Genetic basis for spontaneous hybrid genome doubling during allopolyploid speciation of common wheat shown by natural variation analyses of the paternal species

Rmg10, a Novel Wheat Blast Resistance Gene Derived from Aegilops tauschii

Wheat blast, caused by Pyricularia oryzae (syn. Magnaporthe oryzae) pathotype Triticum (MoT), is a devastating disease that can result in up to 100% yield loss in affected fields. To find new resistance genes against wheat blast, we screened 199 accessions of Aegilops tauschii, the D genome progenitor of common wheat (Triticum aestivum), by seedling inoculation assays with Brazilian MoT isolate Br48 and found 14 resistant accessions. A synthetic hexaploid wheat line (Ldn/KU-2097) derived from a cross between the T. turgidum 'Langdon' (Ldn) and resistant A. tauschii accession KU-2097 exhibited resistance in seedlings and spikes against Br48. In an F2 population derived from 'Chinese Spring' × Ldn/KU-2097, resistant and susceptible individuals segregated in a 3:1 ratio, suggesting that the resistance from KU-2097 is controlled by a single dominant gene. We designated this gene Rmg10. Genetic mapping using an F2:3 population from the same cross mapped the RMG10 locus to the short arm of chromosome 2D. Rmg10 was ineffective against Bangladesh isolates but effective against Brazilian isolates. Field tests in Bolivia showed increased spike resistance in a synthetic octaploid wheat line produced from a cross between common wheat cultivar 'Gladius' and KU-2097. These results suggest that Rmg10 would be beneficial in farmers' fields in South America.

Wild grass-derived alleles represent a genetic architecture for the resilience of modern common wheat to stresses

This review explores the integration of wild grass-derived alleles into modern bread wheat breeding to tackle the challenges of climate change and increasing food demand. With a focus on synthetic hexaploid wheat, this review highlights the potential of genetic variability in wheat wild relatives, particularly Aegilops tauschii, for improving resilience to multifactorial stresses like drought, heat, and salinity. The evolutionary journey of wheat (Triticum spp.) from diploid to hexaploid species is examined, revealing significant genetic contributions from wild grasses. We also emphasize the importance of understanding incomplete lineage sorting in the genomic evolution of wheat. Grasping this information is crucial as it can guide breeders in selecting the appropriate alleles from the gene pool of wild relatives to incorporate into modern wheat varieties. This approach improves the precision of phylogenetic relationships and increases the overall effectiveness of breeding strategies. This review also addresses the challenges in utilizing the wheat wild genetic resources, such as the linkage drag and cross-compatibility issues. Finally, we culminate the review with future perspectives, advocating for a combined approach of high-throughput phenotyping tools and advanced genomic techniques to comprehensively understand the genetic and regulatory architectures of wheat under stress conditions, paving the way for more precise and efficient breeding strategies.

Enhancing Wheat Flour Quality Through Introgression of High-Molecular-Weight Glutenin Subunits From Aegilops tauschii Accessions

Narrow genetic diversity in the wheat gene pool restricts the improvement of wheat quality traits. Aegilops tauschii possesses valuable genetic diversity that can be used to improve not only biotic and abiotic stresses in arid regions but also wheat yield and quality. Our study, which used 392 multiple synthetic derivatives (MSD) panel developed with Ae. tauschii Coss. introgressions, had three main aims: to explore the genetic diversity of high-molecular-weight glutenin subunits (HMW-GS), to investigate the dough strength and the relationship between protein content and grain yield, and to identify lines with a good flour quality. A wide range of allelic diversity was observed at the Glu-D1 locus, reflecting the impact of the different introgressed portions of Ae. tauschii, and a wide variation was found in dough strength even between lines having the same composition of HMW-GS. We report a negative impact on dough strength of subunit 5t+10t from Ae. tauschii and a relatively positive impact of subunit 2t+12.1t. We identified four MSD lines with significantly enhanced flour quality. Regressing the grain yield of the MSD lines against protein content showed no correlation between the two traits and identified lines with comparable grain yield to the recurrent parent and higher protein content. The identified MSD lines could provide a valuable genetic resource for enhancing the end-use quality of flour without any loss in productivity.

Genomic and Meiotic Changes Accompanying Polyploidization

Hybridization and polyploidy have been considered as significant evolutionary forces in adaptation and speciation, especially among plants. Interspecific gene flow generates novel genetic variants adaptable to different environments, but it is also a gene introgression mechanism in crops to increase their agronomical yield. An estimate of 9% of interspecific hybridization has been reported although the frequency varies among taxa. Homoploid hybrid speciation is rare compared to allopolyploidy. Chromosome doubling after hybridization is the result of cellular defects produced mainly during meiosis. Unreduced gametes, which are formed at an average frequency of 2.52% across species, are the result of altered spindle organization or orientation, disturbed kinetochore functioning, abnormal cytokinesis, or loss of any meiotic division. Meiotic changes and their genetic basis, leading to the cytological diploidization of allopolyploids, are just beginning to be understood especially in wheat. However, the nature and mode of action of homoeologous recombination suppressor genes are poorly understood in other allopolyploids. The merger of two independent genomes causes a deep modification of their architecture, gene expression, and molecular interactions leading to the phenotype. We provide an overview of genomic changes and transcriptomic modifications that particularly occur at the early stages of allopolyploid formation.

