Discovery and mapping of wheat Ph1 suppressors

Pairing between wheat (Triticum turgidum and T. aestivum) homeologous chromosomes is prevented by the expression of the Ph1 locus on the long arm of chromosome 5B. The genome of Aegilops speltoides suppresses Ph1 expression in wheat x Ae. speltoides hybrids. Suppressors with major effects were mapped as Mendelian loci on the long arms of Ae. speltoides chromosomes 3S and 7S. The chromosome 3S locus was designated Su1-Ph1 and the chromosome 7S locus was designated Su2-Ph1. A QTL with a minor effect was mapped on the short arm of chromosome 5S and was designated QPh.ucd-5S. The expression of Su1-Ph1 and Su2-Ph1 increased homeologous chromosome pairing in T. aestivum x Ae. speltoides hybrids by 8.4 and 5.8 chiasmata/cell, respectively. Su1-Ph1 was completely epistatic to Su2-Ph1, and the two genes acting together increased homeologous chromosome pairing in T. aestivum x Ae. speltoides hybrids to the same level as Su1-Ph1 acting alone. QPh.ucd-5S expression increased homeologous chromosome pairing by 1.6 chiasmata/cell in T. aestivum x Ae. speltoides hybrids and was additive to the expression of Su2-Ph1. It is hypothesized that the products of Su1-Ph1 and Su2-Ph1 affect pairing between homeologous chromosomes by regulating the expression of Ph1 but the product of QPh.ucd-5S may primarily regulate recombination between homologous chromosomes.     

Pairing affinities of the B- and G-genome chromosomes of polyploid wheats with those of Aegilops speltoides.

Chromosome pairing at metaphase I was studied in different interspecific hybrids involving Aegilops speltoides (SS) and polyploid wheats Triticum timopheevii (AtAtGG), T. turgidum (AABB), and T. aestivum (AABBDD) to study the relationships between the S, G, and B genomes. Individual chromosomes and their arms were identified by means of C-banding. Pairing between chromosomes of the G and S genomes in T. timopheevii x Ae. speltoides (AtGS) hybrids reached a frequency much higher than pairing between chromosomes of the B and S genomes in T. turgidum x Ae. speltoides (ABS) hybrids and T. aestivum x Ae. speltoides (ABDS) hybrids, and pairing between B- and G-genome chromosomes in T. turgidum x T. timopheevii (AAtBG) hybrids or T. aestivum x T. timopheevii (AAtBGD) hybrids. These results support a higher degree of closeness of the G and S genomes to each other than to the B genome. Such relationships are consistent with independent origins of tetraploid wheats T. turgidum and T. timopheevii and with a more recent formation of the timopheevi lineage.     

Repetitive DNas of wild emmer wheat (Triticum dicoccoides) and their relation to S-genome species: molecular cytogenetic analysis.

We have analyzed the chromosomal GISH molecular banding patterns of three populations of the wild allopolyploid wheat Triticum dicoccoides in an attempt to unravel the evolutionary relationships between highly repetitive DNA fractions of T. dicoccoides and proposed diploid progenitors of the B genome. Aegilops speltoides showed almost complete affinity of its repetitive DNA to C-heterochromatin of T. dicoccoides, whereas other S-genome species demonstrated relatedness only to distal heterochromatin. This substantiates the priority of Ae. speltoides as the most similar to the wheat B-genome donor in comparison with other Sitopsis species. Using molecular banding technique with DNA of different Aegilops species as a probe permits tracing of the origin of each heterochromatin cluster. Molecular banding analysis reveals polymorphism between three wild emmer wheat populations. Comparison of molecular banding patterns with chromosomal distribution of the Ty1-copia retrotransposons, which constitute a large share of T. dicoccoides genome, makes it possible to propose that the activity of transposable elements may lie in the background of observed intraspecific polymorphism.     

