Chromosome substitutions of Triticum timopheevii in common wheat and some observations on the evolution of polyploid wheat species

Identification of Fusarium Head Blight Resistance in Triticum timopheevii Accessions and Characterization of Wheat-T. timopheevii Introgression Lines for Enhanced Resistance

A diverse panel of wheat wild relative species was screened for resistance to Fusarium head blight (FHB) by spray inoculation. The great majority of species and accessions were susceptible or highly susceptible to FHB. Accessions of Triticum timopheevii (P95-99.1-1), Agropyron desertorum (9439957), and Elymus vaillantianus (531552) were highly resistant to FHB while additional accessions of T. timopheevii were found to be susceptible to FHB. A combination of spray and point inoculation assessments over two consecutive seasons indicated that the resistance in accession P95-99.1-1 was due to enhanced resistance to initial infection of the fungus (type 1 resistance), and not to reduction in spread (type 2 resistance). A panel of wheat-T. timopheevii (accession P95-99.1-1) introgression lines was screened for FHB resistance over two consecutive seasons using spray inoculation. Most introgression lines were similar in susceptibility to FHB as the wheat recipient (Paragon) but substitution of the terminal portion of chromosome 3BS of wheat with a similar-sized portion of 3G of T. timopheevii significantly enhanced FHB resistance in the wheat background.

Genetic diversity, distribution and domestication history of the neglected GGAtAt genepool of wheat

We present a comprehensive survey of cytogenetic and genomic diversity of the GGA^tA^t genepool of wheat, thereby unlocking these plant genetic resources for wheat improvement. Wheat yields are stagnating around the world and new sources of genes for resistance or tolerances to abiotic traits are required. In this context, the tetraploid wheat wild relatives are among the key candidates for wheat improvement. Despite its potential huge value for wheat breeding, the tetraploid GGA^tA^t genepool is largely neglected. Understanding the population structure, native distribution range, intraspecific variation of the entire tetraploid GGA^tA^t genepool and its domestication history would further its use for wheat improvement. The paper provides the first comprehensive survey of genomic and cytogenetic diversity sampling the full breadth and depth of the tetraploid GGA^tA^t genepool. According to the results obtained, the extant GGA^tA^t genepool consists of three distinct lineages. We provide detailed insights into the cytogenetic composition of GGA^tA^t wheats, revealed group- and population-specific markers and show that chromosomal rearrangements play an important role in intraspecific diversity of T. araraticum. The origin and domestication history of the GGA^tA^t lineages is discussed in the context of state-of-the-art archaeobotanical finds. We shed new light on the complex evolutionary history of the GGA^tA^t wheat genepool and provide the basis for an increased use of the GGA^tA^t wheat genepool for wheat improvement. The findings have implications for our understanding of the origins of agriculture in southwest Asia.

Relationship between neural responses to stress and mental health symptoms in psychogenic nonepileptic seizures after traumatic brain injury

OBJECTIVE

To utilize traumatic brain injury (TBI) as a model for investigating functioning during acute stress experiences in psychogenic nonepileptic seizures (PNES) and to identify neural mechanisms underlying the link between changes in processing of stressful experiences and mental health symptoms in PNES.

METHODS

We recruited 94 participants: 50 with TBI only (TBI-only) and 44 with TBI and PNES (TBI + PNES). Participants completed mood (Beck Depression Inventory-II), anxiety (Beck Anxiety Inventory), and posttraumatic stress disorder (PTSD) symptom (PTSD Checklist-Specific Event) assessments before undergoing functional magnetic resonance imaging during an acute psychosocial stress task. Linear mixed-effects analyses identified clusters of significant interactions between group and neural responses to stressful math performance and stressful auditory feedback conditions within limbic brain regions (volume-corrected α = .05). Spearman rank correlation tests compared mean cluster signals to symptom assessments (false discovery rate-corrected α = .05).

RESULTS

Demographic and TBI-related measures were similar between groups; TBI + PNES demonstrated worse clinical symptom severity compared to TBI-only. Stressful math performance induced relatively greater reactivity within dorsomedial prefrontal cortex (PFC) and right hippocampal regions and relatively reduced reactivity within left hippocampal and dorsolateral PFC regions for TBI + PNES compared to TBI-only. Stressful auditory feedback induced relatively reduced reactivity within ventral PFC, cingulate, hippocampal, and amygdala regions for TBI + PNES compared to TBI-only. Changes in responses to stressful math within hippocampal and dorsal PFC regions were correlated with increased mood, anxiety, and PTSD symptom severity.

SIGNIFICANCE

Corticolimbic functions underlying processing of stressful experiences differ between patients with TBI + PNES and those with TBI-only. Relationships between these neural responses and symptom assessments suggest potential pathophysiologic mechanisms in PNES.