Population genomic analysis of Aegilops tauschii identifies targets for bread wheat improvement

Aegilops tauschii, the diploid wild progenitor of the D subgenome of bread wheat, is a reservoir of genetic diversity for improving bread wheat performance and environmental resilience. Here we sequenced 242 Ae. tauschii accessions and compared them to the wheat D subgenome to characterize genomic diversity. We found that a rare lineage of Ae. tauschii geographically restricted to present-day Georgia contributed to the wheat D subgenome in the independent hybridizations that gave rise to modern bread wheat. Through k-mer-based association mapping, we identified discrete genomic regions with candidate genes for disease and pest resistance and demonstrated their functional transfer into wheat by transgenesis and wide crossing, including the generation of a library of hexaploids incorporating diverse Ae. tauschii genomes. Exploiting the genomic diversity of the Ae. tauschii ancestral diploid genome permits rapid trait discovery and functional genetic validation in a hexaploid background amenable to breeding.

New insights into the dispersion history and adaptive evolution of taxon Aegilops tauschii in China

Aegilops tauschii, the wild progenitor of wheat D-genome and a valuable germplasm for wheat improvement, has a wide natural distribution from eastern Turkey to China. However, the phylogenetic relationship and dispersion history of Ae. tauschii in China has not been scientifically clarified. In this study, we genotyped 208 accessions (with 104 in China) using ddRAD sequencing and 55K SNP array, and classified the population into six sublineages. Three possible spreading routes or events were identified, resulting in specific distribution patterns, with four sublineages found in Xinjiang, one in Qinghai, two in Shaanxi and one in Henan. We also established the correlation of SNP-based, karyotype-based and spike-morphology-based techniques to demonstrate the internal classification of Ae. tauschii, and developed consensus dataset with 1245 putative accessions by merging data previously published. Our analysis suggested that eight inter-lineage accessions could be assigned to the putative Lineage 3 and these accessions would help to conserve the genetic diversity of the species. By developing the consensus phylogenetic relationships of Ae. tauschii, our work validated the hypothesis on the dispersal history of Ae. tauschii in China, and contributed to the efficient and comprehensive germplasm-mining of the species.

Origin and Dynamics of Rwt6, a Wheat Gene for Resistance to Nonadapted Pathotypes of Pyricularia oryzae

Avirulence of Eleusine isolates of Pyricularia oryzae on common wheat is conditioned by at least five avirulence genes. One is PWT3 corresponding to resistance gene Rwt3 located on chromosome 1D. We identified a resistance gene corresponding to a second avirulence gene, PWT6, and named it Rmg9 (Rwt6). Rwt6 was closely linked to Rwt3. A survey of the population of Aegilops tauschii, the D genome donor to common wheat, revealed that some accessions from the southern coastal region of the Caspian Sea, the birthplace of common wheat, carried both genes. Rwt6 and Rwt3 carriers accounted for 65 and 80%, respectively, of accessions in a common wheat landrace collection. The most likely explanation of our results is that both resistance genes were simultaneously introduced into common wheat at the time of hybridization of Triticum turgidum and A. tauschii. However, a prominent difference was recognized in their geographical distributions in modern wheat; Rwt3 and Rwt6 co-occurred at high frequencies in regions to the east of the Caspian Sea, whereas Rwt6 occurred at a lower frequency than Rwt3 in regions to the west. This difference was considered to be associated with range of pathotypes to which these genes were effective. A. tauschii accessions carrying Rwt3 and Rwt6 also carried Rwt4, another resistance gene involved in the species specificity. We suggest that the gain of the D genome should have given an adaptive advantage to the genus Triticum by conferring disease resistance.

