Flowering time diversification and dispersal in central Eurasian wild wheat Aegilops tauschii Coss.: genealogical and ecological framework

Timing of flowering is a reproductive trait that has significant impact on fitness in plants. In contrast to recent advances in understanding the molecular basis of floral transition, few empirical studies have addressed questions concerning population processes of flowering time diversification within species. We analyzed chloroplast DNA genealogical structure of flowering time variation in central Eurasian wild wheat Aegilops tauschii Coss. using 200 accessions that represent the entire species range. Flowering time measured as days from germination to flowering varied from 144.0 to 190.0 days (average 161.3 days) among accessions in a common garden/greenhouse experiment. Subsequent genealogical and statistical analyses showed that (1) there exist significant longitudinal and latitudinal clines in flowering time at the species level, (2) the early-flowering phenotype evolved in two intraspecific lineages, (3) in Asia, winter temperature was an environmental factor that affected the longitudinal clinal pattern of flowering time variation, and (4) in Transcaucasus-Middle East, some latitudinal factors affected the geographic pattern of flowering time variation. On the basis of palaeoclimatic, biogeographic, and genetic evidence, the northern part of current species' range [which was within the temperate desert vegetation (TDV) zone at the Last Glacial Maximum] is hypothesized to have harbored species refugia. Postglacial southward dispersal from the TDV zone seems to have been driven by lineages that evolved short-flowering-time phenotypes through different genetic mechanisms in Transcaucasus-Middle East and Asia.     

Population structure of wild wheat D‐genome progenitor Aegilops tauschii Coss.: implications for intraspecific lineage diversification and evolution of common wheat

Aegilops tauschii Coss. is the D‐genome progenitor of hexaploid wheat. Aegilops tauschii, a wild diploid species, has a wide natural species range in central Eurasia, spreading from Turkey to western China. Amplified fragment length polymorphism (AFLP) analysis using a total of 122 accessions of Ae. tauschii was conducted to clarify the population structure of this widespread wild wheat species. Phylogenetic and principal component analyses revealed two major lineages in Ae. tauschii. Bayesian population structure analyses based on the AFLP data showed that lineages one (L1) and two (L2) were respectively significantly divided into six and three sublineages. Only four out of the six L1 sublineages were diverged from those of western habitats in the Transcaucasia and northern Iran region to eastern habitats such as Pakistan and Afghanistan. Other sublineages including L2 were distributed to a limited extent in the western region. Subspecies strangulata seemed to be differentiated in one sublineage of L2. Among three major haplogroups (HG7, HG9 and HG16) previously identified in the Ae. tauschii population based on chloroplast variation, HG7 accessions were widely distributed to both L1 and L2, HG9 accessions were restricted to L2, and HG16 accessions belonged to L1, suggesting that HG9 and HG16 were formed from HG7 after divergence of the first two lineages of the nuclear genome. These results on the population structure of Ae. tauschii and the genealogical relationship among Ae. tauschii accessions should provide important agricultural and evolutionary knowledge on genetic resources and conservation of natural genetic diversity.     

Pm34: a new powdery mildew resistance gene transferred from Aegilops tauschii Coss. to common wheat (Triticum aestivum L.)

Powdery mildew is a major fungal disease in wheat growing areas worldwide. A novel source of resistance to wheat powdery mildew present in the germplasm line NC97BGTD7 was genetically characterized as a monogenic trait in greenhouse and field trials using F₂ derived lines from a NC97BGTD7 X Saluda cross. Microsatellite markers were used to map and tag this resistance gene, now designated Pm34. Three co-dominant microsatellite markers linked to Pm34 were identified and their most likely order was established as: Xbarc177-5D, 5.4cM, Pm34, 2.6cM, Xbarc144-5D, 14cM, Xgwm272-5D. These microsatellite markers were previously mapped to the long arm of the 5D chromosome and their positions were confirmed using Chinese Spring nullitetrasomic Nulli5D-tetra5A and ditelosomic Dt5DL lines. Pm2, the only other known Pm gene on chromosome 5D, has been mapped to the short arm and its specificity is different from that of Pm34.     

Assessment of Aegilops tauschii for resistance to biotypes of wheat curl mite (Acari: Eriophyidae)

Aegilops tauschii, the wild diploid D-genome progenitor of wheat, Triticum aestivum L., is an important source of resistance to several arthropod pests and pathogens. A total of 108 Ae. tauschii accessions from different geographic regions were evaluated for resistance to biotypes of the wheat curl mite, Aceria tosichella Keifer, from Kansas, Nebraska, and Montana. The wheat curl mite is the only vector known to transmit wheat streak mosaic virus. Wheat curl mite resistance was detected in germplasm from all the geographic locations represented. The highest percentage of resistant accessions originated from Turkey, followed by Afghanistan and the Caspian Sea region of Iran. Sixty-seven percent of the accessions exhibited resistance to at least one wheat curl mite biotype and 19% were resistant to all the three biotyopes. Resistance to the accessions tested occurred more frequently in the Nebraska and Kansas biotypes (69% and 64%, respectively) than did resistance to the Montana biotype (42%), although the frequency of resistance was not significant. The differential reactions of accessions to the different wheat curl mite biotypes suggests that Ae. tauschii has at least five different genes for resistance to mite colonization. Ae. tauschii continues to be a very useful source for wheat curl mite resistance genes for bread wheat improvement.     