Construction and study of leaf rust-resistant common wheat lines with translocations of Aegilops speltoides Tausch

Genotyping was performed for the leaf rust-resistant line 73/00i (Triticum aestivum x Aegilops speltoides). Fluorescence in situ hybridization (FISH) with probes Spelt1 and pSc119.2 in combination with microsatellite analysis were used to determine the locations and sizes of the Ae. speltoides genetic fragments integrated into the line genome. Translocations were identified in the long arms of chromosomes 5B and 6B and in the short arm of chromosome 1B. The Spelt1 and pSc119.2 molecular cytological markers made it possible to rapidly establish lines with single translocation in the long arms of chromosomes 5B and 6B. The line carrying the T5BS x 5BL-5SL translocation was highly resistant to leaf rust, and the lines carrying the T6BS x 6BL-6SL translocation displayed moderate resistance. The translocations differed in chromosomal location from known leaf resistance genes transferred into common wheat from Ae. speltoides. Hence, it was assumed that new genes were introduced into the common wheat genome from Ae. speltoides. The locus that determined high resistance to leaf rust and was transferred into the common wheat genome from the long arm of Ae. speltoides chromosome 5S by the T5BS x 5BL-5SL translocation was preliminarily designated as LrAsp5.     

Molecular analysis of the phylogenetic relationships among the diploid aegilops species of the section Sitopsis

RAPD analysis was used to study the intraspecific variation and phylogenetic relationships of S-genome diploid Aegilops species regarded as potential donors of the B genome of cultivated wheat. In total, 21 DNA specimens from six S-genome diploid species were examined. On a dendrogram, Ae. speltoides and Ae. aucheri formed the most isolated cluster. Among the other species, Ae. searsii was the most distant while Ae. longissima and Ae. sharonensis were the closest species. The maximum difference between individual accessions within one species was approximately the same (0.18-0.22) in Ae. bicornis, Ae. longissima. Ae. sharonensis, and Ae. searsii. The difference between the clusters of questionable species Ae. speltoides and Ae. aucheri corresponded to the intraspecific level; the difference between closely related Ae. longissima and Ae. sharonensis corresponded to the interspecific level. The section Sitopsis of the genus Aegilops includes six diploid species containing the S genome, which is regarded as an ancestor of the B genome of cultivated wheat. The species of the section are thought to be closest to the genus Triticum. Note that the taxonomic status of some forms of the section Sitopsis is questionable. For instance, Ae. speltoides and Ae. aucheri are variously considered as individual species or as a single species, Ae. speltoides. The situation with Ae. longissima and Ae. sharonensis is similar. Thus, although the group includes only diploid species and is well studied morphologically, its phylogeny and taxonomy are still questionable.     

The phylogenetic relationships among the possible donors of B-genome of common wheat based on internal transcribed spacer (ITS) sequences

The ITS regions of 5 species in Aegilops sect. Sitopsis, the possible donors of Bgenome of common wheat, were amplified by PCR, cloned and sequenced. The phylogenetic relationships among 5 species in Aegilops sect. Sitopsis were constructed based on ITS1 + ITS2 sequences. The results demonstrated that Ae. speltoides was a distinct species in Aegilops sect. Sitopsis. The average of the pairwise distances between Ae. speltoides and the other four species was three times as high as that among the latter four. Ae. speltoides was the earliest lineage of the section under question. Relationship between Ae. longissima and Ae. sharonensis was the closest in Aegilops sect. Sitopsis. Sequence of ITS regioncould be used as a molecular marker to identify origin of B-genome in polyploid wheats.     