Molecular cytological analysis of alien introgressions in common wheat lines derived from the cross of TRITICUM AESTIVUM with T. kiharae

BACKGROUND

Triticum kiharae (A^tA^tGGDD, 2n = 42) is of interest for the improvement of bread wheat as a source of high grain protein and gluten content, as well as resistance to many diseases. The use of T. kiharae for the improvement of T. aestivum L. is complicated by the fact that the homology degree of their genomes is low and this leads to an unbalanced set of chromosomes in the gametes of its first generations and the elimination of some genotypes. The aim of this study was to analyze the nature of alien introgressions and their effect on the cytological stability of hybrids obtained from crossing of bread wheat varieties with T. kiharae.

RESULTS

Using C-banding, the presence of entire chromosomes of T. kiharae in the karyotypes of hybrid lines (intergenomic substitution 2G/2B), chromosome arms (centric translocation Т2A^tS:2AL) and large inserts in the form of terminal translocations involving chromosomes of 1st, 3rd and 5th homoeologous groups of B- and G-genomes were found. Molecular markers revealed short introgression of T. kiharae into the genome of common wheat varieties. The highest introgression frequency was shown for 1A, 1B, 2A, 5B, and 6A chromosomes, while no foreign chromatin was detected in 4A and 4B chromosomes. A high level of cytological stability (a meiotic index of 88.18-93.0%) was noted for the majority of introgression lines. An exception was found for the lines containing the structural reorganization of chromosome 5B, affecting the main genes of chromosome synapsis in terms of their functioning.

CONCLUSIONS

During the stabilization of hybrid karyotypes, the introgression of genetic material from T. kiharae into the genome of T. aestivum occurs in the form of short fragments detectable only by molecular markers and in the form of whole chromosomes (intergenomic substitution) and their large fragments (centric and terminal translocations). The level of cytological stability achieved in F₁₀ by the majority of introgression lines ensures the formation of functional gametes sufficient for the successful reproduction of the obtained hybrids.

Tight repulsion linkage between Sr36 and Sr39 was revealed by genetic, cytogenetic and molecular analyses

The shortening of Aegilops speltoides segment did not facilitate recombination between stem rust resistance genes Sr36 and Sr39 . Robustness of marker rwgs28 for marker-assisted selection of Sr39 was demonstrated. Stem rust resistance genes Sr39 and Sr36 were transferred from Aegilops speltoides and Triticum timopheevii, respectively, to chromosome 2B of wheat. Genetic stocks RL6082 and RWG1 carrying Sr39 on a large and a shortened Ae. speltoides segments, respectively, and the Sr36-carrying Australian wheat cultivar Cook were used in this study. This investigation was planned to determine the genetic relationship between these genes. Stem rust tests on F₃ populations derived from RL6082/Cook and RWG1/Cook crosses showed tight repulsion linkage between Sr39 and Sr36. The genomic in situ hybridization analysis of heterozygous F₃ family from the RWG1/Cook population showed that the translocated segments do not overlap. Meiotic analysis on the F₁ plant from RWG1/Cook showed two univalents at the metaphase and anaphase stages in a majority of the cells indicating absence of pairing. Since meiotic pairing has been reported to initiate at the telomere, pairing and recombination may be inhibited due to very little wheat chromatin in the distal end of the chromosome arm 2BS in RWG1. The Sr39-carrying large Ae. speltoides segment transmitted preferentially in the RL6082/Cook F₃ population, whereas the Sr36-carrying T. timopheevii segment over-transmitted in the RWG1/Cook cross. Genotyping with the co-dominant Sr39- and Sr36-linked markers rwgs28 and stm773-2, respectively, matched the phenotypic classification of F₃ families. The RWG1 allele amplified by rwgs28 was diagnostic for the shortened Ae. speltoides segment and alternate alleles were amplified in 29 Australian cultivars. Marker rwgs28 will be useful in marker-assisted pyramiding of Sr39 with other genes.

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.