Genome-Wide Association Study of Morpho-Physiological Traits in Aegilops tauschii to Broaden Wheat Genetic Diversity

Aegilops tauschii, the D-genome donor of bread wheat, is a storehouse of genetic diversity that can be used for wheat improvement. This species consists of two main lineages (TauL1 and TauL2) and one minor lineage (TauL3). Its morpho-physiological diversity is large, with adaptations to a wide ecological range. Identification of allelic diversity in Ae. tauschii is of utmost importance for efficient breeding and widening of the genetic base of wheat. This study aimed at identifying markers or genes associated with morpho-physiological traits in Ae. tauschii, and at understanding the difference in genetic diversity between the two main lineages. We performed genome-wide association studies of 11 morpho-physiological traits of 343 Ae. tauschii accessions representing the entire range of habitats using 34,829 DArTseq markers. We observed a wide range of morpho-physiological variation among all accessions. We identified 23 marker-trait associations (MTAs) in all accessions, 15 specific to TauL1 and eight specific to TauL2, suggesting independent evolution in each lineage. Some of the MTAs could be novel and have not been reported in bread wheat. The markers or genes identified in this study will help reveal the genes controlling the morpho-physiological traits in Ae. tauschii, and thus in bread wheat even if the plant morphology is different.

Reproductive and genetic roles of the maternal progenitor in the origin of common wheat (Triticum aestivum L.)

Common wheat (Triticum aestivum L., AABBDD genome) is thought to have emerged through natural hybridization between Triticum turgidum L. (AABB genome) and Aegilops tauschii Coss. (DD genome). Hybridization barriers and doubling of the trihaploid F1 hybrids' genome (ABD) via unreduced gamete fusion had key roles in the process. However, how T. turgidum, the maternal progenitor, was involved in these mechanisms remains unknown. An artificial cross-experiment using 46 cultivated and 31 wild T. turgidum accessions and a single Ae. tauschii tester with a very short genetic distance to the common wheat D genome was conducted. Cytological and quantitative trait locus analyses of F1 hybrid genome doubling were performed. The crossability and ability to cause hybrid inviability did not greatly differ between the cultivars and wild accessions. The ability to cause hybrid genome doubling was higher in the cultivars. Three novel T. turgidum loci for hybrid genome doubling, which influenced unreduced gamete production in F1 hybrids, were identified. Cultivated T. turgidum might have increased the probability of the emergence of common wheat through its enhanced ability to cause genome doubling in F1 hybrids with Ae. tauschii. The ability enhancement might have involved alterations at a relatively small number of loci.

Development and characterization of Triticum turgidum - Aegilops comosa and T. turgidum - Ae. markgrafii amphidiploids

Aegilops comosa and Ae. markgrafii are diploid progenitors of polyploidy species of Aegilops sharing M and C genomes, respectively. Transferring valuable genes/traits from Aegilops into wheat is an alternative strategy for wheat genetic improvement. The amphidiploids between diploid species of Aegilops and tetraploid wheat can act as bridges to overcome obstacles from direct hybridization and can be developed by the union of unreduced gametes. In this study, we developed seven Triticum turgidum - Ae. comosa and two T. turgidum - Ae. markgrafii amphidiploids. The unreduced gametes mechanisms, including first-division restitution (FDR) and single-division meiosis (SDM), were observed in triploid F₁ hybrids of T. turgidum - Ae. comosa (STM) and T. turgidum - Ae. markgrafii (STC). Only FDR was observed in STC hybrids, whereas FDR or both FDR and SDM were detected in the STM hybrids. All seven pairs of M chromosomes of Ae. comosa and C chromosomes of Ae. markgrafii were distinguished by fluorescent in situ hybridization (FISH) probes pSc119.2 and pTa71 combinations with pTa-535 and (CTT)₁₂/(ACT)₇, respectively. Meanwhile, the chromosomes of tetraploid wheat and diploid Aegilops parents were distinguished by the same FISH probes. The amphidiploids possessed specific valuable traits such as multiple tillers, large seed size related traits, and stripe rust resistance that could be utilized in the genetic improvement of wheat.

An Update of Recent Use of Aegilops Species in Wheat Breeding

Aegilops species have significantly contributed to wheat breeding despite the difficulties involved in the handling of wild species, such as crossability and incompatibility. A number of biotic resistance genes have been identified and incorporated into wheat varieties from Aegilops species, and this genus is also contributing toward improvement of complex traits such as yield and abiotic tolerance for drought and heat. The D genome diploid species of Aegilops tauschii has been utilized most often in wheat breeding programs. Other Aegilops species are more difficult to utilize in the breeding because of lower meiotic recombination frequencies; generally they can be utilized only after extensive and time-consuming procedures in the form of translocation/introgression lines. After the emergence of Ug99 stem rust and wheat blast threats, Aegilops species gathered more attention as a form of new resistance sources. This article aims to update recent progress on Aegilops species, as well as to cover new topics around their use in wheat breeding.

