Genetic diversity of <Emphasis Type="Italic">avenin-like b</Emphasis> genes in <Emphasis Type="Italic">Aegilops tauschii</Emphasis> Coss

Avenin-like storage proteins influence the rheological properties and processing quality in common wheat, and the discovery of new alleles will benefit wheat quality improvement. In this study, 13 avenin-like b alleles (TaALPb7D-A–M) were discovered in 108 Aegilops tauschii Coss. accessions. Ten alleles were reported for the first time, while the remaining three alleles were the same as alleles in other species. A total of 15 nucleotide changes were detected in the 13 alleles, resulting in only 11 amino acid changes because of synonymous mutations. Alleles TaALPb7D-E, TaALPb7D-G, and TaALPb7D-J encoded the same protein. These polymorphic sites existed in the N-terminus, Repetitive region (Left), Repetitive region (Right) and C-terminus domains, with no polymorphisms in the signal peptide sequence nor in those encoding the 18 conserved cysteine residues. Phylogenetic analysis divided the TaALPb7Ds into four clades. The Ae. tauschii alleles were distributed in all four clades, while the alleles derived from common wheat, TaALPb7D-G and TaALPb7D-C, belonged to clade III and IV, respectively. Alleles TaALPb7D-G and TaALPb7D-C were the most widely distributed, being present in nine and six countries, respectively. Iran and Turkey exhibited the highest genetic diversity with respect to TaALPb7D alleles, accessions from these countries carrying seven and six alleles, respectively, which implied that these countries were the centers of origin of the avenin-like b gene. The new alleles discovered and the phylogenetic analysis of avenin-like b genes will provide breeding materials and a theoretical basis for wheat quality improvement.     

Identification of <Emphasis Type="Italic">Pm58</Emphasis> from <Emphasis Type="Italic">Aegilops tauschii</Emphasis>

Key message

A novel powdery mildew-resistance gene, designated Pm58, was introgressed directly from Aegilops tauschii to hexaploid wheat, mapped to chromosome 2DS, and confirmed to be effective under field conditions. Selectable KASP? markers were developed for MAS.

Abstract

Powdery mildew caused by Blumeria graminis (DC.) f. sp. tritici (Bgt) remains a significant threat to wheat (Triticum aestivum L.) production. The rapid breakdown of race-specific resistance to Bgt reinforces the need to identify novel sources of resistance. The d-genome species, Aegilops tauschii, is an excellent source of disease resistance that is transferrable to T. aestivum. The powdery mildew-resistant Ae. tauschii accession TA1662 (2n?=?2x?=?DD) was crossed directly with the susceptible hard white wheat line KS05HW14 (2n?=?6x?=?AABBDD) followed by backcrossing to develop a population of 96 BC₂F₄ introgression lines (ILs). Genotyping-by-sequencing was used to develop a genome-wide genetic map that was anchored to the Ae. tauschii reference genome. A detached-leaf Bgt assay was used to screen BC₂F_4:6 ILs, and resistance was found to segregate as a single locus (χ?=?2.0, P value?=?0.157). The resistance gene, referred to as Pm58, mapped to chromosome 2DS. Pm58 was evaluated under field conditions in replicated trials in 2015 and 2016. In both years, a single QTL spanning the Pm58 locus was identified that reduced powdery mildew severity and explained 21% of field variation (P value?<?0.01). KASP? assays were developed from closely linked GBS-SNP markers, a refined genetic map was developed, and four markers that cosegregate with Pm58 were identified. This novel source of powdery mildew-resistance and closely linked genetic markers will support efforts to develop wheat varieties with powdery mildew resistance.