Identification and Mapping of Two New Genes Conferring Resistance to Powdery Mildew from Aegilops tauschii （Coss,） Schmal

Two powdery mildew resistance genes were Identified from Aegilops tauschll accessions Y201 and Y212 and mapped using two different F2 populations derived from the crosses between susceptible accession Y2272 and Y201, and susceptible accession Y2263 and Y212. Genetic analysis of resistance to powdery mildew Indicated that the resistance of Y201 was controlled by a single dominant gene, whereas the resistance of Y212 was controlled by a single recessive gene. We have temporarily designated these genes as PmY201 and PmY212, respectively. By bulk segregation analysis, six mlcrosatelllte markers Including Xgwm174, cfd26, cfd57, cfdl02, Xgwm583 and Xgwm639 were found to be linked to PraY201 with genetic distances of 5.2, 7.7, 9.6, 12.5, 20.2 and 22.1 cM, respectively. Five SSR markers, including cfd57, Xgwm182, cfd7, cfd102, and cfd12, were found to be linked to PmY212 with distances of 5.6, 7.2, 11.5, 14.7, and 18.5 cM, respectively. According to the locations of the linked markers, the two resistance genes were located In the 5DL region. Based on the chromosomal locations and the resistance patterns of the two genes, we propose that PmY201 and PmY212 are two novel powdery mildew resistance genes, and are suitable for marker-assisted selection.     

Identification of a UDP-glucosyltransferase conferring deoxynivalenol resistance in Aegilops tauschii and wheat

Aegilops tauschii is the diploid progenitor of the wheat D subgenome and a valuable resource for wheat breeding, yet, genetic analysis of resistance against Fusarium head blight (FHB) and the major Fusarium mycotoxin deoxynivalenol (DON) is lacking. We treated a panel of 147 Ae. tauschii accessions with either Fusarium graminearum spores or DON solution and recorded the associated disease spread or toxin-induced bleaching. A k-mer-based association mapping pipeline dissected the genetic basis of resistance and identified candidate genes. After DON infiltration nine accessions revealed severe bleaching symptoms concomitant with lower conversion rates of DON into the non-toxic DON-3-O-glucoside. We identified the gene AET5Gv20385300 on chromosome 5D encoding a uridine diphosphate (UDP)-glucosyltransferase (UGT) as the causal variant and the mutant allele resulting in a truncated protein was only found in the nine susceptible accessions. This UGT is also polymorphic in hexaploid wheat and when expressed in Saccharomyces cerevisiae only the full-length gene conferred resistance against DON. Analysing the D subgenome helped to elucidate the genetic control of FHB resistance and identified a UGT involved in DON detoxification in Ae. tauschii and hexaploid wheat. This resistance mechanism is highly conserved since the UGT is orthologous to the barley UGT HvUGT13248 indicating descent from a common ancestor of wheat and barley.     

Characterization of a dominant mutation for the liguleless trait: Aegilops tauschii liguleless (Lgt)

Background

Leaves of Poaceae have a unique morphological feature: they consist of a proximal sheath and a distal blade separated by a ligular region. The sheath provides structural support and protects young developing leaves, whereas the main function of the blade is photosynthesis. The auricles allow the blade to tilt back for optimal photosynthesis and determine the angle of a leaf, whereas the ligule protects the stem from the entry of water, microorganisms, and pests. Liguleless variants have an upright leaf blade that wraps around the culm. Research on liguleless mutants of maize and other cereals has led to identification of genes that are involved in leaf patterning and differentiation.

Results

We characterized an induced liguleless mutant (LM) of Aegilops tauschii Coss., a donor of genome D of bread wheat Triticum aestivum L.. The liguleless phenotype of LM is under dominant monogenic control (Lg^t). To determine precise position of Lg^t on the Ae. tauschii genetic map, highly saturated genetic maps were constructed containing 887 single-nucleotide polymorphism (SNP) markers derived via diversity arrays technology (DArT)seq. The Lg^t gene was mapped to chromosome 5DS. Taking into account coordinates of the SNP markers, flanking Lg^t, on the pseudomolecule 5D, a chromosomal region that contains this gene was determined, and a list of candidate genes was identified. Morphological features of the LM phenotype suggest that Lg^t participates in the control of leaf development, mainly, in leaf proximal–distal patterning, and its dominant mutation causes abnormal ligular region but does not affect reproductive development.