Genomic Instability in Wheat Induced by Chromosome 6b(s) of Triticum Speltoides

A massive restructuring of chromosomes was observed during the production of a substitution of chromosome 6B(s) from Triticum speltoides (Tausch) Gren. ex Richter for chromosome 6B of Chinese Spring wheat (Triticum aestivum L.). Deletions, translocations, ring chromosomes, dicentric chromosomes and a paracentric inversion were observed. Chromosome rearrangements occurred in both euchromatic and heterochromatic regions. Chromosome rearrangements were not observed either in the amphiploid between Chinese Spring and T. speltoides or in Chinese Spring. No chromosome rearrangements were observed in the backcross derivatives; however, after self-pollination of a monosomic substitution (2n = 41) of chromosome 6B(s) for wheat chromosome 6B, 49 of the 138 plants carried chromosome aberrations. Chromosome rearrangements were observed in both wheat and T. speltoides chromosomes. The frequency of chromosome rearrangements was high among the B-genome chromosomes, moderate among the A-genome chromosomes, and low among the D-genome chromosomes. In the B genome, the rearrangements were nonrandom, occurring most frequently in chromosomes 1B and 5B. Chromosome rearrangements were also frequent for the 6B(s) chromosome of T. speltoides. An intriguing aspect of these observations is that they indicate that wheat genomes can be subject to uneven rates of structural chromosome differentiation in spite of being in the same nucleus.     

用ITS序列确定小麦B基因组的可能供体间的关系

对小麦B基因组的可能供体山羊草属Aegilops sect．Sitopsis的5个种的核糖体DNA的内部转 录区(ITS)进行了PCR扩增和克隆,井测定ITSl和ITS2的序列,用ITSl+ITS2的序列重建了 Aegilops sect．Sitopsis中5个种的系统发育关系。结果表明,斯卑尔脱山羊草Ae．speltoides是sect． Sitopsis中特殊的一个种,它与该组其余4种间的平均遗传距离是后者彼此间平均遗传距离的3倍,Ae． speltoides与同组其余4个种的分离要比后者相互间的分离早得多;在拟斯卑尔脱组Sect．Sitopsis的5 个种中,长柱山羊草Ae．longissima与沙融山羊草Ae．sharonensis的关系最近。ITS序列可以进一步用来作为确定B基因组起源的分子标记。     

Detection of alien chromosomes from S-genome species in the addition/substitution lines of bread wheat and visualization of A-, B- and D-genomes by GISH

A modified approach based on the GISH technique for detecting introgressed chromosomes/chromosome arms from closely related S-genome species to wheat genome and for visualization of A-, B- and D-genomes of Triticum aestivum L. (genome AABBDD, 2n = 6x = 42) is presented. For detecting alien chromosomes we investigated two lines of bread wheat, one is an addition line with a pair of chromosome No. 4 short arms from Aegilops searsii (4SsS) and a wheat substitution line with a pair of chromosomes No. 6 from Ae. longissima (6S1). A hybridization mixture consists of two differently labelled DNAs, one from the line used for chromosome spread preparations, and the second from origin species of alien chromosomes. The latter adds different color in the regions of its hybridization showing the presence of alien chromosomes by creating a strong and easily detected combined signal. For discriminating A-, B-, and D-genome chromosomes, the hybridization mixture of differently labelled total DNA from Ae. tauschii--the proposed progenitor of D-genome (detected red) and T. dicoccoides (genome AABB) (detected green) were used. The high temperature of hybridization allows high precision annealing of chromosome/probe sequences and at the same time it sharpens differences between reassociation kinetics of eu- and heterochromatin revealing chromosome substructure. A pre-annealing step increases probe specificity. As a result, we observed brown chromosomes of A-genome, banded green chromosomes of B-genome and red chromosomes of D-genome. Inter genomic invasion of the sequences from A/B-genomes to D-genome has been detected.     

The origin of the B-genome of bread wheat (Triticum aestivum L.)

Understanding the origin of cultivated wheats would further their genetic improvement. The hexaploid bread wheat (Triticum aestivum L., AABBDD) is believed to have originated through one or more rare hybridization events between Aegilops tauschii (DD) and the tetraploid T. turgidum (AABB). Progenitor, of the A-genome of the tetraploid and hexaploid wheats has generally been accepted to be T. urartu. In spite of the large number of attempts and published reports about the origin of the B-genome in cultivated wheats, the donor of the B-genome is still relatively unknown and controversial and, hence, remains open. This genome has been found to be closely related to the S-genome of the Sitopsis section (Ae. speltoides, Ae. longissima, Ae. sharonensis, Ae. searsii, and Ae. bicornis) of the genus Aegilops L. Among Sitopsis species, the most positive evidence has been accumulated for Ae. speltoides as the progenitor of the B-genome. Therefore, one or more of the Sitopsis species were proposed frequently as the B-genome donor. Although several reviews have been written on the origin of the genomes of wheat over the years, this paper will attempt for the first time to review the immense literature on the subject, with a particular emphasis on the B-genome which has attracted a huge attention over some 100 years. The ambiguity and conflicting results in most of the methods employed in deducing the precise B-genome donor/s to bread wheat are also discussed.     