Hybrid incompatibilities in interspecific crosses between tetraploid wheat and its wild diploid relative Aegilops umbellulata

Hybrid abnormalities, severe growth abortion and grass-clump dwarfism, were found in the tetraploid wheat/Aegilops umbellulata hybrids, and the gene expression changes were conserved in the hybrids with those in other wheat synthetic hexaploids. Aegilops umbellulata Zhuk., a diploid goatgrass species with a UU genome, has been utilized as a genetic resource for wheat breeding. Here, we examine the reproductive barriers between tetraploid wheat cultivar Langdon (Ldn) and various Ae. umbellulata accessions by conducting interspecific crossings. Through systematic cross experiments, three types of hybrid incompatibilities were found: seed production failure in crosses, hybrid growth abnormalities and sterility in the ABU hybrids. Hybrid incompatibilities were widely distributed over the entire range of the natural species, and in about 50% of the cross combinations between tetraploid Ldn and Ae. umbellulata accessions, ABU F₁ hybrids showed one of two abnormal growth phenotypes: severe growth abortion (SGA) or grass-clump dwarfism. Expression of the shoot meristem maintenance-related and cell cycle-related genes was markedly repressed in crown tissues of hybrids showing SGA, suggesting dysfunction of mitotic cell division in the shoot apices. The grass-clump dwarf phenotype may be explained by down-regulation of wheat APETALA1-like MADS box genes, which act as flowering promoters, and altered expression in crown tissues of the miR156/SPLs module, which controls tiller number and branching. These gene expression changes in growth abnormalities were well conserved between the Ldn/Ae. umbellulata plants and interspecific hybrids from crosses of Ldn and wheat D-genome progenitor Ae. tauschii.

The role of reproductive isolation in allopolyploid speciation patterns: empirical insights from the progenitors of common wheat

The ability to cause reproductive isolation often varies among individuals within a plant species. We addressed whether such polymorphism influenced speciation of the allopolyploid common wheat (Triticum aestivum L., AABBDD genome) by evaluating the expression of pre-pollination (outcrossing potential) and post-pollination (crossability) barriers in Aegilops tauschii Coss. (the D genome progenitor). In total, 201 Ae. tauschii accessions representing the entire natural habitat range of the species were used for anther length measurement and artificial crosses with a Triticum turgidum L. (the AB genome progenitor) tester. Intraspecific comparisons showed that both barriers were more strongly expressed in the TauL1 lineage than in the TauL2 lineage. The ability of Ae. tauschii to cause reproductive isolation in the hybridisation with T. turgidum might have markedly influenced common wheat’s speciation by inducing lineage-associated patterns of gene flow. The TauL2 accessions with high potential for natural hybridisation with T. turgidum clustered in the southern coastal Caspian region. This provided phenotypic support for the derivation of the D genome of common wheat from southern Caspian populations. The present study underscored the importance of approaches that incorporate the genealogical and geographic structure of the parental species’ reproductive isolation in understanding the mechanism of plant allopolyploid speciation.

Genetic mapping reveals a dominant awn-inhibiting gene related to differentiation of the variety anathera in the wild diploid wheat Aegilops tauschii

Aegilops tauschii, a wild wheat relative, is the D-genome donor of common wheat. Subspecies and varieties of Ae. tauschii are traditionally classified based on differences in their inflorescence architecture. However, the genetic information for their diversification has been quite limited in the wild wheat relatives. The variety anathera has no awn on the lemma, but the genetic basis for this diagnostic character is unknown. Wide variations in awn length traits at the top and middle spikes were found in the Ae. tauschii core collection, and the awn length at the middle spike was significantly smaller in the eastward-dispersed sublineage than in those in other sublineages. To clarify loci controlling the awnless phenotype of var. anathera, we measured awn length of an intervariety F₂ mapping population, and found that the F₂ individuals could be divided into two groups mainly based on the awn length at the middle of spike, namely short and long awn groups, significantly fitting a 3:1 segregation ratio, which indicated that a single locus controls the awnless phenotype. The awnless locus, Anathera (Antr), was assigned to the distal region of the short arm of chromosome 5D. Quantitative trait locus analysis using the awn length data of each F₂ individual showed that only one major locus was at the same chromosomal position as Antr. These results suggest that a single dominant allele determines the awnless diagnostic character in the variety anathera. The Antr dominant allele is a novel gene inhibiting awn elongation in wheat and its relatives.