Conclusions

Here we report characterization of a liguleless Ae. tauschii mutant, whose phenotype is under control of a dominant mutation of Lg^t. The dominant mode of inheritance of the liguleless trait in a Triticeae species is reported for the first time. The position of the Lg^t locus on chromosome 5DS allowed us to identify a list of candidate genes. This list does not contain Ae. tauschii orthologs of any well-characterized cereal genes whose mutations cause liguleless phenotypes. Thus, the characterized Lg^t mutant represents a new model for further investigation of plant leaf patterning and differentiation.

     

Categories of resistance to greenbug (Homoptera: Aphididae) biotype K in wheat lines containing Aegilops tauschii genes

The wheat lines (cultivars) 'Largo', 'TAM110', 'KS89WGRC4', and 'KSU97-85-3' conferring resistance to greenbug, Schizaphis graminum (Rondani), biotypes E, I, and K were evaluated to determine the categories of resistance in each line to greenbug biotype K. Our results indicated that Largo, TAM110, KS89WGRC4, and KSU97-85-3 expressed both antibiosis and tolerance to biotype K. Largo, KS89WGRC4, and KSU97-85-3, which express antixenosis to biotype I, did not demonstrate antixenosis to biotype K. The results indicate that the same wheat lines may possess different categories of resistance to different greenbug biotypes. A new cage procedure for measuring greenbug intrinsic rate of increase (r(m)) was developed, by using both drinking straw and petri dish cages, to improve the efficiency and accuracy of r(m)-based antibiosis measurements.     

A high-density genetic linkage map of Aegilops tauschii, the D-genome progenitor of bread wheat

 Aegilops tauschii is the diploid D-genome progenitor of bread wheat (Triticum aestivum L. em Thell, 2n=6x=42, AABBDD). A genetic linkage map of the Ae. tauschii genome was constructed, composed of 546 loci. One hundred and thirty two loci (24%) gave distorted segregation ratios. Sixty nine probes (13%) detected multiple copies in the genome. One hundred and twenty three of the 157 markers shared between the Ae. tauschii genetic and T. aestivum physical maps were colinear. The discrepancy in the order of five markers on the Ae. tauschii 3DS genetic map versus the T. aestivum 3D physical map indicated a possible inversion. Further work is needed to verify the discrepancies in the order of markers on the 4D, 5D and 7D Ae. tauschii genetic maps versus the physical and genetic maps of T. aestivum. Using common markers, 164 agronomically important genes were assigned to specific regions on Ae. tauschii linkage, and T. aestivum physical, maps. This information may be useful for map-based cloning and marker-assisted plant breeding. Received: 23 March 1998 / Accepted: 27 October 1998     

Seed protein and isozyme variations in Triticum tauschii (Aegilops squarrosa)

Sixty Triticum tauschii (Aegilops squarrosa, 2n=2x=14, DD) accessions were evaluated for the variability of high-molecular-weight (HMW) glutenins, gliadins and isozymes of seed esterase, -amylase and glucose-phosphate isomerase. Wide variability was observed for HMW-glutenins and gliadins. The implications of unique HMW-glutenin alleles for quality parameters are discussed. Isozyme evaluations indicated more variability for the Est-D ^t 5 locus as compared to the Est-D5 of bread-wheat. The polymorphism for -Amy-D ^t 1 was less than that of -Amy-D1. Similar to the bread-wheat situation, Gpi-D ^t 1 showed no polymorphism. The variability observed with the traits evaluated can be readily observed in T. turgidum x T. tauschii synthetic hexaploids (2n=6x=42, AABBDD) suggesting that T. tauschii accessions may be a rich source for enhancing the genetic variability of T. aestivum cultivars.     

Polycomb proteins regulate the quantitative induction of VERNALIZATION INSENSITIVE 3 in response to low temperatures

Vernalization, the promotion of flowering in response to low temperatures, is one of the best characterized examples of epigenetic regulation in plants. The promotion of flowering is proportional to the duration of the cold period, but the mechanism by which plants measure time at low temperatures has been a long‐standing mystery. We show that the quantitative induction of the first gene in the Arabidopsis vernalization pathway, VERNALIZATION INSENSITIVE 3 (VIN3), is regulated by the components of Polycomb Response Complex 2, which trimethylates histone H3 lysine 27 (H3K27me3). In differentiated animal cells, H3K27me3 is mostly associated with long‐term gene repression, whereas, in pluripotent embyonic stem cells, many cell lineage‐specific genes are inactive but exist in bivalent chromatin that carries both active (H3K4me3) and repressive (H3K27me3) marks on the same molecule. During differentiation, bivalent domains are generally resolved to an active or silent state. We found that H3K27me3 maintains VIN3 in a repressed state prior to cold exposure; this mark is not removed during VIN3 induction. Instead, active VIN3 is associated with bivalently marked chromatin. The continued presence of H3K27me3 ensures that induction of VIN3 is proportional to the duration of the cold, and that plants require prolonged cold to promote the transition to flowering. The observation that Polycomb proteins control VIN3 activity defines a new role for Polycomb proteins in regulating the rate of gene induction.