Visualization of A- and B-genome chromosomes in wheat (Triticum aestivum L.) x jointed goatgrass (Aegilops cylindrica Host) backcross progenies.

Wheat (Triticum aestivum) and jointed goatgrass (Aegilops cylindrica) can cross with each other, and their self-fertile backcross progenies frequently have extra chromosomes and chromosome segments, presumably retained from wheat, raising the possibility that a herbicide resistance gene might transfer from wheat to jointed goatgrass. Genomic in situ hybridization (GISH) was used to clarify the origin of these extra chromosomes. By using T. durum DNA (AABB genome) as a probe and jointed goatgrass DNA (CCDD genome) as blocking DNA, one, two, and three A- or B-genome chromosomes were identified in three BC2S2 individuals where 2n = 29, 30, and 31 chromosomes, respectively. A translocation between wheat and jointed goatgrass chromosomes was also detected in an individual with 30 chromosomes. In pollen mother cells with meiotic configuration of 14 II + 2 I, the two univalents were identified as being retained from the A or B genome of wheat. By using Ae. markgrafii DNA (CC genome) as a probe and wheat DNA (AABBDD genome) as blocking DNA. 14 C-genome chromosomes were visualized in all BC2S2 individuals. The GISH procedure provides a powerful tool to detect the A or B-genome chromatin in a jointed goatgrass background, making it possible to assess the risk of transfer of herbicide resistance genes located on the A or B genome of wheat to jointed goatgrass.     

小麦染色体组的起源与进化探讨

对小麦染色体组的起源及其进化进行了全面综述后,提出了一个新的小麦进化途径,并认为：（１）Ｔｒｉｔｉｃｕｍｍｏｎｏｃｏｃｕｍｖａｒｕｒａｒｔｕ是多倍体小麦Ａ组的原初供体,在Ａ组进入多倍体小麦后有Ｔｍｏｎｏｖａｒｂｏｅｏｔｉｃｕｍ的基因渗入;（２）Ｂ和Ｇ组的原初供体是Ｔｓｐｅｌｔｏｉｄｅｓ的Ｓ组,在该Ｓ组进入多倍体小麦后有两个进化方向,即Ｓ组结构分化形成Ｇ组和Ｓ组经外源染色体代换及重组等而进化成Ｂ组;（３）Ｔｔｕｒｇｉｄｕｍ和Ｔｔｉｍｏｐｈｅｖｉ都是来自Ｔｓｐｅｌｔｏｉｄｅｓ为母本与Ｔｍｏｎｏｖａｒｕｒａｒｔｕ杂交后并双二倍化而形成的原初四倍体小麦（ＳＳＡＡ）,并由它分别经遗传渗入和结构分化而成;（４）Ｔｚｈｕｋｏｖｓｋｙｉ是Ｔｔｉｍｏｐｈｅｖｉ作母本与Ｔｍｏｎｏｖａｒｂｏｅｏｔｉｃｕｍ杂交并双二倍化而形成,故它具有分别来自Ｔｍｏｎｏｖａｒｕｒａｒｔｕ和Ｔｍｏｎｏｖａｒｂｏｅｏｔｉｃｕｍ的两类Ａ组;（５）Ｔａｅｓｔｉｖｕｍ的Ｄ组来自Ｔｔａｕｓｃｈｉ;（６）无论Ａ组、Ｂ组、Ｄ组、Ｇ组在进入多倍体小麦后均有相当分化,同时在其供体种中也有一定分化。     

Coevolution of A and B genomes in allotetraploid Triticum dicoccoides.