Quantitative trait locus analysis for spikelet shape-related traits in wild wheat progenitor Aegilops tauschii: Implications for intraspecific diversification and subspecies differentiation

Wild diploid wheat Aegilops tauschii, the D-genome progenitor of common wheat, carries large genetic variation in spikelet and grain morphology. Two differentiated subspecies of Ae. tauschii, subspecies tauschii and strangulata, have been traditionally defined based on differences in spikelet morphology. Here, we first assessed six spikelet shape-related traits among 199 Ae. tauschii accessions, and found that the accessions belonging to TauL1major lineage produced significantly longer spikes, higher spikelet density, and shorter, narrower spikelets than another major lineage, TauL2, in which the strangulata accessions are included. Next, we performed quantitative trait locus (QTL) analysis of the spikelet and grain shape using three mapping populations derived from interlineage crosses between TauL1 and TauL2 to identify the genetic loci for the morphological variations of the spikelet and grain shape in Ae. tauschii. Three major QTL regions for the examined traits were detected on chromosomes 3D, 4D and 7D. The 3D and 4D QTL regions for several spikelet shape-related traits were conserved in the three mapping populations, which indicated that the 3D and 4D QTLs contribute to divergence of the two major lineages. The 7D QTLs were found only in a mapping population from a cross of the two subspecies, suggesting that these 7D QTLs may be closely related to subspecies differentiation in Ae. tauschii. Thus, QTL analysis for spikelet and grain morphology may provide useful information to elucidate the evolutionary processes of intraspecific differentiation.

Salt tolerance during germination and seedling growth of wild wheat Aegilops tauschii and its impact on the species range expansion

Adaptation to edaphic stress may have a key role in plant species range expansion. Aegilops tauschii Coss., the common wheat’s D-genome progenitor native to the Transcaucasus-Middle East region, is a good model to study the relationships between soil salinity and plant distributions: one of its intraspecific sublineages, TauL1b, drove the long-distance eastward expansion of this species range reaching semi-arid-central Asia. Salt tolerance during germination and seedling growth was evaluated in 206 Ae. tauschii accessions by treating seeds with NaCl solutions differing in concentrations. Differences in natural variation patterns were analyzed between sublineages and associated with natural edaphic condition variables, and then compared with reproductive trait variation patterns. The natural variations observed in NaCl-induced-stress tolerance had clear geographic and genetic structure. Seedling growth significantly increased in the TauL1b accessions that were collected from salt-affected soil habitats, whereas germinability did not. Principal component analysis suggested that the NaCl-induced-stress tolerances and reproductive traits might have had a similar degree of influence on Ae. tauschii’s eastward range expansion. Adaptation to salt-affected soils through increased seedling growth was an important factor for the species’ successful colonization of the semi-arid central Asian habitats. TauL1b accessions might provide useful genetic resources for salt-tolerant wheat breeds.

Fine mapping and genetic association analysis of Net2, the causative D-genome locus of low temperature-induced hybrid necrosis in interspecific crosses between tetraploid wheat and Aegilops tauschii

Hybrid necrosis has been observed in many interspecific hybrids from crosses between tetraploid wheat and the wheat D-genome donor Aegilops tauschii. Type II necrosis is a kind of hybrid incompatibility that is specifically characterized by low-temperature induction and growth suppression. Two complementary genes, Net1 on the AB genome and Net2 on the D genome, putatively control type II necrosis in ABD triploids and synthetic hexaploid wheat. Toward map-based cloning of Net2, a fine map around the Net2 region on 2DS was constructed in this study. Using the draft genome sequence of Ae. tauschii and the physical map of the barley genome, the Net2 locus was mapped within a 0.6 cM interval between two closely linked markers. Although local chromosomal rearrangements were observed in the Net2-corresponding region between the barley/Brachypodium and Ae. tauschii genomes, the two closely linked markers were significantly associated with type II necrosis in Ae. tauschii. These results suggest that these markers will aid efficient selection of Net2 non-carrier individuals from the Ae. tauschii population and intraspecific progeny, and could help with introgression of agriculturally important genes from Ae. tauschii to common wheat.

Quantitative trait locus analysis for flowering-related traits using two F2 populations derived from crosses between Japanese common wheat cultivars and synthetic hexaploids

Flowering time is an important trait for Japanese wheat breeding. Aegilops tauschii, the D-genome donor of hexaploid wheat, is a useful resource to enlarge the D-genome diversity of common wheat. Previously, we identified flowering-related QTLs in F2 populations of synthetic hexaploid wheat lines between the tetraploid wheat cultivar Langdon and Ae. tauschii accessions. Here, to evaluate the usefulness of the early-flowering alleles from Ae. tauschii for Japanese wheat breeding, QTL analyses were conducted in two F2 populations derived from crosses between Japanese wheat cultivars and early-flowering lines of synthetic hexaploid wheat. Only two chromosomal regions controlling flowering-related traits were identified, on chromosomes 2DS and 5AL in the mapping populations, and no previously identified QTLs were found in the synthetic hexaploid lines. The strong effect of the 2DS QTL, putatively corresponding to Ppd-D1, was considered to hide any significant expression of other QTLs with small effects on flowering-related traits. When F2 individuals carrying Ae. tauschii-homozygous alleles around the 2DS QTL region were selected, the Ae. tauschii-derived alleles of the previously identified flowering QTLs partly showed an early-flowering phenotype compared with the Japanese wheat-derived alleles. Thus, some early-flowering alleles from Ae. tauschii may be useful for production of early-flowering Japanese wheat cultivars.

