Role of cytoplasm-specific introgression in the evolution of the polyploid wheats

Chromosome translocation affects multiple phenotypes, causes genome-wide dysregulation of gene expression, and remodels metabolome in hexaploid wheat

Chromosomal rearrangements (CRs) may occur in newly formed polyploids due to compromised meiotic fidelity. Moreover, CRs can be more readily tolerated in polyploids allowing their longer-term retention and hence potential spreading/fixation within a lineage. The direct functional consequences of CRs in plant polyploids remain unexplored. Here, we identified a heterozygous individual from a synthetic allohexaploid wheat in which the terminal parts of the long-arms of chromosomes 2D (approximately 193 Mb) and 4A (approximately 167 Mb) were reciprocally translocated. Five homogeneous translocation lines including both unbalanced and balanced types were developed by selfing fertilization of the founder mutant (RT [2DL; 4AL]-ter/1, reciprocal translocation). We investigated impacts of these translocations on phenotype, genome-wide gene expression and metabolome. We find that, compared with sibling wild-type, CRs in the form of both unbalanced and balanced translocations induced substantial changes of gene expression primarily via trans-regulation in the nascent allopolyploid wheat. The CRs also manifested clear phenotypic and metabolic consequences. In particular, the genetically balanced, stable reciprocal translocations lines showed immediate enhanced reproductive fitness relative to wild type. Our results underscore the profound impact of CRs on gene expression in nascent allopolyploids with wide-ranging phenotypic and metabolic consequences, suggesting CRs are an important source of genetic variation that can be exploited for crop breeding.

Single-gene FISH maps and major chromosomal rearrangements in Elymus sibiricus and E. nutans

BACKGROUND

Chromosomal variations have been revealed in both E. sibiricus and E. nutans, but chromosomal structural variations, such as intra-genome translocations and inversions, are still not recognized due to the cytological limitations of previous studies. Furthermore, the syntenic relationship between both species and wheat chromosomes remains unknown.

RESULTS

Fifty-nine single-gene fluorescence in situ hybridization (FISH) probes, including 22 single-gene probes previously mapped on wheat chromosomes and other newly developed probes from the cDNA of Elymus species, were used to characterize the chromosome homoeologous relationship and collinearity of both E. sibiricus and E. nutans with those of wheat. Eight species-specific chromosomal rearrangements (CRs) were exclusively identified in E. sibiricus, including five pericentric inversions in 1H, 2H, 3H, 6H and 2St; one possible pericentric inversion in 5St; one paracentric inversion in 4St; and one reciprocal 4H/6H translocation. Five species-specific CRs were identified in E. nutans, including one possible pericentric inversion in 2Y, three possible pericentric multiple-inversions in 1H, 2H and 4Y, and one reciprocal 4Y/5Y translocation. Polymorphic CRs were detected in three of the six materials in E. sibiricus, which were mainly represented by inter-genomic translocations. More polymorphic CRs were identified in E. nutans, including duplication and insertion, deletion, pericentric inversion, paracentric inversion, and intra- or inter-genomic translocation in different chromosomes.

CONCLUSIONS

The study first identified the cross-species homoeology and the syntenic relationship between E. sibiricus, E. nutans and wheat chromosomes. There are distinct different species-specific CRs between E. sibiricus and E. nutans, which may be due to their different polyploidy processes. The frequencies of intra-species polymorphic CRs in E. nutans were higher than that in E. sibiricus. To conclude, the results provide new insights into genome structure and evolution and will facilitate the utilization of germplasm diversity in both E. sibiricus and E. nutans.

Genetic diversity, distribution and domestication history of the neglected GGAtAt genepool of wheat

We present a comprehensive survey of cytogenetic and genomic diversity of the GGA^tA^t genepool of wheat, thereby unlocking these plant genetic resources for wheat improvement. Wheat yields are stagnating around the world and new sources of genes for resistance or tolerances to abiotic traits are required. In this context, the tetraploid wheat wild relatives are among the key candidates for wheat improvement. Despite its potential huge value for wheat breeding, the tetraploid GGA^tA^t genepool is largely neglected. Understanding the population structure, native distribution range, intraspecific variation of the entire tetraploid GGA^tA^t genepool and its domestication history would further its use for wheat improvement. The paper provides the first comprehensive survey of genomic and cytogenetic diversity sampling the full breadth and depth of the tetraploid GGA^tA^t genepool. According to the results obtained, the extant GGA^tA^t genepool consists of three distinct lineages. We provide detailed insights into the cytogenetic composition of GGA^tA^t wheats, revealed group- and population-specific markers and show that chromosomal rearrangements play an important role in intraspecific diversity of T. araraticum. The origin and domestication history of the GGA^tA^t lineages is discussed in the context of state-of-the-art archaeobotanical finds. We shed new light on the complex evolutionary history of the GGA^tA^t wheat genepool and provide the basis for an increased use of the GGA^tA^t wheat genepool for wheat improvement. The findings have implications for our understanding of the origins of agriculture in southwest Asia.

Preferential meiotic chromosome pairing among homologous chromosomes with cryptic sequence variation in tetraploid maize

Meiotic chromosome pairing between homoeologous chromosomes was reported in many nascent allopolyploids. Homoeologous pairing is gradually eliminated and replaced by exclusive homologous pairing in well-established allopolyploids, an evolutionary process referred to as the diploidization of allopolyploids. A fundamental question of the diploidization of allopolyploids is whether and to what extent the DNA sequence variation among homoeologous chromosomes contribute to the establishment of exclusive homologous chromosome pairing. We developed aneuploid tetraploid maize lines that contain three copies of chromosome 10 derived from inbred lines B73 and H99. We were able to identify the parental origin of each copy of chromosome 10 in the materials using oligonucleotide-based haplotype-specific chromosome painting. We demonstrate that the two identical copies of chromosome 10 from H99 pair preferentially over chromosome 10 from B73 in different stages of prophase I and metaphase I during meiosis. Thus, homologous chromosome pairing is favored to partners with the most similar DNA sequences and can be discriminated based on cryptic sequence variation. We propose that innate preference of homologous chromosome pairing exists in nascent allopolyploids and serves as the first layer that would eventually block all homoeologous chromosome pairing in allopolyploids.