     

Production of intergeneric hybrid between dwarfing polish wheat (<Emphasis Type="Italic">Triticum polonicum</Emphasis> L.) and <Emphasis Type="Italic">Aegilops tauschii</Emphasis> Cosson. with reference to wheat origin

Dwarfing polish wheat is a dwarfing accession of Triticum polonicum L. from Xinjiang of China. In the present study, the artificial hybridization between dwarfing polish wheat and two accessions of Aegilops tauschii Cosson. (AS60 and AS65) was carried out, and the F₁ hybrids were obtained successfully without using embryo rescue techniques for the first time. The crossabilities of hybrids T. polonicum × Ae. tauschii (AS60) and T. polonicum × Ae. tauschii (AS65) were 1.67% and 0.60% respectively. Only the hybrids of T. polonicum × Ae. tauschii (AS60) germinated well, and 24 F₁ hybrid plants were obtained. All the F₁ hybrid plants grew vigorously, and the morphological traits were similar to bread wheat. The F₁ plants had some obvious traits inherited from T. polonicum and Ae. tauschii and were completely sterile. Chromosome pairing in the hybrid was characterized by a large number of univalents, with an average of 20.56 and 0.22 bivalents per PMC, and no ring bivalents and multivalents were observed. Furthermore, the potential value of the F1 hybrids between T. polonicum and Ae. tauschii for studying wheat origin and breeding are discussed. The article is published in the original.     

Genetic diversity of Iranian <Emphasis Type="Italic">Aegilops tauschii</Emphasis> Coss. using microsatellite molecular markers and morphological traits

Aegilops tauschii Coss. is a diploid (2n = 2x = 14,DD) goat grass species which has contributed the D genome in common wheat. Genetic variations in 28 accessions of Aegilops tauschii belonged to different provinces of Iran, were evaluated using 16 morphological traits and 19 SSR markers. In number of spikelet per spike and plant height, there was a high variation in ssp. tauschii and ssp. strangulata respectively and for days to mature a low variation in both subspecies was found. Discriminant function analysis showed that 67.9% of original grouped cases correctly classified. Factor analysis indicated that three factor explain 66.49% of total variation. The three clusters revealed by the cluster analysis were not consistent with their geographical distributions. We determined 208 alleles using 19 microsatellites. Average of alleles for every locus was 10.94. The total average of PIC was 0.267. 2261 bands produced for total of genotypes and Chinese Spring had the highest bands (95 alleles). The range of similarity coefficients was between 0.23 and 0.73. Genotypes were clustered using UPGMA method. The accessions did not match according to morphological cluster and geographical regions. 51.2% of total variations were related to 9 principle components.     

Manipulating the Length of the <Emphasis Type="Italic">b</Emphasis> Subunit F<Subscript>1</Subscript> Binding Domain in F<Subscript>1</Subscript>F<Subscript>0</Subscript> ATP Synthase from <Emphasis Type="Italic">Escherichia coli</Emphasis>

The peripheral stalk of F₁F₀ ATP synthase is composed of a parallel homodimer of b subunits that extends across the cytoplasmic membrane in F₀ to the top of the F₁ sector. The stalk serves as the stator necessary for holding F₁ against movement of the rotor. A series of insertions and deletions have been engineered into the hydrophilic domain that interacts with F₁. Only the hydrophobic segment from {val-121} to {ala-132} and the extreme carboxyl terminus proved to be highly sensitive to mutation. Deletions in either site apparently abolished enzyme function as a result of defects is assembly of the F₁F₀ complex. Other mutations manipulating the length of the sequence between these two areas had only limited effects on enzyme function. Expression of a b subunit with insertions with as few as two amino acids into the hydrophobic segment also resulted in loss of F₁F₀ ATP synthase. However, a fully defective b subunit with seven additional amino acids could be stabilized in a heterodimeric peripheral stalk within a functional F₁F₀ complex by a normal b subunit.     