Sister chromatid separation and monopolar spindle organization in the first meiosis as two mechanisms of unreduced gametes formation in wheat-rye hybrids

KEY MESSAGE

Unreduced gametes. The absence of a strict pachytene checkpoint in plants presents an opportunity to study meiosis in polyhaploid organisms. In the present study, we demonstrate that meiosis is coordinated in hybrids between disomic wheat-rye substitution lines 1Rv(1A), 2R(2D), 5R(5D), 6R(6A) and rye (Triticum aestivum L. × Secale cereale L., 4x = 28, ABDR). By using in situ hybridization with a centromere pAet6-09 probe and immunostaining with H3Ser10ph-, CENH3-, and α-tubulin-specific antibodies, we distinguished four chromosome behaviour types. The first one is a mitotic-like division that is characterized by mitotic centromere architecture, robust bipolar spindle, one-step loss of arm and centromere cohesion, and sister chromatid separation in the first and only meiotic division. The second type involves a monopolar spindle formation, which appears as a hat-shaped group of chromosomes moving in one direction, wherein MT bundles are co-oriented polewards. It prevents chromosome segregation in meiosis I, with a bipolar spindle distributing sister chromatids to the poles in meiosis II. These events subsequently result in the formation of unreduced microspores. The other two meiotic-like chromosome segregation patterns known as reductional and equational plus reductional represent stand-alone types of cell division rather than intermediate steps of meiosis I. Only sterile pollen is produced as a result of such meiotic-like chromosome behaviours. Slightly variable meiotic phenotypes are reproducibly observed in hybrids under different growth conditions. The 2R(2D)xR genotype tends to promote reductional division. In contrast, the genotypes 1Rv(1A)xR, 5R(5D)xR, and 6R(6A)xR promote equational chromosome segregation and monopolar spindle formation in addition to reductional and equational plus reductional division types.

Unreduced gamete formation in wheat × Aegilops spp. hybrids is genotype specific and prevented by shared homologous subgenomes

The presence of homologous subgenomes inhibited unreduced gamete formation in wheat × Aegilops interspecific hybrids. Unreduced gamete rates were under the control of the wheat nuclear genome. Production of unreduced gametes is common among interspecific hybrids, and may be affected by parental genotypes and genomic similarity. In the present study, five cultivars of Triticum aestivum and two tetraploid Aegilops species (i.e. Ae. triuncialis and Ae. cylindrica) were reciprocally crossed to produce 20 interspecific hybrid combinations. These hybrids comprised two different types: T. aestivum × Aegilops triuncialis; 2n = ABDU(t)C(t) (which lack a common subgenome) and T. aestivum × Ae. cylindrica; 2n = ABDD(c)C(c) (which share a common subgenome). The frequency of unreduced gametes in F1 hybrids was estimated in sporads from the frequency of dyads, and the frequency of viable pollen, germinated pollen and seed set were recorded. Different meiotic abnormalities recorded in the hybrids included precocious chromosome migration to the poles at metaphase I and II, laggards in anaphase I and II, micronuclei and chromosome stickiness, failure in cell wall formation, premature cytokinesis and microspore fusion. The mean frequency of restitution meiosis was 10.1 %, and the mean frequency of unreduced viable pollen was 4.84 % in T. aestivum × Ae. triuncialis hybrids. By contrast, in T. aestivum × Ae. cylindrica hybrids no meiotic restitution was observed, and a low rate of viable gametes (0.3 %) was recorded. This study present evidence that high levels of homologous pairing between the D and D(c) subgenomes may interfere with meiotic restitution and the formation of unreduced gametes. Variation in unreduced gamete production was also observed between T. aestivum × Ae. triuncialis hybrid plants, suggesting genetic control of this trait.

Complete chloroplast genomes of Aegilops tauschii Coss. and Ae. cylindrica Host sheds light on plasmon D evolution