The Independent Domestication of Timopheev's Wheat: Insights from Haplotype Analysis of the Brittle rachis 1 (BTR1-A) Gene

Triticum turgidum and T. timopheevii are two tetraploid wheat species sharing T. urartu as a common ancestor, and domesticated accessions from both of these allopolyploids exhibit nonbrittle rachis (i.e., nonshattering spikes). We previously described the loss-of-function mutations in the Brittle Rachis 1 genes BTR1-A and BTR1-B in the A and B subgenomes, respectively, that are responsible for this most visible domestication trait in T. turgidum. Resequencing of a large panel of wild and domesticated T. turgidum accessions subsequently led to the identification of the two progenitor haplotypes of the btr1-A and btr1-B domesticated alleles. Here, we extended the haplotype analysis to other T. turgidum subspecies and to the BTR1 homologues in the related T. timopheevii species. Our results showed that all the domesticated wheat subspecies within T. turgidum share common BTR1-A and BTR1-B haplotypes, confirming their common origin. In T. timopheevii, however, we identified a novel loss-of-function btr1-A allele underlying a partially brittle spike phenotype. This novel recessive allele appeared fixed within the pool of domesticated Timopheev’s wheat but was also carried by one wild timopheevii accession exhibiting partial brittleness. The promoter region for BTR1-B could not be amplified in any T. timopheevii accessions with any T. turgidum primer combination, exemplifying the gene-level distance between the two species. Altogether, our results support the concept of independent domestication processes for the two polyploid, wheat-related species.

Genetic diversity of Ethiopian durum wheat landraces

Genetic diversity and population structure assessment in crops is essential for marker trait association, marker assisted breeding and crop germplasm conservation. We analyzed a set of 285 durum wheat accessions comprising 215 Ethiopian durum wheat landraces, 10 released durum wheat varieties, 10 advanced durum wheat lines from Ethiopia, and 50 durum wheat lines from CIMMYT. We investigated the genetic diversity and population structure for the complete panel as well as for the 215 landraces, separately based on 11,919 SNP markers with known physical positions. The whole panel was clustered into two populations representing on the one hand mainly the landraces, and on the other hand mainly released, advanced and CIMMYT lines. Further population structure analysis of the landraces uncovered 4 subgroups emphasizing the high degree of genetic diversity within Ethiopian durum landraces. Population structure based AMOVA for both sets unveiled significant (P < 0.001) variation between populations and within populations. Total variation within population accessions (81%, 76%) was higher than total variation between populations (19%, 24%) for both sets. Population structure analysis based genetic differentiation (FST) and gene flow (Nm) for the whole set and the Ethiopian landraces were 0.19 and 0.24, 1.04, and 0.81, respectively indicating high genetic differentiation and limited gene flow. Diversity indices verify that the landrace panel was more diverse with (I = 0.7, He = 0.46, uHe = 0.46) than the advanced lines (I = 0.6, He = 0.42, uHe = 0.42). Similarly, differences within the landrace clusters were observed. In summary a high genetic diversity within Ethiopian durum wheat landraces was detected, which may be a target for national and international wheat improvement programs to exploit valuable traits for biotic and abiotic stresses.

The in silico identification and characterization of a bread wheat/Triticum militinae introgression line

The capacity of the bread wheat (Triticum aestivum) genome to tolerate introgression from related genomes can be exploited for wheat improvement. A resistance to powdery mildew expressed by a derivative of the cross-bread wheat cv. Tähti × T. militinae (Tm) is known to be due to the incorporation of a Tm segment into the long arm of chromosome 4A. Here, a newly developed in silico method termed rearrangement identification and characterization (RICh) has been applied to characterize the introgression. A virtual gene order, assembled using the GenomeZipper approach, was obtained for the native copy of chromosome 4A; it incorporated 570 4A DArTseq markers to produce a zipper comprising 2132 loci. A comparison between the native and introgressed forms of the 4AL chromosome arm showed that the introgressed region is located at the distal part of the arm. The Tm segment, derived from chromosome 7G, harbours 131 homoeologs of the 357 genes present on the corresponding region of Chinese Spring 4AL. The estimated number of Tm genes transferred along with the disease resistance gene was 169. Characterizing the introgression's position, gene content and internal gene order should not only facilitate gene isolation, but may also be informative with respect to chromatin structure and behaviour studies.

Reconciling the evolutionary origin of bread wheat (Triticum aestivum)

The origin of bread wheat (Triticum aestivum; AABBDD) has been a subject of controversy and of intense debate in the scientific community over the last few decades. In 2015, three articles published in New Phytologist discussed the origin of hexaploid bread wheat (AABBDD) from the diploid progenitors Triticum urartu (AA), a relative of Aegilops speltoides (BB) and Triticum tauschii (DD). Access to new genomic resources since 2013 has offered the opportunity to gain novel insights into the paleohistory of modern bread wheat, allowing characterization of its origin from its diploid progenitors at unprecedented resolution. We propose a reconciled evolutionary scenario for the modern bread wheat genome based on the complementary investigation of transposable element and mutation dynamics between diploid, tetraploid and hexaploid wheat. In this scenario, the structural asymmetry observed between the A, B and D subgenomes in hexaploid bread wheat derives from the cumulative effect of diploid progenitor divergence, the hybrid origin of the D subgenome, and subgenome partitioning following the polyploidization events.