Characterization of low-molecular-weight glutenin genes in <Emphasis Type="Italic">Aegilops tauschii</Emphasis>

This paper reports the characterization of the low-molecular-weight (LMW) glutenin gene family of Aegilops tauschii (syn. Triticum tauschii), the D-genome donor of hexaploid wheat. By analysis of bacterial artificial chromosome (BAC) clones positive for hybridization with an LMW glutenin probe, seven unique LMW glutenin genes were identified. These genes were sequenced, including their untranslated 3 and 5 flanking regions. The deduced amino acid sequences of the genes revealed four putative active genes and three pseudogenes. All these genes had a very high level of similarity to LMW glutenins characterized in hexaploid wheat. The predicted molecular weights of the mature proteins were between 32.2 kDa and 39.6 kDa, and the predicted isoelectric points of the proteins were between 7.53 and 8.06. All the deduced proteins were of the LMW-m type. The organization of the seven LMW glutenin genes appears to be interspersed over at least several hundred kilo base pairs, as indicated by the presence of only one gene or pseudogene per BAC clone. Southern blot analysis of genomic DNA of Ae. tauschii and the BAC clones containing the seven LMW glutenin genes indicated that the BAC clones contained all LMW glutenin-hybridizing bands present in the genome. Two-dimensional gel electrophoresis of an LMW glutenin extract from Ae. tauschii was conducted and showed the presence of at least 11 distinct proteins. Further analysis indicated that some of the observed proteins were modified gliadins. These results suggest that the actual number of typical LMW glutenins may in fact be much lower than previously thought, with a number of modified gliadins also being present in the polymeric fraction.     

Fine genetic mapping of greenbug aphid-resistance gene <Emphasis Type="Italic">Gb3</Emphasis> in <Emphasis Type="Italic">Aegilops tauschii</Emphasis>

The greenbug, Schizaphis graminum (Rondani), is an important aphid pest of small grain crops especially wheat (Triticum aestivum L., 2n = 6x = 42, genomes AABBDD) in many parts of the world. The greenbug-resistance gene Gb3 originated from Aegilops tauschii Coss. (2n = 2x = 14, genome D^tD^t) has shown consistent and durable resistance against prevailing greenbug biotypes in wheat fields. We previously mapped Gb3 in a recombination-rich, telomeric bin of wheat chromosome arm 7DL. In this study, high-resolution genetic mapping was carried out using an F_2:3 segregating population derived from two Ae. tauschii accessions, the resistant PI 268210 (original donor of Gb3 in the hexaploid wheat germplasm line ‘Largo’) and susceptible AL8/78. Molecular markers were developed by exploring bin-mapped wheat RFLPs, SSRs, ESTs and the Ae. tauschii physical map (BAC contigs). Wheat EST and Ae. tauschii BAC end sequences located in the deletion bin 7DL3-0.82–1.00 were used to design STS (sequence tagged site) or CAPS (Cleaved Amplified Polymorphic Sequence) markers. Forty-five PCR-based markers were developed and mapped to the chromosomal region spanning the Gb3 locus. The greenbug-resistance gene Gb3 now was delimited in an interval of 1.1 cM by two molecular markers (HI067J6-R and HI009B3-R). This localized high-resolution genetic map with markers closely linked to Gb3 lays a solid foundation for map based cloning of Gb3 and marker-assisted selection of this gene in wheat breeding.     

The <Emphasis Type="Italic">b</Emphasis><Subscript>arg36</Subscript> contributes to efficient coupling in F<Subscript>1</Subscript>F<Subscript>O</Subscript> ATP synthase in <Emphasis Type="Italic">Escherichia coli</Emphasis>

In Escherichia coli, the F₁F_O ATP synthase b subunits house a conserved arginine in the tether domain at position 36 where the subunit emerges from the membrane. Previous experiments showed that substitution of isoleucine or glutamate result in a loss of enzyme activity. Double mutants have been constructed in an attempt to achieve an intragenic suppressor of the b _arg36→ile and the b _arg36→glu mutations. The b _arg36→ile mutation could not be suppressed. In contrast, the phenotypic defect resulting from the b _arg36→glu mutation was largely suppressed in the b _{arg36→glu,glu39→arg} double mutant. E. coli expressing the b _{arg36→glu,glu39→arg} subunit grew well on succinate-based medium. F₁F_O ATP synthase complexes were more efficiently assembled and ATP driven proton pumping activity was improved. The evidence suggests that efficient coupling in F₁F_O ATP synthase is dependent upon a basic amino acid located at the base of the peripheral stalk.     