Hexaploid wheat (Triticum aestivum L., genomes AABBDD) originated in South Caucasus by allopolyploidization of the cultivated Emmer wheat T. dicoccum (genomes AABB) with the Caucasian Ae. tauschii ssp strangulata (genomes DD). Genetic variation of Ae. tauschii is an important natural resource, that is why it is of particular importance to investigate how this variation was formed during Ae. tauschii evolutionary history and how it is presented through the species area. The D genome is also found in tetraploid Ae. cylindrica Host (2n = 28, CCDD). The plasmon diversity that exists in Triticum and Aegilops species is of great significance for understanding the evolution of these genera. In the present investigation the complete nucleotide sequence of plasmon D (chloroplast DNA) of nine accessions of Ae. tauschii and two accessions of Ae. cylindrica are presented. Twenty-eight SNPs are characteristic for both TauL1 and TauL2 accessions of Ae. tauschii using TauL3 as a reference. Four SNPs are additionally observed for TauL2 lineage. The longest (27 bp) indel is located in the intergenic spacer Rps15-ndhF of SSC. This indel can be used for simple determination of TauL3 lineage among Ae. tauschii accessions. In the case of Ae. cylindrica additionally 7 SNPs were observed. The phylogeny tree shows that chloroplast DNA of TauL1 and TauL2 diverged from the TauL3 lineage. TauL1 lineage is relatively older then TauL2. The position of Ae. cylindrica accessions on Ae. tauschii phylogeny tree constructed on chloroplast DNA variation data is intermediate between TauL1 and TauL2. The complete nucleotide sequence of chloroplast DNA of Ae. tauschii and Ae. cylindrica allows to refine the origin and evolution of D plasmon of genus Aegilops.

Intraspecific lineage divergence and its association with reproductive trait change during species range expansion in central Eurasian wild wheat Aegilops tauschii Coss. (Poaceae)

Background

How species ranges form in landscapes is a matter of long-standing evolutionary interest. However, little is known about how natural phenotypic variations of ecologically important traits contribute to species range expansion. In this study, we examined the phylogeographic patterns of phenotypic changes in life history (seed production) and phenological (flowering time) traits during the range expansion of Aegilops tauschii Coss. from the Transcaucasus and Middle East to central Asia.

Results

Our comparative analyses of the patterns of natural variations for those traits and their association with the intraspecific lineage structure showed that (1) the eastward expansion to Asia was driven by an intraspecific sublineage (named TauL1b), (2) high seed production ability likely had an important role at the initial dispersal stage of TauL1b’s expansion to Asia, and (3) the phenological change to early flowering phenotypes was one of the key adaptation events for TauL1b to further expand its range in Asia.

Conclusions

This study provides for the first time a broad picture of the process of Ae. tauschii’s eastward range expansion in which life history and phenological traits may have had respective roles in its dispersal and adaptation in Asia. The clear association of seed production and flowering time patterns with the intraspecific lineage divergence found in this study invites further genetic research to bring the mechanistic understanding of the changes in these key functional traits during range expansion within reach.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0496-9) contains supplementary material, which is available to authorized users.

Accelerated senescence and enhanced disease resistance in hybrid chlorosis lines derived from interspecific crosses between tetraploid wheat and Aegilops tauschii

Hybrid chlorosis, a type of hybrid incompatibility, has frequently been reported in inter- and intraspecific crosses of allopolyploid wheat. In a previous study, we reported some types of growth abnormalities such as hybrid necrosis and observed hybrid chlorosis with mild or severe abnormalities in wheat triploids obtained in crosses between tetraploid wheat cultivar Langdon and four Ae. tauschii accessions and in their derived synthetic hexaploids. However, the molecular mechanisms underlying hybrid chlorosis are not well understood. Here, we compared cytology and gene expression in leaves to characterize the abnormal growth in wheat synthetics showing mild and severe chlorosis. In addition, we compared disease resistance to wheat blast fungus. In total 55 and 105 genes related to carbohydrate metabolism and 53 and 89 genes for defense responses were markedly up-regulated in the mild and severe chlorosis lines, respectively. Abnormal chloroplasts formed in the mesophyll cells before the leaves yellowed in the hybrid chlorosis lines. The plants with mild chlorosis showed increased resistance to wheat blast and powdery mildew fungi, although significant differences only in two, third internode length and maturation time, out of the examined agricultural traits were found between the wild type and plants showing mild chlorosis. These observations suggest that senescence might be accelerated in hybrid chlorosis lines of wheat synthetics. Moreover, in wheat synthetics showing mild chlorosis, the negative effects on biomass can be minimized, and they may show substantial fitness under pathogen-polluted conditions.

Line differences in Cor/Lea and fructan biosynthesis-related gene transcript accumulation are related to distinct freezing tolerance levels in synthetic wheat hexaploids

In common wheat, cultivar differences in freezing tolerance are considered to be mainly due to allelic differences at two major loci controlling freezing tolerance. One of the two loci, Fr-2, is coincident with a cluster of genes encoding C-repeat binding factors (CBFs), which induce downstream Cor/Lea genes during cold acclimation. Here, we conducted microarray analysis to study comprehensive changes in gene expression profile under long-term low-temperature (LT) treatment and to identify other LT-responsive genes related to cold acclimation in leaves of seedlings and crown tissues of a synthetic hexaploid wheat line. The microarray analysis revealed marked up-regulation of a number of Cor/Lea genes and fructan biosynthesis-related genes under the long-term LT treatment. For validation of the microarray data, we selected four synthetic wheat lines that contain the A and B genomes from the tetraploid wheat cultivar Langdon and the diverse D genomes originating from different Aegilops tauschii accessions with distinct levels of freezing tolerance after cold acclimation. Quantitative RT-PCR showed increased transcript levels of the Cor/Lea, CBF, and fructan biosynthesis-related genes in more freezing-tolerant lines than in sensitive lines. After a 14-day LT treatment, a significant difference in fructan accumulation was observed among the four lines. Therefore, the fructan biosynthetic pathway is associated with cold acclimation in development of wheat freezing tolerance and is another pathway related to diversity in freezing tolerance, in addition to the CBF-mediated Cor/Lea expression pathway.