Data is presented on the coevolution of A and B genomes in allotetraploid wheat Triticum dicoccoides (2n = 4x = 28, genome AABB) obtained by genomic in situ hybridization (GISH). Probing chromosomes of T. dicoccoides with DNA from the proposed A/B diploid genome ancestors shows evidence of enriching A-genome with repetitive sequences of B-genome type. Thus, ancestral S-genome sequences have spread throughout the AB polyploid genome to a greater extent than have ancestral A-genome sequences. The substitution of part of the A-genome heterochromatin clusters by satellite DNA of the B genome is detected by using the molecular banding technique. The cause may be interlocus concerted evolution and (or) colonization. We propose that the detected high level of intergenomic invasion in old polyploids might reflect general tendencies in speciation and stabilization of the allopolyploid genome.     

Independent wheat B and G genome origins in outcrossing Aegilops progenitor haplotypes

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

Molecular evidence for Triticum speltoides as a B-genome progenitor of wheat (Triticum aestivum).

In polyploid wheat, the origin of the B-genome donor has remained relatively unknown in spite of a number of investigations attempting to identify the parental species. A project was designed to isolate and clone a genome-specific DNA sequence from Triticum speltoides L. to determine if that species could be the B-genome donor. A cloning scheme involving the prescreening of 1-kb fragments followed by colony, dot blot, and Southern blot hybridization screenings was used to isolate a speltoides-specific sequence (pSp89.XI). The methods used allowed for rapid isolation of a genome-specific sequence when screened against total DNA from closely related species. Subsequent analyses showed that the sequence was barely detected in any of the other genomes of the annual Sitopsis section. The results of dot blot and Southern blot analyses established that (i) the sequence pSP89.XI, specific to T. speltoides relative to the other species of the Sitopsis section, was present in the genomes of tetraploid and hexaploid wheat, (ii) the relative abundance of pSp89.XI seemed to decrease from the diploid to the polyploid wheats, and (iii) the existence of a related, but modified B genome in polyploid wheat compared with that in modern T. speltoides was probable. Key words : genome-specific, DNA.     

Homoeologous relationships of Aegilops speltoides chromosomes to bread wheat

 Homoeologous pairing at metaphase I was analysed in the standard-type, ph2b and ph1b hybrids of Triticum aestivum (AABBDD) and Aegilops speltoides (SS). Data from relative pairing affinities were used to predict homoeologous relationships of Ae. speltoides chromosomes to wheat. Chromosomes of both species, and their arms, were identified by C-banding. The Ae. speltoides genotype carried genes that induced a high level of homoeologous pairing in the three types of hybrids analyzed. All arms of the seven chromosomes of the S genome showed normal homoeologous pairing, which implies that no apparent chromosome rearrangements occurred in the evolution of Ae. speltoides relative to wheat. A pattern of preferential pairing of two types, A-D and B-S, confirmed that the S genome is very closely related to the B genome of wheat. Although this pairing pattern was also reported in hybrids of wheat with Ae. longissima and Ae. sharonensis, a different behaviour was found in group 5 chromosomes. In the hybrids of Ae. speltoides, chromosome 5B-5S pairing was much more frequent than 5D-5S, while these chromosome associations reached similar frequencies in the hybrids of Ae. longissima and Ae. sharonensis. These results are in agreement with the hypothesis that the B genome of wheat is derived from Ae. speltoides. Received: 8 January 1998 / Accepted: 4 February 1998     

Supernumerary chromosomes in the second outbreeding species of the wheat group

Meiosis was examined in PMC of 138 plants of the outbreeder Triticum speltoides (=Aegilops speltoides) and 175 plants of the closely related T. longissimum (= Ae. longissima), which is a selfer. Nine of the T. speltoides plants, originating fra two of the five Israeli populations under study, contained B chromosomes in addition to the normal complements (2n = 14). No supernumerary chromosomes were found in T. longissimum. — Plants were found carrying 1, 2 or 3 B chromosomes, which seem to be stable among PMC. Their size is 2/3 of the average length of the regular chromosomes, but otherwise they are similar in appearance. Pairing between Bs is common as bivalents and trivalents, but whether this association is ehiasmatic is not clear. There is a slight reduction in chiasma frequency of the regular complement in B-carriers compared to other plants from the same population. — The only other species in the wheat group reported to have B chromosomes in natural populations is also an outbreeder, namely T. tripsacoides (=Ae. mutica). The connection between outbreeding and B chromosomes might be significant.     