Recurrence of Chromosome Rearrangements and Reuse of DNA Breakpoints in the Evolution of the Triticeae Genomes

Chromosomal rearrangements (CRs) play important roles in karyotype diversity and speciation. While many CR breakpoints have been characterized at the sequence level in yeast, insects, and primates, little is known about the structure of evolutionary CR breakpoints in plant genomes, which are much more dynamic in genome size and sequence organization. Here, we report identification of breakpoints of a translocation between chromosome arms 4L and 5L of Triticeae, which is fixed in several species, including diploid wheat and rye, by comparative mapping and analysis of the draft genome and chromosome survey sequences of the Triticeae species. The wheat translocation joined the ends of breakpoints downstream of a WD40 gene on 4AL and a gene of the PMEI family on 5AL. A basic helix-loop-helix transcription factor gene in 5AL junction was significantly restructured. Rye and wheat share the same position for the 4L breakpoint, but the 5L breakpoint positions are not identical, although very close in these two species, indicating the recurrence of 4L/5L translocations in the Triticeae. Although barley does not carry the translocation, collinearity across the breakpoints was violated by putative inversions and/or transpositions. Alignment with model grass genomes indicated that the translocation breakpoints coincided with ancient inversion junctions in the Triticeae ancestor. Our results show that the 4L/5L translocation breakpoints represent two CR hotspots reused during Triticeae evolution, and support breakpoint reuse as a widespread mechanism in all eukaryotes. The mechanisms of the recurrent translocation and its role in Triticeae evolution are also discussed.

Characterization of new allele influencing flowering time in bread wheat introgressed from Triticum militinae

Flowering time variation was identified within a mapping population of doubled haploid lines developed from a cross between the introgressive line 8.1 and spring bread wheat cv. Tähti. The line 8.1 carried introgressions from tetraploid Triticum militinae in the cv. Tähti genetic background on chromosomes 1A, 2A, 4A, 5A, 7A, 1B and 5B. The most significant QTL for the flowering time variation was identified within the introgressed region on chromosome 5A and its largest effect was associated with the VRN-A1 locus, accounting for up to 70% of phenotypic variance. The allele of T. militinae origin was designated as VRN-A1f-like. The effect of the VRN-A1f-like allele was verified in two other mapping populations. QTL analysis identified that in cv. Tähti and cv. Mooni genetic background, VRN-A1f-like allele incurred a delay of 1.9-18.6 days in flowering time, depending on growing conditions. Sequence comparison of the VRN-A1f-like and VRN-A1a alleles from the parental lines of the mapping populations revealed major mutations in the promoter region as well as in the first intron, including insertion of a MITE element and a large deletion. The sequence variation allowed construction of specific diagnostic PCR markers for VRN-A1f-like allele determination. Identification and quantification of the effect of the VRN-A1f-like allele offers a useful tool for wheat breeding and for studying fine-scale regulation of flowering pathways in wheat.

Wheat syntenome unveils new evidences of contrasted evolutionary plasticity between paleo- and neoduplicated subgenomes

Bread wheat derives from a grass ancestor structured in seven protochromosomes followed by a paleotetraploidization to reach a 12 chromosomes intermediate and a neohexaploidization (involving subgenomes A, B and D) event that finally shaped the 21 modern chromosomes. Insights into wheat syntenome in sequencing conserved orthologous set (COS) genes unravelled differences in genomic structure (such as gene conservation and diversity) and genetical landscape (such as recombination pattern) between ancestral as well as recent duplicated blocks. Contrasted evolutionary plasticity is observed where the B subgenome appears more sensitive (i.e. plastic) in contrast to A as dominant (i.e. stable) in response to the neotetraploidization and D subgenome as supra-dominant (i.e. pivotal) in response to the neohexaploidization event. Finally, the wheat syntenome, delivered through a public web interface PlantSyntenyViewer at http://urgi.versailles.inra.fr/synteny-wheat, can be considered as a guide for accelerated dissection of major agronomical traits in wheat.

Molecular cytogenetic analysis of Agropyron chromatin specifying resistance to barley yellow dwarf virus in wheat

Nine families of bread wheat (TC5, TC6, TC7, TC8, TC9, TC10, TC14, 5395-(243AA), and 5395) with resistance to barley yellow dwarf virus and containing putative translocations between wheat and a group 7 chromosome of Agropyron intermedium (L1 disomic addition line, 7Ai#1 chromosome) induced by homoeologous pairing or tissue culture were analyzed. C-banding, genomic in situ hybridization (GISH), and restriction fragment length polymorphism (RFLP) in combination with repetitive Agropyron-specific sequences and deletion mapping in wheat were used to determine the relative locations of the translocation breakpoints and the size of the transferred alien chromatin segments in hexaploid wheat-Agropyron translocation lines. All homoeologous compensating lines had complete 7Ai#1 or translocated 7Ai#1-7D chromosomes that substitute for chromosome 7D. Two complete 7Ai#1 (7D) substitution lines (5395-(243AA) and 5395), one T1BS-7Ai#1S&#x2219;7Ai#1L addition line (TC7), and two different translocation types, T7DS-7Ai#1S&#x2219;7Ai#1L (TC5, TC6, TC8, TC9, and TC10) and T7DS&#x2219;7DL-7Ai#1L (TC14), substituting for chromosome 7D were identified. The substitution line 5395-(243AA) had a reciprocal T1BS&#x2219;1BL-4BS/T1BL-4BS&#x2219;4BL translocation. TC14 has a 6G (6B) substitution. The RFLP data from deletion mapping studies in wheat using 37 group 7 clones provided 10 molecular tagged chromosome regions for homoeologous and syntenic group 7 wheat or Agropyron chromosomes. Together with GISH we identified three different sizes of the transferred Agropyron chromosome segments with approximate breakpoints at fraction length (FL) 0.33 in the short arm of chromosome T7DS-7Ai#1S&#x2219;7Ai#1L (TC5, TC6, TC8, TC9, and TC10) and another at FL 0.37 of the nonhomoeologous translocated chromosome T1BS-7Ai#1S&#x2219;7Ai#1L (TC7). One breakpoint was identified in the long arm of chromosome T7DS&#x2219;7DL-7Ai#1L (TC14) at FL 0.56. We detected some nonreciprocal translocations for the most proximal region of the chromosome arm of 7DL, which resulted in small duplications. Key words : C-banding, genomic in situ hybridization (GISH), physical mapping, translocation mapping, RFLP analysis.