Low molecular weight glutenin subunit gene composition at <Emphasis Type="Italic">Glu</Emphasis>-<Emphasis Type="Italic">D3</Emphasis> loci of <Emphasis Type="Italic">Aegilops tauschii</Emphasis> and common wheat and a further view of wheat evolution

Key message

A comprehensive comparison of LMW-GS genes between Ae. tauschii and its progeny common wheat.

Abstract

Low molecular weight glutenin subunits (LMW-GSs) are determinant of wheat flour processing quality. However, the LMW-GS gene composition in Aegilops tauschii, the wheat D genome progenitor, has not been comprehensively elucidated and the impact of allohexaploidization on the Glu-D3 locus remains elusive. In this work, using the LMW-GS gene molecular marker system and the full-length gene-cloning method, LMW-GS genes at the Glu-D3 loci of 218 Ae. tauschii and 173 common wheat (Triticum aestivum L.) were characterized. Each Ae. tauschii contained 11 LMW-GS genes, and the whole collection was divided into 25 haplotypes (AeH01–AeH25). The Glu-D3 locus in common wheat lacked the LMW-GS genes D3-417, D3-507 and D3-552, but shared eight genes of identical open reading frame (ORF) sequences when compared to that of Ae. tauschii. Therefore, the allohexaploidization induces deletions, but exerts no influence on LMW-GS gene coding sequences at the Glu-D3 locus. 92.17% Ae. tauschii had 7-9 LMW-GSs, more than the six subunits in common wheat. The haplotypes AeH16, AeH20 and AeH23 of Ae. tauschii ssp. strangulate distributed in southeastern Caspian Iran were the main putative D genome donor of common wheat. These results facilitate the utilization of the Ae. tauschii glutenin gene resources and the understanding of wheat evolution.

     

Introgression of a leaf rust resistance gene from <Emphasis Type="Italic">Aegilops caudata</Emphasis> to bread wheat

Rusts are the most important biotic constraints limiting wheat productivity worldwide. Deployment of cultivars with broad spectrum rust resistance is the only environmentally viable option to combat these diseases. Identification and introgression of novel sources of resistance is a continuous process to combat the ever evolving pathogens. The germplasm of nonprogenitor Aegilops species with substantial amount of variability has been exploited to a limited extent. In the present investigation introgression, inheritance and molecular mapping of a leaf rust resistance gene of Ae. caudata (CC) acc. pau3556 in cultivated wheat were undertaken. An F(2) population derived from the cross of Triticum aestivum cv. WL711 - Ae. caudata introgression line T291-2 with wheat cultivar PBW343 segregated for a single dominant leaf rust resistance gene at the seedling and adult plant stages. Progeny testing in F(3) confirmed the introgression of a single gene for leaf rust resistance. Bulked segregant analysis using polymorphic D-genome-specific SSR markers and the cosegregation of the 5DS anchored markers (Xcfd18, Xcfd78, Xfd81 and Xcfd189) with the rust resistance in the F(2) population mapped the leaf rust resistance gene (LrAC) on the short arm of wheat chromosome 5D. Genetic complementation and the linked molecular markers revealed that LrAC is a novel homoeoallele of an orthologue Lr57 already introgressed from the 5M chromosome of Ae. geniculata on 5DS of wheat.     

Geographic patterns of histone H1 encoding genes allelic variation in <Emphasis Type="Italic">Aegilops tauschii</Emphasis> Coss. (Poaceae)

