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.     

Photoperiod and Vernalization Responses in Triticum turgidumxT. tauschii Synthetic Hexaploid Wheats

Studies of synthetic hexaploid wheat developed from Triticumturgidum(AABB genomes) and T. tauschii(DD genome) can provideinformation on potentially useful characters in T. tauschiiand/or T. turgidum for genetic improvement of hexaploid wheat(T. aestivum). Synthetic hexaploid wheats and the T. turgidumand T. tauschii parents were assessed for their developmentalresponses to photoperiod and vernalization for days to ear emergence,final leaf number and the number of spikelets per spike. Theresponses to photoperiod and vernalization of the synthetichexaploids were generally intermediate between those of theparents but in some instances the levels of expression exhibitedby the T. tauschii or T. turgidum parents were epistatic inthe synthetic hexaploids. The relatively strong photoperiodresponse of the T. tauschii accessions was not expressed inthe synthetic hexaploids, but rather the synthetic hexaploidsreflected the photoperiod response of the respective T. turgidumparents. The synthetic hexaploids had vernalization responsesstronger than those of the T. turgidum and bread wheats usedin the study. The expression of ear emergence in response tovernalization of these synthetic hexaploids appeared to be modifiedby the T. turgidum parent. Copyright 2001 Annals of Botany Company Photoperiod, synthetic hexaploids, Triticum aestivum, Triticum tauschii, Triticum turgidum, vernalization     

Evolutionary History of Triticum petropavlovskyi Udacz. et Migusch. Inferred from the Sequences of the 3-Phosphoglycerate Kinase Gene

Single- and low-copy genes are less likely to be subject to concerted evolution. Thus, they are appropriate tools to study the origin and evolution of polyploidy plant taxa. The plastid 3-phosphoglycerate kinase gene (Pgk-1) sequences from 44 accessions of Triticum and Aegilops, representing diploid, tetraploid, and hexaploid wheats, were used to estimate the origin of Triticum petropavlovskyi. Our phylogenetic analysis was carried out on exon+intron, exon and intron sequences, using maximum likelihood, Bayesian inference and haplotype networking. We found the D genome sequences of Pgk-1 genes from T. petropavlovskyi are similar to the D genome orthologs in T. aestivum, while their relationship with Ae. tauschii is more distant. The A genome sequences of T. petropavlovskyi group with those of T. polonicum, but its Pgk-1 B genome sequences to some extent diverge from those of other species of Triticum. Our data do not support for the origin of T. petropavlovskyi either as an independent allopolyploidization event between Ae. tauschii and T. polonicum, or as a monomendelian mutation in T. aestivum. We suggest that T. petropavlovskyi originated via spontaneous introgression from T. polonicum into T. aestivum. The dating of this introgression indicates an age of 0.78 million years; a further mutation event concerning the B genome occurred 0.69 million years ago.     

Phylogeny and genetic diversity of D-genome species of Aegilops and Triticum (Triticeae, Poaceae) from Iran based on microsatellites, ITS, and trnL-F

Cereal species of the grass tribe Triticeae are economically important and provide staple food for large parts of the human population. The Fertile Crescent of Southwest Asia harbors high genetic and morphological diversity of these species. In this study, we analyzed genetic diversity and phylogenetic relationships among D genome-bearing species of the wheat relatives of the genus Aegilops from Iran and adjacent areas using allelic diversity at 25 nuclear microsatellite loci, nuclear rDNA ITS, and chloroplast trnL-F sequences. Our analyses revealed high microsatellite diversity in Aegilops tauschii and the D genomes of Triticum aestivum and Ae. ventricosa, low genetic diversity in Ae. cylindrica, two different Ae. tauschii gene pools, and a close relationship among Ae. crassa, Ae. juvenalis, and Ae. vavilovii. In the latter species group, cloned sequences revealed high diversity at the ITS region, while in most other polyploids, homogenization of the ITS region towards one parental type seems to have taken place. The chloroplast genealogy of the trnL-F haplotypes showed close relationships within the D genome Aegilops species and T. aestivum, the presence of shared haplotypes in up to three species, and up to three different haplotypes within single species, and indicates chloroplast capture from an unidentified species in Ae. markgrafii. The ITS phylogeny revealed Triticum as monophyletic and Aegilops as monophyletic when Amblyopyrum muticum is included.     