Genetic compensation abilities of Aegilops speltoides chromosomes for homoeologous B-genome chromosomes of polyploid wheat in disomic S(B) chromosome substitution lines

The S genome of Aegilops speltoides is closely related to the B and G genomes of polyploid wheats. However, little work has been reported on the genetic relationships between the S-genome and B-genome chromosomes of polyploid wheat. Here, we report the isolation of a set of disomic substitutions (DS) of S-genome chromosomes for the B-genome chromosomes and their effects on gametophytic and sporophytic development. Ae. speltoides chromosomes were identified by their distinct C-banding and fluorescence in situ hybridization patterns with the Ae. speltoides-derived clone pGc1R-1. Although no large structural differences between S-genome and B-genome chromosomes exist, significant differences in gametophytic compensation were observed for chromosomes 1S, 3S, 5S and 6S. Similarly, chromosomes 1S, 2S, 4S, 5S and 6S affected certain aspects of sporophytic development in relation to spike morphology, fertility and meiotic pairing. The DS5S(5B) had disturbed meiosis with univalents/multivalents and suffered chromosome elimination in the germ tissues leading to haploid spikes in 50% of the plants. The effect of the Ph1 gene on meiosis is well known, and these results provide evidence for the role of Ph1 in the maintenance of polyploid genome integrity. These and other data are discussed in relation to the structural and functional differentiation of S- and B-genome chromosomes and the practical utility of the stocks in wheat improvement.

New insights into the origin of the B genome of hexaploid wheat: evolutionary relationships at the SPA genomic region with the S genome of the diploid relative Aegilops speltoides

Background

Several studies suggested that the diploid ancestor of the B genome of tetraploid and hexaploid wheat species belongs to the Sitopsis section, having Aegilops speltoides (SS, 2n = 14) as the closest identified relative. However molecular relationships based on genomic sequence comparison, including both coding and non-coding DNA, have never been investigated. In an attempt to clarify these relationships, we compared, in this study, sequences of the Storage Protein Activator (SPA) locus region of the S genome of Ae. speltoides (2n = 14) to that of the A, B and D genomes co-resident in the hexaploid wheat species (Triticum aestivum, AABBDD, 2n = 42).

Results

Four BAC clones, spanning the SPA locus of respectively the A, B, D and S genomes, were isolated and sequenced. Orthologous genomic regions were identified as delimited by shared non-transposable elements and non-coding sequences surrounding the SPA gene and correspond to 35 268, 22 739, 43 397 and 53 919 bp for the A, B, D and S genomes, respectively. Sequence length discrepancies within and outside the SPA orthologous regions are the result of non-shared transposable elements (TE) insertions, all of which inserted after the progenitors of the four genomes divergence.

Conclusion

On the basis of conserved sequence length as well as identity of the shared non-TE regions and the SPA coding sequence, Ae speltoides appears to be more evolutionary related to the B genome of T. aestivum than the A and D genomes. However, the differential insertions of TEs, none of which are conserved between the two genomes led to the conclusion that the S genome of Ae. speltoides has diverged very early from the progenitor of the B genome which remains to be identified.

New insights into the origin of the B genome of hexaploid wheat: evolutionary relationships at the SPA genomic region with the S genome of the diploid relative Aegilops speltoides

BACKGROUND

Several studies suggested that the diploid ancestor of the B genome of tetraploid and hexaploid wheat species belongs to the Sitopsis section, having Aegilops speltoides (SS, 2n = 14) as the closest identified relative. However molecular relationships based on genomic sequence comparison, including both coding and non-coding DNA, have never been investigated. In an attempt to clarify these relationships, we compared, in this study, sequences of the Storage Protein Activator (SPA) locus region of the S genome of Ae. speltoides (2n = 14) to that of the A, B and D genomes co-resident in the hexaploid wheat species (Triticum aestivum, AABBDD, 2n = 42).

RESULTS

Four BAC clones, spanning the SPA locus of respectively the A, B, D and S genomes, were isolated and sequenced. Orthologous genomic regions were identified as delimited by shared non-transposable elements and non-coding sequences surrounding the SPA gene and correspond to 35,268, 22,739, 43,397 and 53,919 bp for the A, B, D and S genomes, respectively. Sequence length discrepancies within and outside the SPA orthologous regions are the result of non-shared transposable elements (TE) insertions, all of which inserted after the progenitors of the four genomes divergence.

CONCLUSION

On the basis of conserved sequence length as well as identity of the shared non-TE regions and the SPA coding sequence, Ae speltoides appears to be more evolutionary related to the B genome of T. aestivum than the A and D genomes. However, the differential insertions of TEs, none of which are conserved between the two genomes led to the conclusion that the S genome of Ae. speltoides has diverged very early from the progenitor of the B genome which remains to be identified.

Complex genome rearrangements reveal evolutionary dynamics of pericentromeric regions in the Triticeae

Most pericentromeric regions of eukaryotic chromosomes are heterochromatic and are the most rapidly evolving regions of complex genomes. The closely related genomes within hexaploid wheat (Triticum aestivum L., 2n=6x=42, AABBDD), as well as in the related Triticeae taxa, share large conserved chromosome segments and provide a good model for the study of the evolution of pericentromeric regions. Here we report on the comparative analysis of pericentric inversions in the Triticeae, including Triticum aestivum, Aegilops speltoides, Ae. longissima, Ae. searsii, Hordeum vulgare, Secale cereale, and Agropyron elongatum. Previously, 4 pericentric inversions were identified in the hexaploid wheat cultivar 'Chinese Spring' ('CS') involving chromosomes 2B, 4A, 4B, and 5A. In the present study, 2 additional pericentric inversions were detected in chromosomes 3B and 6B of 'CS' wheat. Only the 3B inversion pre-existed in chromosome 3S, 3Sl, and 3Ss of Aegilops species of the Sitopsis section, the remaining inversions occurring after wheat polyploidization. The translocation T2BS/6BS previously reported in 'CS' was detected in the hexaploid variety 'Wichita' but not in other species of the Triticeae. It appears that the B genome is more prone to genome rearrangements than are the A and D genomes. Five different pericentric inversions were detected in rye chromosomes 3R and 4R, 4Sl of Ae. longissima, 4H of barley, and 6E of Ag. elongatum. This indicates that pericentric regions in the Triticeae, especially those of group 4 chromosomes, are undergoing rapid and recurrent rearrangements.