An electrophoretic analysis of histone H1 of Aegilops tauschii was carried out using the collection of 303 accessions (156 of ssp. tauschii and 147 of ssp. stangulata) representing all the species range. Three, four and six allelic variants were found for Hst1, Hst2 and Hst3 locus, respectively. The level of histone H1 allelic variability in ssp. strangulata was considerably higher than in ssp. tauschii. Expected heterozygosity (H_E) for the loci Hst1, Hst2 and Hst3 made up 0.066, 0.484 and 0.224 respectively in ssp. strangulata vs. 0.024, 0.051 and 0.214 in ssp. tauschii. Besides the most common haplotype, Hst1 ¹⁰⁰⁰, Hst2 ¹⁰⁰⁰, Hst3 ¹⁰⁰⁰, five other haplotypes with frequencies of occurrence higher than 0.02 were found in ssp. strangulata, and only one such haplotype—in ssp. tauschii. The most part of histone H1 variation in ssp. tauschii was in the western part of the area. In ssp. strangulata, the alleles Hst2 ⁹⁸⁸ and Hst2 ⁹⁷³ were found only in Caucasia, and the allele Hst1 ¹⁰⁴³—only in Precaspian Iran and south-eastern Azerbaijan. Histone H1 variation patterns in Ae. tauschii are very similar to those of non-coding sequences of chloroplast DNA. Therefore, histone H1 allelic variation in this species seems to be mostly neutral. Nevertheless, the evidences were pointed out, revealing that some part of variation at Hst2 locus in ssp. strangulata could be adaptive. It seems that Hst2 ¹⁰²⁶ allele is disadvantageous in western Precaspian Iran, the region with the high annual rainfall, and being eliminated by natural selection.     

Chromosomal location of <Emphasis Type="Italic">Pm35</Emphasis>, a novel <Emphasis Type="Italic">Aegilops tauschii</Emphasis> derived powdery mildew resistance gene introgressed into common wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

A single gene controlling powdery mildew resistance was identified in the North Carolina germplasm line NC96BGTD3 (NCD3) using genetic analysis of F₂ derived lines from a NCD3 X Saluda cross. Microsatellite markers linked to this Pm gene were identified and their most likely order was Xcfd7, 10.3 cM, Xgdm43, 8.6 cM, Xcfd26, 11.9 cM, Pm gene. These markers and the Pm gene were assigned to chromosome 5DL by means of Chinese Spring Nullitetrasomic (Nulli5D-tetra5A) and ditelosomic (Dt5DL) lines. A detached leaf test showed a distinctive disease reaction to six pathogen isolates among the NCD3 Pm gene, Pm2 (5DS) and Pm34 (5DL). An allelism test showed independence between Pm34 and the NCD3 Pm gene. Together, the tests provided strong evidence for the presence of a novel Pm gene in NCD3, and this gene was designated Pm35.     

Over-expressing a UDP-glucosyltransferase gene (<Emphasis Type="Italic">Ta-UGT</Emphasis><Subscript><Emphasis Type="Italic">3</Emphasis></Subscript>) enhances <Emphasis Type="Italic">Fusarium</Emphasis> Head Blight resistance of wheat

Wheat Fusarium Head Blight (FHB), mainly caused by Fusarium graminearum (F.g), is a destructive fungal disease worldwide. FHB can not only cause considerable reduction in yield, but more seriously, can contaminate grain by trichothecene toxins released by the fungus. Here, we report new insights into the function and underlying mechanisms of a UDP-glycosyltransferase gene, Ta-UGT ₃ , that is involved in FHB resistance in wheat. In our previous study, Ta-UGT ₃ was found to enhance host tolerance against deoxynivalenol (DON) in Arabidopsis. In this study, four transgenic lines over-expressing Ta-UGT ₃ in a FHB highly susceptible wheat variety, Alondra’s, were obtained and characterized. 3 years of assays using single floret inoculation with F.g indicated that all four transgenic lines exhibited significantly enhanced type II resistance to FHB and less DON accumulation in the grains compared to the untransformed control. Histological observation using GFP labelled F.g was in agreement with the above test results since over-expression of Ta-UGT ₃ dramatically inhibited expansion of F.g. To explore the putative mechanism of resistance mediated by Ta-UGT ₃ , microarray analysis, qRT-PCR and hormone measurements were performed. Microarray analysis showed that DON up-regulated genes, such as TaNPR1, in the susceptible control, and down-regulated genes in F.g inoculated transgenic lines, while qRT-PCR showed that some defence related genes were up-regulated in F.g inoculated transgenic lines. Ta-UGT ₃ over-expression also changed the contents of the endogenous hormones SA and JA in the spikes. These data suggest that Ta-UGT ₃ positively regulates the defence responses to F.g, perhaps by regulating defence-related and DON-induced downstream genes.     

