000 simple sequence repeat (SSR) sites.

CONCLUSIONS

De novo transcriptome assemblies of two accessions of the diploid wheat T. monococcum provide new empirical transcriptome references for improving Triticeae genome annotations, and insights into transcriptional programming during photomorphogenesis. The SNP and SSR sites identified in our analysis provide additional resources for the development of molecular markers.

Molecular, physicochemical and rheological characteristics of introgressive Triticale/Triticum monococcum ssp. monococcum lines with wheat 1D/1A chromosome substitution

Three sets of hexaploid introgressive triticale lines, with Triticum monococcum ssp. monococcum (cultivated einkorn wheat) genes and a bread wheat chromosome 1D substituted for chromosome 1A, and one set of secondary triticale lines were evaluated for grain and flour physicochemical and dough rheological characteristics in two generations (F7 and F8). Genomic in situ hybridization (GISH) and fluorescence in situ hybridization (FISH) confirmed the 1D/1A chromosome substitution. The presence or absence of einkorn high-molecular-weight (HMW) glutenin subunits and the wheat Glu-D1d locus encoding the 5 + 10 subunits was assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), capillary zone electrophoresis, and allele-specific molecular markers. Significant differences were found among physicochemical properties (with the exception of the Hagberg falling number) of all introgressive Triticale/T. monococcum lines and the secondary triticale lines. The wheat 1D/1A chromosome substitution also affected these properties. The results showed that in all introgressive triticale lines, the protein and gluten content, Zeleny sedimentation value, and water absorption capacity, were increased. The rheological parameters estimated using micro-farinograph, reomixer, and Kieffer dough extensibility systems also showed an appreciable increase in dough-mixing properties, maximum resistance to extension (Rmax), and dough extensibility. Introgressive Triticale/T. monococcum lines with 5 + 10 subunits have particularly favorable rheological parameters. The results obtained in this study suggest that the cultivated einkorn genome Am, in the context of hexaploid secondary triticale lines and with a wheat 1D/1A substitution, has the potential to improve gluten polymer interactions and be a valuable genetic resource for triticale quality improvement.

Identification of Ug99 stem rust resistance loci in winter wheat germplasm using genome-wide association analysis

The evolution of a new race of stem rust, generally referred to as Ug99, threatens global wheat production because it can overcome widely deployed resistance genes that had been effective for many years. To identify loci conferring resistance to Ug99 in wheat, a genome-wide association study was conducted using 232 winter wheat breeding lines from the International Winter Wheat Improvement Program. Breeding lines were genotyped with diversity array technology, simple sequence repeat and sequence-tagged site markers, and phenotyped at the adult plant stage for resistance to stem rust in the stem rust resistance screening nursery at Njoro, Kenya during 2009-2011. A mixed linear model was used for detecting marker-trait associations. Twelve loci associated with Ug99 resistance were identified including markers linked to known genes Sr2 and Lr34. Other markers were located in the chromosome regions where no Sr genes have been previously reported, including one each on chromosomes 1A, 2B, 4A and 7B, two on chromosome 5B and four on chromosome 6B. The same data were used for investigating epistatic interactions between markers with or without main effects. The marker csSr2 linked to Sr2 interacted with wPt4930 on 6BS and wPt729773 in an unknown location. Another marker, csLV34 linked to Lr34, also interacted with wPt4930 on 6BS and wPt4916 on 2BS. The frequent involvement of wPt4916 on 2BS and wPt4930 on 6BS in interactions with other significant loci on the same or different chromosomes suggested complex genetic control for adult plant resistance to Ug99 in winter wheat germplasm.