Homoeologous recombination, chromosome engineering and crop improvement

Sears (1956) pioneered plant chromosome engineering 50 years ago by directed transfer of a leaf rust resistance gene from an alien chromosome to a wheat chromosome using X-ray irradiation and an elegant cytogenetic scheme. Since then many other protocols have been reported, but the one dealing with induced homoeologous pairing and recombination is the most powerful, and has been extensively used in wheat. Here, we briefly review the current status of homoeologous recombination-based chromosome engineering research in plants with a focus on wheat, and demonstrate that integrated use of cytogenetic stocks and molecular resources can enhance the efficiency and precision of homoeologus-based chromosome engineering. We report the results of an experiment on homoeologous recombination-based transfer of virus resistance from an alien chromosome to a wheat chromosome, its characterization, and the prospects for further engineering by a second round of recombination. A proposal is presented for genome-wide, homoeologous recombination-based engineering for efficient mining of gene pools of wild relatives for crop improvement.

Transferability of wheat microsatellites to diploid Aegilops species and determination of chromosomal localizations of microsatellites in the S genome

Overall, 253 genomic wheat (Triticum aestivum) microsatellite markers were studied for their transferability to the diploid species Aegilops speltoides, Aegilops longissima, and Aegilops searsii, representing the S genome. In total, 88% of all the analyzed primer pairs of markers derived from the B genome of hexaploid wheat amplified DNA fragments in the genomes of the studied species. The transferability of simple sequence repeat (SSR) markers of the T. aestivum A and D genomes totaled 74%. Triticum aestivum-Ae. speltoides, T. aestivum-Ae. longissima, and T. aestivum-Ae. searsii chromosome addition lines allowed us to determine the chromosomal localizations of 103 microsatellite markers in the Aegilops genomes. The majority of them were localized to homoeologous chromosomes in the genome of Aegilops. Several instances of nonhomoeologous localization of T. aestivum SSR markers in the Aegilops genome were considered to be either amplification of other loci or putative translocations. The results of microsatellite analysis were used to study phylogenetic relationships among the 3 species of the Sitopsis section (Ae. speltoides, Ae. longissima, and Ae. searsii) and T. aestivum. The dendrogram obtained generally reflects the current views on phylogenetic relationships among these species.

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.

Wheat cytogenetics in the genomics era and its relevance to breeding

Hexaploid wheat is a species that has been subjected to most extensive cytogenetic studies. This has contributed to understanding the mechanism of the evolution of polyploids involving diploidization through genetic restriction of chromosome pairing to only homologous chromosomes. The availability of a variety of aneuploids and the ph mutants (Ph1 and Ph2) in bread wheat also allowed chromosome manipulations leading to the development of alien addition/substitution lines and the introgression of alien chromosome segments into the wheat genome. More recently in the genomics era, molecular tools have been used extensively not only for the construction of molecular maps, but also for identification/isolation of genes/QTLs (including epistatic QTLs, eQTLs and PQLs) for several agronomic traits. It has also been possible to identify gene-rich regions and recombination hot spots in the wheat genome, which are now being subjected to sequencing at the genome level, through development of BAC libraries. In the EST database also, among all plants wheat ESTs are the highest in number, and are only next to those for human, mouse, Ciona intestinalis (a chordate), rat and zebrafish genomes. These ESTs and sequences of several genomic regions have been subjected to a variety of applications including development of perfect markers and establishment of microcollinearity. The technique of in situ hybridization (including FISH, GISH and McFISH) and the development of deletion stocks also facilitated the preparation of physical maps. Molecular markers are also used for marker-assisted selection in wheat breeding programs in several countries. Construction of a wheat DNA chip, which will also become available soon, may further facilitate wheat genomics research. These enormous resources, knowledge base and the fast development of additional molecular tools and high throughput approaches for genotyping will prove extremely useful in future wheat research and will lead to development of improved wheat cultivars.

Simultaneous painting of three genomes in hexaploid wheat by BAC-FISH

Fluorescence in situ hybridization (FISH) is widely used in the physical mapping of genes and chromosome landmarks in plants and animals. Bacterial artificial chromosomes (BACs) contain large inserts, making them amenable for FISH mapping. In our BAC-FISH experiments, we selected 56 restriction fragment length polymorphism (RFLP)-locus-specific BAC clones from the libraries of Triticum monococcum and Aegilops tauschii, which are the A- and D-genome donors of wheat (Triticum aestivum, 2n = 6x = 42), respectively. The BAC clone 676D4 from the T. monococcum library contains a dispersed repeat that preferentially hybridizes to A-genome chromosomes, and two BAC clones, 9I10 and 9M13, from the Ae. tauschii library contain a dispersed repeat that preferentially hybridizes to the D-genome chromosomes. These repeats are useful in simultaneously discriminating the three different genomes in hexaploid wheat, and in identifying intergenomic translocations in wheat or between wheat and alien chromosomes. Sequencing results show that both of these repeats are transposable elements, indicating the importance of transposable elements, especially retrotransposons, in the genome evolution of wheat.Key words: bacterial artificial chromosome (BAC), fluorescence in situ hybridization (FISH), transposable elements (TEs), wheat, Triticum aestivum.

The synaptic behaviour of the wild forms of Triticum turgidum and T. timopheevii

Different wild allopolyploid species of Triticeae show extensive bivalent formation at zygotene while a considerable number of multivalents is present in cultivated polyploid wheats. To study the chromosome behaviour at early meiotic stages in wild forms of tetraploid wheats Triticum turgidum and T timopheevii (2n = 4x = 28) we have analysed the synaptic pattern in fully traced spread nuclei at mid- and late zygotene and at pachytene of wild accessions of these species. The mean number of synaptonemal complex (SC) bivalents at mid-zygotene ranged from 12.22 to 13.14 among the accessions studied indicating a strong restriction of synapsis initiation to homologous chromosomes. The mean of bivalents increased at pachytene because of the transformation of multivalents into bivalents. Ring bivalents observed at metaphase I support that SC bivalents were formed by homologous chromosomes. The average values of SC bivalents at mid-zygotene in the wild forms are much higher than the average values observed in the cultivated tetraploid wheats but similar to that of a mutant line of T turgidum with a duplication that includes Ph1, the major homoeologous pairing suppressor locus. These results suggest that the efficiency of the mechanism operating in the homologous recognition for synapsis is higher in wild wheat populations than in cultivated varieties. Apparently, a relatively detrimental modification of the pairing regulating genetic system accompanied the domestication of the wild wheat forms.