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     

Comparative RFLP mapping of Hordeum vulgare and Triticum tauschii

Hordeum vulgare (barley) and Triticum tauschii are related, but sexually incompatible, species. This study was conducted to determine the extent of homology between the genomes of barley and T. tauschii using a common set of restriction fragment length polymorphism (RFLP) markers. Results showed that >95% of low-copy sequences are shared, but 42% of the conserved sequences showed copy-number differences. Sixty-three loci were mapped in T. tauschii using RFLP markers previously mapped in barley. A comparison of RFLP marker order showed that, in general, barley and T. tauschii have conserved linkage groups, with markers in the same linear orders. However, six of the seven linkage groups of T. tauschii contained markers which mapped to unrelated (i.e., non-homoeologous) barley chromosomes. Additionally, four of the T. tauschii linkage groups contained markers that were switched in order with respect to barley. All the chromosome segments differing between T. tauschii and barley contained markers that were detected by multi-copy probes. The results suggest that the observed differences between the T. tauschii and barley genomes were brought about by duplications or deletions of segments in one or both species. The implications of these findings for genetic mapping, breeding, and plant genome evolution are discussed.Published with the approval of the Director of the Colorado State University/Agricultural Experiment Station     

BAC libraries of Triticum urartu, Aegilops speltoides and Ae. tauschii, the diploid ancestors of polyploid wheat

Triticum urartu, Aegilops speltoides and Ae. tauschii are respectively the immediate diploid sources, or their closest relatives, of the A, B and D genomes of polyploid wheats. Here we report the construction and characterization of arrayed large-insert libraries in a bacterial artificial chromosome (BAC) vector, one for each of these diploid species. The libraries are equivalent to 3.7, 5.4 and 4.1 of the T. urartu, Ae. speltoides, Ae. tauschii genomes, respectively. The predicted levels of genome coverage were confirmed by library hybridization with single-copy genes. The libraries were used to estimate the proportion of known repeated nucleotide sequences and gene content in each genome by BAC-end sequencing. Repeated sequence families previously detected in Triticeae accounted for 57, 61 and 57% of the T. urartu, Ae. speltoides and Ae. tauschii genomes, and coding regions accounted for 5.8, 4.5 and 4.8%, respectively.     

Genetic Basis for Spontaneous Hybrid Genome Doubling during Allopolyploid Speciation of Common Wheat Shown by Natural Variation Analyses of the Paternal Species

The complex process of allopolyploid speciation includes various mechanisms ranging from species crosses and hybrid genome doubling to genome alterations and the establishment of new allopolyploids as persisting natural entities. Currently, little is known about the genetic mechanisms that underlie hybrid genome doubling, despite the fact that natural allopolyploid formation is highly dependent on this phenomenon. We examined the genetic basis for the spontaneous genome doubling of triploid F₁ hybrids between the direct ancestors of allohexaploid common wheat (Triticum aestivum L., AABBDD genome), namely Triticum turgidum L. (AABB genome) and Aegilops tauschii Coss. (DD genome). An Ae. tauschii intraspecific lineage that is closely related to the D genome of common wheat was identified by population-based analysis. Two representative accessions, one that produces a high-genome-doubling-frequency hybrid when crossed with a T . turgidum cultivar and the other that produces a low-genome-doubling-frequency hybrid with the same cultivar, were chosen from that lineage for further analyses. A series of investigations including fertility analysis, immunostaining, and quantitative trait locus (QTL) analysis showed that (1) production of functional unreduced gametes through nonreductional meiosis is an early step key to successful hybrid genome doubling, (2) first division restitution is one of the cytological mechanisms that cause meiotic nonreduction during the production of functional male unreduced gametes, and (3) six QTLs in the Ae . tauschii genome, most of which likely regulate nonreductional meiosis and its subsequent gamete production processes, are involved in hybrid genome doubling. Interlineage comparisons of Ae . tauschii ’s ability to cause hybrid genome doubling suggested an evolutionary model for the natural variation pattern of the trait in which non-deleterious mutations in six QTLs may have important roles. The findings of this study demonstrated that the genetic mechanisms for hybrid genome doubling could be studied based on the intrinsic natural variation that exists in the parental species.     