Genome differentiation in Aegilops. 1. Distribution of highly repetitive DNA sequences on chromosomes of diploid species.

Genome differentiation in 12 diploid Aegilops species was analyzed using in situ hybridization with the highly repetitive DNA sequences pSc119 and pAs1 and C-banding. Chromosomes of all these diploid Aegilops species hybridized with the pSc119 probe; however, the level of hybridization and labeling patterns differed among genomes. Only four species (Ae. squarrosa, Ae. comosa, Ae. heldreichii, and Ae. uniaristata) showed distinct hybridization with pAs1. The labeling patterns were species-specific and chromosome-specific. Differences in in situ hybridization (ISH) patterns, also observed by C-banding, exist between the karyotypes of Ae. comosa and Ae. heldreichii, suggesting that they are separate, although closely related, subspecies. The S genome of Ae. spelioides was most similar to the B and G genomes of polyploid wheats on the basis of both C-banding and ISH patterns, but was different from other species of section Sitopsis. These species had different C-banding patterns but they were similar to each other and to Ae. mutica in the distribution of pSc119 hybridization sites. Two types of labeling were detected in Ae. squarrosa with the pAs1 probe. The first resembled that of the D-genome chromosomes of bread wheat, Triticum aestivum L. em. Thell., while the second was similar to the D genome of some of the polyploid Aegilops species. Relationships among diploid Aegilops species and the possible mechanisms of genome differentiation are discussed. Key words : wheat, Triticum, Aegilops, in situ hybridization, C-banding, evolution.     

A chromosome-specific sequence common to the B genome of polyploid wheat and Aegilops searsii

A low-copy, non-coding chromosome-specific DNA sequence, isolated from common wheat, was physically mapped to the distal 19% region of the long arm of chromosome 3B (3BL) of common wheat. This sequence, designated WPG118, was then characterized by Southern hybridization, PCR amplification and sequence comparison using a large collection of polyploid wheats and diploid Triticum and Aegilops species. The data show that the sequence exists in all polyploid wheats containing the B genome and absent from those containing the G genome. At the diploid level, it exists only in Ae. searsii, a diploid species of section Sitopsis, and not in other diploids including Ae. speltoides, the closest extant relative to the donor of the B genome of polyploid wheat. This finding may support the hypothesis that the B-genome of polyploid wheat is of a polyphyletic origin, i.e. it is a recombined genome derived from two or more diploid Aegilops species.     

Microsatellite markers reveal chimeric origin of redesignated chromosome 4A of wheat from Triticum urartu and other species.

Although a new nomenclature has been adopted for wheat in which chromosome 4A (4AO) has been renamed 4B (4BN) and chromosome 4B (4BO) has been renamed 4A (4AN), their specific origin remains uncertain. The use of wheat microsatellite (WMS) markers mapped to chromosomes 4AN and 4BN in a set of polyploid wheats and diploid genome donors has unequivocally indicated that the entire short arm of 4AN, some part of 4ANL. and a segment of 4BNL were derived from Triticum urartu. The presence of a T. urartu-specific allele at locus gwm368 on 4BNL and of an Aegilops speltoides allele at locus gwm397 on 4ANL suggests the possibility of a reciprocal translocation between 4ANL and 4BNL. The subcentromeric and telomeric regions of 4ANL corresponding to heterochromatic C-bands were derived neither from diploid wheats nor from Ae. speltoides or Aegilops longissima.