Collinearity-based marker mining for the fine mapping of Pm6, a powdery mildew resistance gene in wheat

The genome sequences of rice (Oryza sativa L.) and Brachypodium distachyon and the comprehensive Triticeae EST (Expressed Sequence Tag) resources provide invaluable information for comparative genomics analysis. The powdery mildew resistance gene, Pm6, which was introgressed into common wheat from Triticum timopheevii, was previously mapped to the wheat chromosome bin of 2BL [fraction length (FL) 0.50-1.00] with limited DNA markers. In this study, we saturated the Pm6 locus in wheat using the collinearity-based markers by extensively exploiting these genomic resources. All wheat ESTs located in the bin 2BL FL 0.50-1.00 and their corresponding orthologous genes on rice chromosome 4 were firstly used to develop STS (Sequence Tagged Site) markers. Those identified markers that flanked the Pm6 locus were then used to identify the collinear regions in the genomes of rice and Brachypodium. Triticeae ESTs with orthologous genes in these collinear regions were further used to develop new conserved markers for the fine mapping of Pm6. Using two F(2) populations derived from crosses of IGVI-465 × Prins and IGVI-466 × Prins, we mapped a total of 29 markers to the Pm6 locus. Among them, 14 markers were co-segregated with Pm6 in the IGVI-466/Prins population. Comparative genome analysis showed that the collinear region of the 29 linked markers covers a ~5.6-Mb region in chromosome 5L of Brachypodium and a ~6.0-Mb region in chromosome 4L of rice. The marker order is conserved between rice and Brachypodium, but re-arrangements are present in wheat. Comparative mapping in the two populations showed that two conserved markers (CINAU123 and CINAU127) flanked the Pm6 locus, and an LRR-receptor-like protein kinase cluster was identified in the collinear regions of Brachypodium and rice. This putative resistance gene cluster provides a potential target site for further fine mapping and cloning of Pm6. Moreover, the newly developed conserved markers closely linked to Pm6 can be used for the marker-assisted selection (MAS) of Pm6 in wheat breeding programs.

Independent Wheat B and G Genome Origins in Outcrossing Aegilops Progenitor Haplotypes

The origin of modern wheats involved alloploidization among related genomes. To determine if Aegilops speltoides was the donor of the B and G genomes in AABB and AAGG tetraploids, we used a 3-tiered approach. Using 70 amplified fragment length polymorphism (AFLP) loci, we sampled molecular diversity among 480 wheat lines from their natural habitats encompassing all S genome Aegilops, the putative progenitors of wheat B and G genomes. Fifty-nine Aegilops representatives for S genome diversity were compared at 375 AFLP loci with diploid, tetraploid, and 11 nulli-tetrasomic Triticum aestivum wheat lines. B genome-specific markers allowed pinning the origin of the B genome to S chromosomes of A. speltoides, while excluding other lineages. The outbreeding nature of A. speltoides influences its molecular diversity and bears upon inferences of B and G genome origins. Haplotypes at nuclear and chloroplast loci ACC1, G6PDH, GPT, PGK1, Q, VRN1, and ndhF for approximately 70 Aegilops and Triticum lines (0.73 Mb sequenced) reveal both B and G genomes of polyploid wheats as unique samples of A. speltoides haplotype diversity. These have been sequestered by the AABB Triticum dicoccoides and AAGG Triticum araraticum lineages during their independent origins.

Multiple genetic pathways for seed shattering in the grasses

Shattering is an essential seed dispersal mechanism in wild species. It is believed that independent mutations at orthologous loci led to convergent domestication of cereal crops. To investigate genetic relationships of Triticeae shattering genes with those of other grasses, we mapped spike-, barrel- (B-type), and wedge-type (W-type) spikelet disarticulation genes in wheat and its wild relatives. The Br1 gene for W-type disarticulation was mapped to a region delimited by Xpsr598 and Xpsr1196 on the short arm of chromosomes 3A in Triticum timopheevii and 3S in Aegilops speltoides. The spike- and W-type disarticulation genes are allelic at Br1 in Ae. speltoides. The B-type disarticulation gene, designated as Br2, was mapped to an interval of 4.4 cM between Xmwg2013 and Xpsr170 on the long arm of chromosome 3D in Aegilops tauschii, the D-genome donor of common wheat. Therefore, B- and W-type disarticulations are governed by two different orthologous loci on group-3 chromosomes. Based on map position, orthologs of Br1 and Br2 were not detected in barley, maize, rice, and sorghum, indicating multiple genetic pathways for shattering in grasses. The implications of the mapping results are discussed with regard to the evolution of polyploid wheat and domestication of cereals.

Plant cytogenetics at the dawn of the 21st century

The years 1996-1997 saw advances in plant chromosome handling, structure, behaviour and manipulation. Improved protocols were developed for flow sorting, microdissection and microcloning. Fibre FISH was used to map a range of DNA sequences at a resolution of a few kilobases. Over 400 wheat deletion stocks were reported and healing of broken chromosomes by de novo addition of telomeric sequences was demonstrated. Centromeric DNA sequences were identified. The role of telomeric ends in pairing was demonstrated. Apparently unusually long chromosome arms can interfere with mitosis. Novel phenomena and potential of wide hybrids for genome analysis were noteworthy.