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.

Chromosome structure of Triticum timopheevii relative to T. turgidum

The chromosome structure of four different wild populations and a cultivated line of Triticum timopheevii (2n = 28, AtAtGG) relative to Triticum turgidum (2n = 28, AABB) was studied, using genomic in situ hybridisation (GISH) and C-banding analysis of meiotic configurations in interspecific hybrids. Two wild accessions and the cultivated line showed the standard C-banding karyotype. The other two accessions are homozygous for translocation 5At/3G and translocations 1G/2G and 5G/6G. GISH analysis revealed that all the T. timopheevii accessions carry intergenome translocations 6At/1G and 1G/4G and identified the position of the breakpoint in translocation 5At/3G. C-banding analysis of pairing at metaphase I in the hybrids with T. turgidum provides evidence that four species-specific translocations (6AtS/1GS, 1GS/4GS, 4GS/4AtL, and 4AtL/3AtL) exist in T. timopheevii, and that T. timopheevii and T. turgidum differ in the pericentric inversion of chromosome 4A. Bridge plus acentric fragment configurations involving 4AL and 4AtL were identified in cells at anaphase I. This result suggests that the paracentric inversion of 4AL from T. turgidum does not exist in T. timopheevii. Both tetraploid species have undergone independent and distinct evolutionary chromosomal rearrangements. The position, intercalary or subdistal, of the breakpoints in species-specific translocations and inversions contrasts with the position, at or close to the centromere, of intraspecific translocations. Different mechanisms for intraspecific and species-specific chromosome rearrangements are suggested.Key words: Triticum timopheevii, chromosome pairing, translocation, evolution, C-banding, GISH.

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.

Chromosome substitutions of Triticum timopheevii in common wheat and some observations on the evolution of polyploid wheat species

Whether the two tetraploid wheat species, the well known Triticum turgidum L. (macaroni wheat, AABB genomes) and the obscure T. timopheevii Zhuk. (A(t)A(t)GG), have monophyletic or diphyletic origin from the same or different diploid species presents an interesting evolutionary problem. Moreover, T. timopheevii and its wild form T. araraticum are an important genetic resource for macaroni and bread-wheat improvement. To study these objectives, the substitution and genetic compensation abilities of individual T. timopheevii chromosomes for missing chromosomes of T. aestivum 'Chinese Spring' (AABBDD) were analyzed. 'Chinese Spring' aneuploids (nullisomic-tetrasomics) were crossed with a T. timopheevii x Aegilops tauschii amphiploid to isolate T. timopheevii chromosomes in a monosomic condition. The F1 hybrids were backcrossed one to four times to Chinese Spring aneuploids without selection for the T. timopheevii chromosome of interest. While spontaneous substitutions involving all A(t)- and G-genome chromosomes were identified, the targeted T. timopheevii chromosome was not always recovered. Lines with spontaneous substitutions from T. timopheevii were chosen for further backcrossing. Six T. timopheevii chromosome substitutions were isolated: 6A(t) (6A), 2G (2B), 3G (3B), 4G (4B), 5G (5B) and 6G (6B). The substitution lines had normal morphology and fertility. The 6A(t) of T. timopheevii was involved in a translocation with chromosome 1G, resulting in the transfer of the group-1 gliadin locus to 6A(t). Chromosome 2G substituted for 2B at a frequency higher than expected and may carry putative homoeoalleles of gametocidal genes present on group-2 chromosomes of several alien species. Our data indicate a common origin for tetraploid wheat species, but from separate hybridization events because of the presence of a different spectrum of intergenomic translocations.

Molecular evidence forTriticum speltoidesas 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.

Variability in wheat based on low-copy DNA sequence comparisons

The chromosomes of the B genome of hexaploid wheat (AABBDD) do not pair completely with those of any of the diploid species with genomes similar to B. Various biochemical and molecular analyses have suggested that each of the five diploid species in section Sitopsis of Triticum are ancestral to B. These observations have led to the hypothesis that the B genome may be polyphyletic, descending from more than one diploid ancestor. This hypothesis may account for differences between the wheat B genome and the diploids and also for variability that currently exists among different wheat accessions. In this study, we cloned and compared nucleotide sequences for three low-copy DNA fragments from the B and D genomes of several wheat accessions and from diploid relatives of the B and D genomes. Our results suggested that the amount of DNA sequence variability in wheat is low, although somewhat more variability existed in the B genome than in the D genome. The B genome of wheat was significantly diverged from all the Sitopsis diploid species, and Triticum speltoides was closer to B than to other members of this section. The D genome of wheat was very similar to that of its progenitor, Triticum tauschii. No evidence for a polyphyletic origin of the B genome was found. A more parsimonious hypothesis is that the wheat B genome diverged from its diploid ancestor after the original hybridization event occurred.

Standard karyotype of Triticum searsii and its relationship with other S-genome species and common wheat

C-banding polymorphism was analyzed in 14 accessions of Triticum searsii from Israel, and a generalized idiogram of the species was established. One accession was homozygous for whole arm translocations T1S(s)S·4S(s)S and T1S(s)L·4S(s)L. C-banding analysis was also used to identify 7 T. aestivum cv 'Chinese Spring'-T. searsii disomic chromosome addition lines, 14 ditelosomic chromosome addition lines, 21 disomic whole chromosome, and 31 ditelosomic chromosome substitution lines. The identity of these lines was further confirmed by meiotic pairing analysis. Sporophytic and gametophytic compensation tests were used to determine the homoeologous relationships of the T. searsii chromosomes. The results show that the T. searsii chromosomes do not compensate well for their wheat homoeologues. The C-banding patterns of T. searsii chromosomes are distinct from those of other S-genome species and from the B-genome chromosomes of wheat, indicating that T. searsii is not a direct B-genome donor species of T. turgidum and T. aestivum.