The cuticular wax inhibitor locus Iw2 in wild diploid wheat Aegilops tauschii: phenotypic survey, genetic analysis, and implications for the evolution of common wheat

BACKGROUND

Cuticular wax production on plant surfaces confers a glaucous appearance and plays important roles in plant stress tolerance. Most common wheat cultivars, which are hexaploid, and most tetraploid wheat cultivars are glaucous; in contrast, a wild wheat progenitor, Aegilops tauschii, can be glaucous or non-glaucous. A dominant non-glaucous allele, Iw2, resides on the short arm of chromosome 2D, which was inherited from Ae. tauschii through polyploidization. Iw2 is one of the major causal genes related to variation in glaucousness among hexaploid wheat. Detailed genetic and phylogeographic knowledge of the Iw2 locus in Ae. tauschii may provide important information and lead to a better understanding of the evolution of common wheat.

RESULTS

Glaucous Ae. tauschii accessions were collected from a broad area ranging from Armenia to the southwestern coastal part of the Caspian Sea. Linkage analyses with five mapping populations showed that the glaucous versus non-glaucous difference was mainly controlled by the Iw2 locus in Ae. tauschii. Comparative genomic analysis of barley and Ae. tauschii was then used to develop molecular markers tightly linked with Ae. tauschii Iw2. Chromosomal synteny around the orthologous Iw2 regions indicated that some chromosomal rearrangement had occurred during the genetic divergence leading to Ae. tauschii, barley, and Brachypodium. Genetic associations between specific Iw2-linked markers and respective glaucous phenotypes in Ae. tauschii indicated that at least two non-glaucous accessions might carry other glaucousness-determining loci outside of the Iw2 locus.

CONCLUSION

Allelic differences at the Iw2 locus were the main contributors to the phenotypic difference between the glaucous and non-glaucous accessions of Ae. tauschii. Our results supported the previous assumption that the D-genome donor of common wheat could have been any Ae. tauschii variant that carried the recessive iw2 allele.

QTug.sau-3B is a major quantitative trait locus for wheat hexaploidization

Meiotic nonreduction resulting in unreduced gametes is thought to be the predominant mechanism underlying allopolyploid formation in plants. Until now, however, its genetic base was largely unknown. The allohexaploid crop common wheat (Triticum aestivum L.), which originated from hybrids of T. turgidum L. with Aegilops tauschii Cosson, provides a model to address this issue. Our observations of meiosis in pollen mother cells from T. turgidum×Ae. tauschii hybrids indicated that first division restitution, which exhibited prolonged cell division during meiosis I, was responsible for unreduced gamete formation. A major quantitative trait locus (QTL) for this trait, named QTug.sau-3B, was detected on chromosome 3B in two T. turgidum×Ae. tauschii haploid populations. This QTL is situated between markers Xgwm285 and Xcfp1012 and covered a genetic distance of 1 cM in one population. QTug.sau-3B is a haploid-dependent QTL because it was not detected in doubled haploid populations. Comparative genome analysis indicated that this QTL was close to Ttam-3B, a collinear homolog of tam in wheat. Although the relationship between QTug.sau-3B and Ttam requires further study, high frequencies of unreduced gametes may be related to reduced expression of Ttam in wheat.

Genetic mechanisms of allopolyploid speciation through hybrid genome doubling: novel insights from wheat (Triticum and Aegilops) studies

Polyploidy, which arises through complex genetic and ecological processes, is an important mode of plant speciation. This review provides an overview of recent advances in understanding why plant polyploid species are so ubiquitous and diverse. We consider how the modern framework for understanding genetic mechanisms of speciation could be used to study allopolyploid speciation that occurs through hybrid genome doubling, that is, whole genome doubling of interspecific F1 hybrids by the union of male and female unreduced gametes. We outline genetic and ecological mechanisms that may have positive or negative impacts on the process of allopolyploid speciation through hybrid genome doubling. We also discuss the current status of studies on the underlying genetic mechanisms focusing on the wheat (Triticum and Aegilops) hybrid-specific reproductive phenomena that are well known but deserve renewed attention from an evolutionary viewpoint.