Comparison of the primary structure ofwaxy proteins (granule-bound starch synthase) between polyploid wheats and related diploid species

Thewaxy proteins encoded by the genomes A, B, and D in polyploid wheats and related diploid species were isolated by SDS-PAGE. The N-terminal amino acid sequences of mature proteins and V8 protease-induced fragments were determined. A total of five amino acid substitutions was detected in these sequences, which represent about 10% of the whole sequences of thewaxy proteins. A comparison of these sequences in polyploid wheats with those in related diploid species revealed the following: (i)waxy proteins encoded by the A genome of polyploid wheats were identical to that ofTriticum monococcum, (ii) thewaxy protein encoded by the B genome ofT. turgidum was identical to that ofT. searsii, but differed from those ofT. speltoides andT. longissimum by one amino acid substitution, (iii) thewaxy protein encoded by the B genome ofT. aestivum differed from that encoded by the B genome ofT. turgidum by one amino acid substitution, and (iv) thewaxy protein encoded by the D genome ofT. aestivum was identical to that ofT. tauschii.     

Genome differentiation in Aegilops. 3. Evolution of the D-genome cluster

 Six polyploid Aegilops species containing the D genome were studied by C-banding and fluorescence in situ hybridization (FISH) using clones pTa71 (18S-5.8S-26S rDNA), pTa794 (5S rDNA), and pAs1 (non-coding repetitive DNA sequence) as probes. The C-banding and pAs1-FISH patterns of Ae. cylindrica chromosomes were identical to those of the parental species. However, inactivation of the NOR on chromosome 5D with a simultaneous decrease in the size of the pTa71-FISH site was observed. The N^v and D^v genomes of Ae. ventricosa were somewhat modified as compared with the N genome of Ae. uniaristata and the D genome of Ae. tauschii. Modifications included minor changes in the C-banding and pAs1-FISH patterns, complete deletion of the NOR on chromosome 5D^v, and the loss of several minor 18S-5.8S-26S rDNA loci on N^v genome chromosomes. According to C-banding and FISH analyses, the D^cr1 genome of Ae. crassa is more similar to the D^v genome of Ae. ventricosa than to the D genome of Ae. tauschii. Mapping of the 18S-5.8S-26S rDNA and 5S rDNA loci by multicolor FISH suggests that the second (X^cr) genome of tetraploid Ae. crassa is a derivative of the S genome (section Emarginata of the Sitopsis group). Both genomes of Ae. crassa were significantly modified as the result of chromosomal rearrangements and redistribution of highly repetitive DNA sequences. Hexaploid Ae. crassa and Ae. vavilovii arose from the hybridization of chromosomal type N of tetraploid Ae. crassa with Ae. tauschii and Ae. searsii, respectively. Chromosomal type T1 of tetraploid Ae. crassa and Ae. umbellulata were the ancestral forms of Ae. juvenalis. The high level of genome modification in Ae. juvenalis indicates that it is the oldest hexaploid species in this group. The occurrence of hexaploid Ae. crassa was accompanied by a species-specific translocation between chromosomes 4D^cr1 and 7X^cr. No chromosome changes relative to the parental species were detected in Ae. vavilovii, however, its intraspecific diversity was accompanied by a translocation between chromosomes 3X^cr and 3D^cr1. Received July 24, 2001 Accepted October 1, 2001     

GluDy allele variations in Aegilops tauschii and Triticum aestivum: implications for the origins of hexaploid wheats

To investigate the evolution and geographical origins of hexaploid wheat, we examined a 284 bp sequence from the promoter region of the GluDy locus, coding for the y subunit of high-molecular-weight glutenin. Fourteen different alleles were found in 100 accessions of Aegilops tauschii and 169 of Triticum aestivum. Two alleles were present in both species; the other 7 alleles from Ae. tauschii and 5 from T. aestivum were unique to their respective species. The two shared alleles differed at only one nucleotide position within the region sequenced, but their apparent association with the common haplotypes GluD1a and GluD1d, which have substantial differences within their GluDy coding regions, makes it unlikely that the alleles evolved independently in Ae. tauschii and T. aestivum. The results therefore support previous studies which suggest that there were at least two Ae. tauschii sources that contributed germplasm to the D genome of T. aestivum. The number of alleles present in T. aestivum, and the nucleotide diversity of these alleles, indicates that this region of the D genome has undergone relatively rapid change since polyploidisation. Ae. tauschii from Syria and Turkey had relatively high nucleotide diversity and possessed all the major GluDy alleles, indicating that these populations are probably ancient and not the result of adventive spread. The presence in the Turkish population of both of the shared alleles suggests that hexaploid wheat is likely to have originated in southeast Turkey or northern Syria, within the Fertile Crescent and near to the farming villages at which archaeological remains of hexaploid wheats are first found. A second, more recent, hexaploidisation probably occurred in Iran.     