Comparison of genetic and physical maps of group 7 chromosomes from Triticum aestivum L

We present a high density physical map of homoeologous group 7 chromosomes from Triticum aestivum L. using a series of 54 deletion lines, 6 random amplified polymorphic DNA (RAPD) markers and 91 cDNA or genomic DNA clones from wheat, barley and oat. So far, 51 chromosome segments have been distinguished by molecular markers, and 54 homoeoloci have been allocated among chromosomes 7A, 7B and 7D. The linear order of molecular markers along the chromosomes is almost identical in the A- B- and D-genome of wheat. In addition, there is colinearity between the physical and genetic maps of chromosomes 7A, 7B and 7D from T. aestivum, indicating gene synteny among the Triticeae. However, comparison of the physical map of chromosome 7D from T. aestivum with the genetic map from Triticum tauschii some markers have been shown to be physically allocated with distortion in more distal chromosome regions. The integration of genetic and physical maps could assist in estimating the frequency and distribution of recombination in defined regions along the chromosome. Physical distance did not correlate with genetic distance. A dense map facilitates the detection of multiple rearrangements. We present the first evidence for an interstitial inversion either on chromosome arm 7AS or 7DS of Chinese Spring. Molecularly tagged chromosome regions (MTCRs) provide landmarks for long-range mapping of DNA fragments.

Cytogenetic investigation of Triticum timopheevii (Zhuk.) Zhuk. and related species using the C-banding technique

Triticum timopheevii and related species T. militinae (2n=28, A(t)G) and T. zhukovskyi (2n=42, A(m)A(t)G), hybrids T. kiharae, T. miguschovae, the amphidiploid T. timopheevii x T. tauschii (all 2n=42, A(t)GD), T. fungicidum (ABA(t)G) and T. timonovum (2n=56, A(t)A(t)GG) were analyzed using the C-banding technique. Chromosomes of the A(m) and A(t) genomes in the karyotype of T. zhukovskyi differed in their C-banding pattern. Partial substitutions of A(t)-genome chromosomes and a complete substitution of the G-genome chromosomes by homoeologous chromosomes of an unidentified tetraploid wheat species with an AB genome composition were found in the T. timonovum karyotype. A(t)- and G-genome chromosomes in the karyotypes of all studied species had similar C-banding patterns and were characterized by a low level of polymorphism. The comparative stability of the A(t) and G genomes is determined by the origin and specifity of cultivation of studied species.

New 18S.26S ribosomal RNA gene loci: chromosomal landmarks for the evolution of polyploid wheats

Three new 18S.26S rRNA gene loci were identified in common wheat by sequential N-banding and in situ hybridization (ISH) analysis. Locus Nor-A7 is located at the terminal area of the long arm of 5A in both diploid and polyploid wheats. Locus Nor-B6 is located in N-band 1BL2.5 of the long arm of chromosome 1B in Triticum turgidum and Triticum aestivum. ISH sites, similar to Nor-B6, were also detected on the long arms of chromosomes 1G in Triticum timopheevii and 1S in Aegilops speltoides, but their locations on the chromosomes were different from that of Nor-B6, indicating possible chromosome rearrangements in 1GL and 1BL during evolution. The third new locus, Nor-D8, was only found on the short arm of chromosome 3D in the common wheat Wichita. The loss of rRNA gene locus Nor-A3 and gain of repetitive DNA sequence pSc119 on the terminal part of 5AS suggest a structural modification of 5AS. Comparative studies of the location of the 18S.26S rRNA gene loci in polyploid wheats and putative A and B (G) genome progenitor species support the idea that: (1) Triticum monococcum subsp. urartu is the donor of both the A and A(t) genome of polyploid wheats. (2) Ae. speltoides is closer to the B and G genome of polyploid wheats than Aegilops longissima and is the most probable progenitor of these two genomes.

Different species-specific chromosome translocations in Triticum timopheevii and T. turgidum support the diphyletic origin of polyploid wheats

Triticum timopheevii ssp. timopheevii and T. timopheevii ssp. araraticum were analysed by sequential N-banding and genomic in situ hybridization. Three chromosomes, 6At, 1G and 4G, were involved in At-G intergenomic translocations in all six lines analysed. These chromosomes may be derived from a cyclic translocation that is species-specific to T. timopheevii. In contrast, Triticum turgidum has a species-specific cyclic translocation involving chromosomes 4A, 5A and 7B. The discovery of different species-specific chromosome translocations supports the diphyletic hypothesis of the evolution of tetraploid wheats. The results from genomic blocking analysis also revealed that the chromosomes of Aegilops speltoides are closer to the G genome than the B genome chromosomes. The possible role of species-specific translocations in the evolution of wheat is discussed.

Stabilization of tetraploid triticale with chromosomes from Triticum aestivum (ABD)(ABD)RR (2n = 28)

F1 hybrids with the genome constitution ABDERR (2n = 6x = 42) or ABDE(AB)RR (2n = 7x = 49), selected from crosses between either an octoploid Triticum aestivum/Thinopyrum elongatun amphiploid and tetraploid Secale cereale (AABBDDEE x RRRR) or autoallohexaploid triticale [AABBDDEE x (AB)(AB)RRRR], were backcrossed to tetraploid triticale (AB)(AB)RR and selfed for six generations. Thirty-three different tetraploid F6 progenies were karyotyped using C-banding. The aneuploidy frequency was 6.6% with 4.0% hypoploids and 2.6% hyperploids. Among 71 plants with 28 chromosomes, 53.5% had a stabilized karyotype while 46.5% were unstabilized with at least one homoeologous group segregating for A-, B-, or D-genome chromosomes. The stabilized plants represent 19 different tetraploid karyotypes with six of them not containing any detectable D-genome chromosomes from T. aestivum or E-genome chromosome from Th. elongatum. Thirteen lines were (ABD)(ABD)RR tetraploids with one-to-three disomic substitutions of D-genome chromosomes for A or B-genome chromosomes. No disomic substitution of E-genome chromosomes was identified. On average 0.58 D substitutions per line were determined. Of the seven D-genome chromosomes only four, 1D, 2D, 5D, and 7D, were present in their disomic state. In unstabilized karyotypes, chromosomes 3D, 4D, and 6D were present in their monosomic state. Among all 30 viable plants (42.3%), the order of decreasing frequency of Dgenome chromosomes was 5D (25.0%), 1D (20.0%), 2D (10.0%), 6D (5.0%), and 3D (1.7%). Plants with 4D and 7D chromosomes were not viable. An increase in the number of D-genome chromosomes in the (ABD) genome is associated with a decrease in viability and fertility. Minor differences in the C-banding of chromosomes in homoeologous groups 1, 5, and 6 indicate the possibility of translocations between A-, B-, D-, and E-genome chromosomes. Evolutionary and breeding aspects of tetraploid triticale with mixed genomes are discussed.