Expression of α-amylase inhibitors in diploid Triticum species

The aim of the work was to characterize the expression of various α-amylase inhibitors (αAIs), well known anti-nutritional compounds, for the development of healthier diploid wheat-based functional foods. The salt-soluble protein fractions from the seeds of 53 accessions among Triticum monococcum subsp. monococcum (T.m.), T. monococcum subsp. boeoticum (T.b.) and Triticum urartu (T.u.) were analyzed by immunoblotting after SDS-PAGE and Urea-PAGE using polyclonal antibodies (PABs) raised against 0.19 and 0.28 αAIs expressed in bread-wheat. Reverse zymography with human saliva and Tenebrio molitor α-amylases was used to assay inhibition activity. A great variability of the expression of αAI-related proteins was observed among T.b. and T.u. PABs, and reverse zymography revealed different bands, often not correlating with those present in bread-wheat. Two-dimensional electrophoresis followed by immunoblotting and mass spectrometric analysis identified these proteins as αAIs. Interestingly, no signal was observed within T.m. accessions. This makes T.m. an important candidate for the production of novel functional foods.

Genetic analysis of abiotic stress tolerance in crops

Abiotic stress tolerance is complex, but as phenotyping technologies improve, components that contribute to abiotic stress tolerance can be quantified with increasing ease. In parallel with these phenomics advances, genetic approaches with more complex genomes are becoming increasingly tractable as genomic information in non-model crops increases and even whole crop genomes can be re-sequenced. Thus, genetic approaches to elucidating the molecular basis to abiotic stress tolerance in crops are becoming more easily achievable.

Association genetics of carbon isotope discrimination, height and foliar nitrogen in a natural population of Pinus taeda L

Loblolly pine, Pinus taeda L., is one of the most widely planted, commercially and ecologically important tree species in North America. We took an association genetics approach, using an unimproved population of 380 clonally replicated unrelated trees, to test 3,938 single nucleotide polymorphisms (SNPs) in as many genes for association with phenotypic variation in carbon isotope discrimination, foliar nitrogen concentration and total tree height after two growing seasons. Best linear unbiased prediction (BLUP) was used with a spatial adjustment to remove environmental variation from phenotypic data derived from a common garden experiment. After correction for multiple testing, a total of 14 SNPs were associated with the traits of carbon isotope discrimination (n = 7), height (n = 1) and foliar nitrogen concentration (n = 6) using 380 clones. Tails of the population phenotypic distribution were compared for allele frequency differences, revealing 10 SNPs with allele frequency in at least one tail significantly different from the overall population. Eight associated SNPs were in sequences similar to known genes, such as an AP2 transcription factor related to carbon isotope discrimination and glutamate decarboxylase associated with foliar nitrogen concentration, and others were from unknown genes without homologs in Arabidopsis.

DArT markers: diversity analyses, genomes comparison, mapping and integration with SSR markers in Triticum monococcum

BACKGROUND

Triticum monococcum (2n = 2x = 14) is an ancient diploid wheat with many useful traits and is used as a model for wheat gene discovery. DArT (Diversity Arrays Technology) employs a hybridisation-based approach to type thousands of genomic loci in parallel. DArT markers were developed for T. monococcum to assess genetic diversity, compare relationships with hexaploid genomes, and construct a genetic linkage map integrating DArT and microsatellite markers.

RESULTS

A DArT array, consisting of 2304 hexaploid wheat, 1536 tetraploid wheat, 1536 T. monococcum as well as 1536 T. boeoticum representative genomic clones, was used to fingerprint 16 T. monococcum accessions of diverse geographical origins. In total, 846 polymorphic DArT markers were identified, of which 317 were of T. monococcum origin, 246 of hexaploid, 157 of tetraploid, and 126 of T. boeoticum genomes. The fingerprinting data indicated that the geographic origin of T. monococcum accessions was partially correlated with their genetic variation. DArT markers could also well distinguish the genetic differences amongst a panel of 23 hexaploid wheat and nine T. monococcum genomes. For the first time, 274 DArT markers were integrated with 82 simple sequence repeat (SSR) and two morphological trait loci in a genetic map spanning 1062.72 cM in T. monococcum. Six chromosomes were represented by single linkage groups, and chromosome 4Am was formed by three linkage groups. The DArT and SSR genetic loci tended to form independent clusters along the chromosomes. Segregation distortion was observed for one third of the DArT loci. The Ba (black awn) locus was refined to a 23.2 cM region between the DArT marker locus wPt-2584 and the microsatellite locus Xgwmd33 on 1Am; and the Hl (hairy leaf) locus to a 4.0 cM region between DArT loci 376589 and 469591 on 5Am.