Expression of Vernalization Requirement and Spikelet Number in Synthetic Hexaploid Wheats and their Triticum tauschii and Tetraploid Wheat Parents

Vernalization requirement, as measured by days from sowing toear emergence (plants grown under an 18-h photoperiod), andspikelet number per ear were recorded for 17 synthetic hexaploidwheats and the six tetraploid (Triticum durum) and the ninediploid T. tauschii parents used to synthesize them. The tetraploid parents and the synthetic hexaploids had springphenotypes (little or no vernalization requirement) whereasthe T. tauschii parents were all winter types (strong vernalizationrequirement). The tetraploid wheats and the synthetic hexaploidsreached ear emergence 50·3 to 63·8 d and 58·2to 75·3 d after sowing, respectively, while the T. tauschiilines reached ear emergence 114·3 to 179·5 d aftersowing. The spring habit of the synthetic hexaploids demonstrates theepistasis of spring over winter habit. It is considered thatwith a presumed single vrn locus in the diploid species T. tauschiithe range of ear emergence in these lines is consistent withthe action of multiple alleles at that locus. Although there was no general epistasis for spikelet number,the tetraploid parents appear to be exerting more influenceover spikelet number in the synthetic hexaploids than T. tauschii.The well established association between the duration from sowingto ear emergence and spikelet number was not evident eitherwithin each ploidy group or when the 32 lines were consideredtogether. Triticum tauschii, Triticum durum, hexaploid wheat, spikelet number, vernalization requirement     

Use of winter wheat x Triticum tauschii backcross populations for germplasm evaluation

The wild diploid goatgrass, Triticum tauschii (Coss.) Schmal., is an important source of genes for resistance to both diseases and insects in common wheat (Triticum aestivum L.) We have evaluated grain yield, kernel weight, protein concentration, and kernel hardness of 641 BC₂ F₁-derived families from direct crosses involving four T. aestivum cultivars and 13 T. tauschii accessions over 2 years and at two Kansas, USA, locations. On average, T. tauschii germplasm depressed grain yield and increased protein concentration, whereas kernel weight was affected either positively or negatively, depending on the T. tauschii parent. Three T. tauschii parents produced a large proportion of families with very soft endosperm. Some variation among progeny of different T. tauschii parents resulted from the segregation of genes for resistance to leaf rust (caused by Puccinia recondita Rob. ex Desm.). This study confirmed that random BC₂-derived families can be used to evaluate the effects of T. tauschii genes in the field. This methodology, although laborious, can provide useful information which is not obtainable by the screening of T. tauschii accessions themselves.Joint contribution of USDA-ARS, the Kansas Agricultural Experiment Station, and the Wheat Genetics Resource Center. Contribution no. 94-242-J. Mention of a proprietary name in the article does not imply approval to the exclusion of other suitable products     

Analysis of 5S rDNA changes in synthetic allopolyploids <Emphasis Type="Italic">Triticum</Emphasis> × <Emphasis Type="Italic">Aegilops</Emphasis>

Changes of 5S rDNA at the early stage of allopolyploidization were investigated in three synthetic allopolyploids: Aegilops sharonensis × Ae. umbellulata (2n = 28), Triticum urartu × Ae. tauschii (2n = 28), and T. dicoccoides × Ae. tauschii (2n = 42). Fluorescent in situ hybridization (FISH) revealed quantitative changes affecting separate loci of one of the parental genomes in S₃ plants of each hybrid combination. Southern hybridization with genomic DNA of the allopolyploid T. urartu × Ae. tauschii (TMU38 × TQ27) revealed a lower intensity of signals from Ae. tauschii fragments compared with those derived from T. urartu. This confirmed the signal reduction revealed for chromosome 1D of this hybrid by FISH. Neither Southern hybridization nor PCR testing of 5–15 plants of the S₂-S₃ generations revealed an appearance of new 5S rDNA fragments or a complete disappearance of parental fragments from the allopolyploids under study. No changes were found by aligning nine 5S rDNA sequences of the allopolyploid TMU38 × TQ27 with corresponding sequences of the parental species. The similarity between one of the synthetic allopolyploids examined and a natural allopolyploid with the same genome composition points to an early formation of the 5S rDNA organization unique for each allopolyploid.     