Metaphase-I bound-arm frequency and genome analysis in wheat-Aegilops hybrids. 2. Cytogenetical evidence for excluding Ae. Sharonensis as the donor of the B genome of polyploid wheats

Genome affinities were analyzed at meiosis in C-banded metaphase-I cells of wheat x Ae. Sharonensis hybrid plants. The results showed that the most frequent type of pairing occurred between chromosomes of the A and D genomes in all plants, as well as in cells with different numbers of associations. These findings clearly indicated that Ae. Sharonensis can be excluded as the donor of the B genome of wheat.

The presence of the enhanced K/Na discrimination trait in diploid Triticum species

A number of accessions of the three species of diploid wheat, Triticum boeoticum, T. monococcum, and T. urartu, were grown in 50 mol m(-3) NaCl+2.5 mol m(-3) CaCl2. Sodium accumulation in the leaves was low and potassium concentrations remained high. This was not the case in T. durum grown under the same conditions, and indicates the presence in diploid wheats of the enhanced K/Na discrimination character which has previously been found in Aegilops squarrosa and hexaploid wheat. None of the accessions of diploid wheat showed poor K/Na discrimination, which suggests that if the A genome of modern tetraploid wheats was derived from a diploid Triticum species, then the enhanced K/Na discrimination character became altered after the formation of the original allopolyploid. Another possibility is that a diploid wheat that did not have the enhanced K/Na discrimination character was involved in the hybridization event which produced tetraploid wheat, and that this diploid is now extinct or has not yet been discovered.

General features of chromosome substitutions in Triticum aestivum x T. timopheevii hybrids

Based on a C-banded chromosome analysis of Triticum aestivum x T. timopheevii hybrid lines, we developed a classification of the A(t) and G genome chromosomes that agrees with the standard genetic nomenclature of T. aestivum chromosomes. Transfer of genetic material from T. timopheevii to T. aestivum involved complete chromosomes, chromosome arms, or segments. Frequencies of chromosome substitution were highest for homoeologous group 2 and lowest for group 4. Moreover, B(G) substitutions were frequent compared to A(A(t)). The pattern of chromosome substitutions and rearrangements were unequal and depended on the genotype of the parental T. aestivum cultivar.

Chromosome pairing relationships among the A, B, and D genomes of bread wheat

Chromosome pairing and chiasma frequency were studied in bread wheat euhaploids (2n = 3x = 21; ABD genomes) with and without the major pairing regulatorPh1. This constitutes the first report of chromosome pairing relationships among the A, B, and D genomes of wheat without the influence of an alien genome. AllPh1 euhaploids had very little pairing, with 0.62-1.05 rod bivalents per cell; ring bivalents were virtually absent and mean arm-binding frequency (c) values ranged from 0.050 to 0.086. In contrast, theph1b euhaploids had extensive homoeologous pairing, with chiasma frequency 7.5-11.6 times higher than that in thePh1 euhaploids. They had 0.53-1.16 trivalents, 1.53-1.74 ring bivalents, and 2.90-3.57 rod bivalents, withc from 0.580 to 0.629. N-banding of meiotic chromosomes showed strongly preferential pairing between chromosomes of the A and D genomes; 80% of the pairing was between these genomes, especially in the presence of theph1b allele. The application of mathematical models to unmarked chromosomes also supported a 2∶1 genomic structure of theph1b euhaploids. Numerical modeling suggested that about 80% of the metaphase I association was between the two most related genomes in the presence ofph1b, but that pairing under Ph1 was considerably more random. The data demonstrate that the A and D genomes are much more closely related to each other than either is to B. These results may have phylogenetic significance and hence breeding implications.

Cytological analysis on the distribution and origin of the alien chromosome pair conferring blue aleurone color in several European common wheat (Triticum aestivum L.) strains

Meiotic chromosome pairing and Giemsa C-banding analyses in crosses of several European blue-grained wheat strains with Chinese Spring double ditelosomic and other aneuploid lines showed that Triticum aestivum Blaukorn strains "Berlin," "Probstdorf," "Tschermak," and "Weihenstephan" are chromosome substitutions, in which the complete wheat chromosome 4A pair is replaced, whereas the strains "Brünn" and "Moskau" are 4B substitutions. The alien chromosome pair in all of these strains is an A genome chromosome (4A) from diploid Triticum monococcum or T. boeoticum not present in common tetraploid and hexaploid cultivated wheats. The Blaukorn strain Weihenstephan "W 70a86" possesses, in addition to a rye chromosome pair 5R compensating for the loss of part of chromosome 5D, a 4A/5DL translocation replacing chromosome pair 4B of wheat.

Chromosome structure of durum wheat

Meiotic pairing was analyzed at metaphase I in ABRR triticale × rye hybrids to identify the arm homoeology of chromosomes ofTriticum turgidum. Wheat chromosomes were identified using a C-banding technique. Results confirmed that the homoeologous relationships between chromosome arms of the A and B genomes inT. turgidum are the same as inT. aestivum, and that a double translocation involving4AL,5AL, and7BS is present inT. turgidum. It is proposed that a pericentric inversion involving a substantial portion of chromosome4A is present inT. turgidum andT. aestivum.