Identification and genetic characterization of an <Emphasis Type="Italic">Aegilops tauschii</Emphasis> ortholog of the wheat leaf rust disease resistance gene <Emphasis Type="Italic">Lr1</Emphasis>

Aegilops tauschii (goat grass) is the progenitor of the D genome in hexaploid bread wheat. We have screened more than 200 Ae. tauschii accessions for resistance against leaf rust (Puccinia triticina) isolates, which are avirulent on the leaf rust resistance gene Lr1. Approximately 3.5% of the Ae. tauschii accessions displayed the same low infection type as the tester line Thatcher Lr1. The accession Tr.t. 213, which showed resistance after artificial infection with Lr1 isolates both in Mexico and in Switzerland, was chosen for further analysis. Genetic analysis showed that the resistance in this accession is controlled by a single dominant gene, which mapped at the same chromosomal position as Lr1 in wheat. It was delimited in a 1.3-cM region between the restriction fragment length polymorphism (RFLP) markers ABC718 and PSR567 on chromosome 5DL of Ae. tauschii. The gene was more tightly linked to PSR567 (0.47 cM) than to ABC718 (0.79 cM). These results indicate that the resistance gene in Ae. tauschii accession Tr.t. 213 is an ortholog of the leaf rust resistance gene Lr1 of bread wheat, suggesting that Lr1 originally evolved in diploid goat grass and was introgressed into the wheat D genome during or after domestication of hexaploid wheat. Compared to hexaploid wheat, higher marker polymorphism and recombination frequencies were observed in the region of the Lr1 ortholog in Ae. tauschii. The identification of Lr1^Ae, the orthologous gene of wheat Lr1, in Ae. tauschii will allow map-based cloning of Lr1 from this genetically simpler, diploid genome.Hong-Qing Ling and Jiwen Qiu have contributed equally to this work     

<Emphasis Type="Italic">Sr33</Emphasis> and <Emphasis Type="Italic">Sr35</Emphasis> gene homolog identification in genomes of cereals related to <Emphasis Type="Italic">Aegilops tauschii</Emphasis> and <Emphasis Type="Italic">Triticum monococcum</Emphasis>

Using bioinformatics analysis, the homologs of genes Sr33 and Sr35 were identified in the genomes of Triticum aestivum, Hordeum vulgare, and Triticum urartu. It is known that these genes confer resistance to highly virulent wheat stem rust races (Ug99). To identify amino acid sites important for this resistance, the found homologs were compared with the Sr33 and Sr35 protein sequences. It was found that sequences S5DMA6 and E9P785 are the closest homologs of protein RGAle, a Sr33 gene product, and sequences M7YFA9 (CNL-C) and F2E9R2 are homologs of protein CNL9, a Sr35 gene product. It is assumed that the homologs of genes Sr33 and Sr35, which were obtained from the wild relatives of wheat and barley, can confer resistance to various forms of stem rust and can be used in the future breeding programs aimed at improvement of national wheat varieties.     

LOSITAN: A workbench to detect molecular adaptation based on a <Emphasis Type="Italic">F</Emphasis><Subscript><Emphasis Type="Italic">st</Emphasis></Subscript>-outlier method

Background  

Testing for selection is becoming one of the most important steps in the analysis of multilocus population genetics data sets. Existing applications are difficult to use, leaving many non-trivial, error-prone tasks to the user.     