The structure of the Aegilops tauschii genepool and the evolution of hexaploid wheat

 Polymorphism in the lengths of restriction fragments at 53 single-copy loci, the rRNA locus Nor3, and the high-molecular-weight glutenin locus Glu1 was investigated in the D genome of hexaploid Triticum aestivum and that of Aegilops tauschii, the source of the T. aestivum D genome. The distribution of genetic variation in Ae. tauschii suggests gene flow between Ae. tauschii ssp. strangulata and ssp. tauschii in Iran but less in Transcaucasia. The “strangulata” genepool is wider than it appears on the basis of morphology and includes ssp. strangulata in Transcaucasia and southeastern (SE) Caspian Iran and ssp. tauschii in north-central Iran and southwestern (SW) Caspian Iran. In the latter region, Ae. tauschii morphological varieties ‘meyeri’ and ‘typica’ are equidistant to ssp. strangulata in Transcaucasia, and both belong to the “strangulata” genepool. A model of the evolution of Ae. tauschii is presented. On the geographic region basis, the D genomes of all investigated forms of T. aestivum are most closely related to the “strangulata” genepool in Transcaucasia, Armenia in particular, and SW Caspian Iran. It is suggested that the principal area of the origin of T. aestivum is Armenia, but the SW coastal area of the Caspian Sea and a corridor between the two areas may have played a role as well. Little genetic differentiation was found among the D genomes of all investigated free-threshing and hulled forms of T. aestivum, and all appear to share a single D-genome genepool, in spite of the fact that several Ae. tauschii parents were involved in the evolution of T. aestivum. Received: 17 November 1997 / Accepted: 17 March 1998     

Simultaneous transfer, introgression, and genomic localization of genes for resistance to stem rust race TTKSK (Ug99) from Aegilops tauschii to wheat

Wheat production is currently threatened by widely virulent races of the wheat stem rust fungus, Puccinia graminis f. sp. tritici, that are part of the TTKSK (also known as ‘Ug99’) race group. The diploid D genome donor species Aegilops tauschii (2n = 2x = 14, DD) is a readily accessible source of resistance to TTKSK and its derivatives that can be transferred to hexaploid wheat, Triticum aestivum (2n = 6x = 42, AABBDD). To expedite transfer of TTKSK resistance from Ae. tauschii, a direct hybridization approach was undertaken that integrates gene transfer, mapping, and introgression into one process. Direct crossing of Ae. tauschii accessions with an elite wheat breeding line combines the steps of gene transfer and introgression while development of mapping populations during gene transfer enables the identification of closely linked markers. Direct crosses were made using TTKSK-resistant Ae. tauschii accessions TA1662 and PI 603225 as males and a stem rust-susceptible T. aestivum breeding line, KS05HW14, as a female. Embryo rescue enabled recovery of F₁ (ABDD) plants that were backcrossed as females to the hexaploid recurrent parent. Stem rust-resistant BC₁F₁ plants from each Ae. tauschii donor source were used as males to generate BC₂F₁ mapping populations. Bulked segregant analysis of BC₂F₁ genotypes was performed using 70 SSR loci distributed across the D genome. Using this approach, stem rust resistance genes from both accessions were located on chromosome arm 1DS and mapped using SSR and EST-STS markers. An allelism test indicated the stem rust resistance gene transferred from PI 603225 is Sr33. Race specificity suggests the stem rust resistance gene transferred from TA1662 is unique and this gene has been temporarily designated SrTA1662. Stem rust resistance genes derived from TA1662 and PI 603225 have been made available with selectable molecular markers in genetic backgrounds suitable for stem rust resistance breeding.