CONCLUSION

DArT is a rapid and efficient approach to develop many new molecular markers for genetic studies in T. monococcum. The constructed genetic linkage map will facilitate localisation and map-based cloning of genes of interest, comparative mapping as well as genome organisation and evolution studies between this ancient diploid species and other crops.

DArT markers: diversity analyses, genomes comparison, mapping and integration with SSR markers in Triticum monococcum

Background

Triticum monococcum (2n = 2x = 14) is an ancient diploid wheat with many useful traits and is used as a model for wheat gene discovery. DArT (Diversity Arrays Technology) employs a hybridisation-based approach to type thousands of genomic loci in parallel. DArT markers were developed for T. monococcum to assess genetic diversity, compare relationships with hexaploid genomes, and construct a genetic linkage map integrating DArT and microsatellite markers.

Results

A DArT array, consisting of 2304 hexaploid wheat, 1536 tetraploid wheat, 1536 T. monococcum as well as 1536 T. boeoticum representative genomic clones, was used to fingerprint 16 T. monococcum accessions of diverse geographical origins. In total, 846 polymorphic DArT markers were identified, of which 317 were of T. monococcum origin, 246 of hexaploid, 157 of tetraploid, and 126 of T. boeoticum genomes. The fingerprinting data indicated that the geographic origin of T. monococcum accessions was partially correlated with their genetic variation. DArT markers could also well distinguish the genetic differences amongst a panel of 23 hexaploid wheat and nine T. monococcum genomes. For the first time, 274 DArT markers were integrated with 82 simple sequence repeat (SSR) and two morphological trait loci in a genetic map spanning 1062.72 cM in T. monococcum. Six chromosomes were represented by single linkage groups, and chromosome 4A^m was formed by three linkage groups. The DArT and SSR genetic loci tended to form independent clusters along the chromosomes. Segregation distortion was observed for one third of the DArT loci. The Ba (black awn) locus was refined to a 23.2 cM region between the DArT marker locus wPt-2584 and the microsatellite locus Xgwmd33 on 1A^m; and the Hl (hairy leaf) locus to a 4.0 cM region between DArT loci 376589 and 469591 on 5A^m.

Conclusion

DArT is a rapid and efficient approach to develop many new molecular markers for genetic studies in T. monococcum. The constructed genetic linkage map will facilitate localisation and map-based cloning of genes of interest, comparative mapping as well as genome organisation and evolution studies between this ancient diploid species and other crops.

Phenotypic and genetic analysis of the Triticum monococcum-Mycosphaerella graminicola interaction

Here, the aim was to understand the cellular and genetic basis of the Triticum monococcum-Mycosphaerella graminicola interaction. Testing for 5 yr under UK field conditions revealed that all 24 T. monococcum accessions exposed to a high level of natural inocula were fully resistant to M. graminicola. When the accessions were individually inoculated in the glasshouse using an attached leaf seeding assay and nine previously characterized M. graminicola isolates, fungal sporulation was observed in only three of the 216 interactions examined. Microscopic analyses revealed that M. graminicola infection was arrested at four different stages post-stomatal entry. When the inoculated leaves were detached 30 d post inoculation and incubated at 100% humidity, abundant asexual sporulation occurred within 5 d in a further 61 interactions. An F(2) mapping population generated from a cross between T. monococcum accession MDR002 (susceptible) and MDR043 (resistant) was inoculated with the M. graminicola isolate IPO323. Both resistance and in planta fungal growth were found to be controlled by a single genetic locus designated as TmStb1 which was linked to the microsatellite locus Xbarc174 on chromosome 7A(m). Exploitation of T. monococcum may provide new sources of resistance to septoria tritici blotch disease.