Chromosomal localization of aromatic alcohol dehydrogenase fast-migrating isoenzyme <Emphasis Type="Italic">Aadh1F</Emphasis> (<Emphasis Type="Italic">CAD1-F</Emphasis>) gene in <Emphasis Type="Italic">Triticum aestivum</Emphasis> L. bread wheat

Differences in isoenzyme pattern of aromatic alcohol dehydrogenase, NADP-AADH or CAD, were found in the Triticum aestivum L. winter bread wheat cultivars by the method of electrophoresis in the starch gel. A standard three-component spectrum is present in the cv. Zitnica (former Yugoslavia); additional fact-migrating isoenzymes appear in the cv. Novosibirskaya 9 (Institute of Cytology and Genetics, Siberian Branch, Russian Academy of Sciences, Russia). The presence of fast-migrating CAD isoenzymes is designated as FF phenotype; their absence, as 00 phenotype. Hybridological analysis was carried out; the excess of “null” genotypes was found in F₂ progenies. Hybridization with nulli-tetrasomic lines of the chromosomes of the fifth homeologous group was conducted for the gene localization. The segregation analysis demonstrated the most probable localization of the CAD1-F gene in the chromosome 5A. The plants with FF and 00 genotypes differed in a number of chemical and anatomical traits, as well as in grain productivity. The results obtained are discussed in connection with the function of this enzyme in the wheat plant tissues.     

The distribution of <Emphasis Type="Italic">F</Emphasis><Subscript><Emphasis Type="Italic">st</Emphasis></Subscript> and other genetic statistics for a class of population structure models

We examine genetic statistics used in the study of structured populations. In a 1999 paper, Wakeley observed that the coalescent process associated with the finite island model can be decomposed into a scattering phase and a collecting phase. This decomposition becomes exact in the large population limit with the coalescent at the end of the scattering phase converging to the Ewens sampling formula and the coalescent during the collecting phase converging to the Kingman coalescent. In this paper we introduce a class of limiting models, which we refer to as G/KC models, that generalize Wakeley’s decomposition. G in G/KC represents a completely general limit for the scattering phase, while KC represents a Kingman coalescent limit for the collecting phase. We show that both the island and two-dimensional stepping stone models converge to G/KC models in the large population limit. We then derive the distribution of the statistic F _st for all G/KC models under a large sample limit for the cases of strong or weak mutation, thereby deriving the large population, large sample limiting distribution of F _st for the island and two-dimensional stepping stone models as a special case of a general formula. Our methods allow us to take the large population and large sample limits simultaneously. In the context of large population, large sample limits, we show that the variance of F _st in the presence of weak mutation collapses as O(\frac1logd){O(\frac{1}{\log d})} where d is the number of demes sampled. Further, we show that this O(\frac1logd){O(\frac{1}{\log d})} is caused by a heavy tail in the distribution of F _st . Our analysis of F _st can be extended to an entire class of genetic statistics, and we use our approach to examine homozygosity measures. Our analysis uses coalescent based methods.     

Virus-Induced Gene Silencing (VIGS) in <Emphasis Type="Italic">Aegilops tauschii</Emphasis> and Its Use in Functional Analysis of <Emphasis Type="Italic">AetDREB2</Emphasis>

Among the available reverse genetic approaches for studying gene function, virus-induced gene silencing (VIGS) has several advantages. It allows rapid characterization of gene function independent of stable transformation, which is basically difficult to achieve in monocots, and offers the potential to silence individual or multiple genes of a gene family. In order to establish a VIGS system in Aegilops tauschii, modified vectors derived from Barley stripe mosaic virus (BSMV) were used for silencing a phytoene desaturase gene that provides a convenient visual reporter for silencing. The results demonstrated a high efficiency of BSMV-VIGS in A. tauschii. Moreover, the BSMV-VIGS system was used to target a 354 bp specific region of the Dehydration-responsive element-binding (AetDreb2) gene, resulting in successful silencing of the gene in A. tauschii plants, as verified by real-time qRT-PCR. Indeed, in comparison with plants that were inoculated with an empty vector (BSMV:00), a faster rate of wilting and a lower relative water content were observed in plants inoculated with BSMV:AetDreb2 when they were exposed to drought stress. Therefore, BSMV-VIGS can be efficiently employed as a novel tool for reverse genetics in A. tauschii. It can also be used to study the effects of polyploidization on the gene function by a comparative analysis between bread wheat and its diploid progenitor.