Genome differentiation in Aegilops. 2. Physical mapping of 5S and 18S-26S ribosomal RNA gene families in diploid species

Intraspecific divergence of diploid grass Aegilopscomosa is associated with structural chromosome changes

Aegilopscomosa Smith in Sibthorp et Smith, 1806 is diploid grass with MM genome constitution occurring mainly in Greece. Two morphologically distinct subspecies - Ae.c.comosa Chennaveeraiah, 1960 and Ae.c.heldreichii (Holzmann ex Boissier) Eig, 1929 are discriminated within Ae.comosa, however, genetic and karyotypic bases of their divergence are not fully understood. We used Fluorescence in situ hybridization (FISH) with repetitive DNA probes and electrophoretic analysis of gliadins to characterize the genome and karyotype of Ae.comosa to assess the level of their genetic diversity and uncover mechanisms leading to radiation of subspecies. We show that two subspecies differ in size and morphology of chromosomes 3M and 6M, which can be due to reciprocal translocation. Subspecies also differ in the amount and distribution of microsatellite and satellite DNA sequences, the number and position of minor NORs, especially on 3M and 6M, and gliadin spectra mainly in the a-zone. Frequent occurrence of hybrids can be caused by open pollination, which, along with genetic heterogeneity of accessions and, probably, the lack of geographic or genetic barrier between the subspecies, may contribute to extremely broad intraspecific variation of GAA_n and gliadin patterns in Ae.comosa, which are usually not observed in endemic plant species.

The launch of satellite: DNA repeats as a cytogenetic tool in discovering the chromosomal universe of wild Triticeae

Fluorescence in situ hybridization is a powerful tool that enables plant researchers to perform systematic, evolutionary, and population studies of wheat wild relatives as well as to characterize alien introgression into the wheat genome. This retrospective review reflects on progress made in the development of methods for creating new chromosomal markers since the launch of this cytogenetic satellite instrument to the present day. DNA probes based on satellite repeats have been widely used for chromosome analysis, especially for "classical" wheat probes (pSc119.2 and Afa family) and "universal" repeats (45S rDNA, 5S rDNA, and microsatellites). The rapid development of new-generation sequencing and bioinformatical tools, and the application of oligo- and multioligonucleotides has resulted in an explosion in the discovery of new genome- and chromosome-specific chromosome markers. Owing to modern technologies, new chromosomal markers are appearing at an unprecedented velocity. The present review describes the specifics of localization when employing commonly used vs. newly developed probes for chromosomes in J, E, V, St, Y, and P genomes and their diploid and polyploid carriers Agropyron, Dasypyrum, Thinopyrum, Pseudoroegneria, Elymus, Roegneria, and Kengyilia. Particular attention is paid to the specificity of probes, which determines their applicability for the detection of alien introgression to enhance the genetic diversity of wheat through wide hybridization. The information from the reviewed articles is summarized into the TRepeT database, which may be useful for studying the cytogenetics of Triticeae. The review describes the trends in the development of technology used in establishing chromosomal markers that can be used for prediction and foresight in the field of molecular biology and in methods of cytogenetic analysis.

Fluorescence In Situ Hybridization (FISH) for the Genotyping of Triticeae Tribe Species and Hybrids

This chapter is dedicated to using fluorescence in situ hybridization (FISH) for the genotyping of Triticeae tribe species and hybrids. The basic method of FISH on metaphase chromosomes is presented with a discussion on its modifications, and deoxyribonucleic acid (DNA) probes that can be useful for genotyping are proposed.

Development and application of specific FISH probes for karyotyping Psathyrostachys huashanica chromosomes

Background

Psathyrostachys huashanica Keng has long been used as a genetic resource for improving wheat cultivar because of its genes mediating the resistance to various diseases (stripe rust, leaf rust, take-all, and powdery mildew) as well as its desirable agronomic traits. However, a high-resolution fluorescence in situ hybridization (FISH) karyotype of P. huashanica remains unavailable.

Results

To develop chromosome-specific FISH markers for P. huashanica, repetitive sequences, including pSc119.2, pTa535, pTa713, pAs1, (AAC)₅, (CTT)₁₂, pSc200, pTa71A-2, and Oligo-44 were used for a FISH analysis. The results indicated that the combination of pSc200, pTa71A-2 and Oligo-44 probes can clearly identify all Ns genomic chromosomes in the two P. huashanica germplasms. The homoeologous relationships between individual P. huashanica chromosomes and common wheat chromosomes were clarified by FISH painting. Marker validation analyses revealed that the combination of pSc200, pTa71A-2, and Oligo-44 for a FISH analysis can distinguish the P. huashanica Ns-genome chromosomes from wheat chromosomes, as well as all chromosomes (except 4Ns) from the chromosomes of diploid wheat relatives carrying St, E, V, I, P and R genomes. Additionally, the probes were applicable for discriminating between the P. huashanica Ns-genome chromosomes in all homologous groups and the corresponding chromosomes in Psathyrostachys juncea and most Leymus species containing the Ns genome. Furthermore, six wheat–P. huashanica chromosome addition lines (i.e., 2Ns, 3Ns, 4Ns, 7Ns chromosomes and chromosomal segments) were characterized using the newly developed FISH markers. Thus, these probes can rapidly and precisely detect P. huashanica alien chromosomes in the wheat background.

Conclusions

The FISH karyotype established in this study lays a solid foundation for the efficient identification of P. huashanica chromosomes in wheat genetic improvement programs.

Genome sequences of three Aegilops species of the section Sitopsis reveal phylogenetic relationships and provide resources for wheat improvement

Aegilops is a close relative of wheat (Triticum spp.), and Aegilops species in the section Sitopsis represent a rich reservoir of genetic diversity for the improvement of wheat. To understand their diversity and advance their utilization, we produced whole-genome assemblies of Aegilops longissima and Aegilops speltoides. Whole-genome comparative analysis, along with the recently sequenced Aegilops sharonensis genome, showed that the Ae. longissima and Ae. sharonensis genomes are highly similar and are most closely related to the wheat D subgenome. By contrast, the Ae. speltoides genome is more closely related to the B subgenome. Haplotype block analysis supported the idea that Ae. speltoides genome is closest to the wheat B subgenome, and highlighted variable and similar genomic regions between the three Aegilops species and wheat. Genome-wide analysis of nucleotide-binding leucine-rich repeat (NLR) genes revealed species-specific and lineage-specific NLR genes and variants, demonstrating the potential of Aegilops genomes for wheat improvement.

Identification and Evaluation of Wheat-Aegilops bicornis Lines with Resistance to Powdery Mildew and Stripe Rust

Wheat pathogens, especially those causing powdery mildew and stripe rust, seriously threaten yield worldwide. Utilizing newly identified disease resistance genes from wheat relatives is an effective strategy to minimize disease damage. In this study, chromosome-specific molecular markers for the 3S^b and 7S^b chromosomes of Aegilops bicornis were developed using PCR-based landmark unique gene primers for screening wheat-A. bicornis progenies. Fluorescence in situ hybridization (FISH) was performed to further identify wheat-A. bicornis progenies using oligonucleotides probes Oligo-pSc119.2-1, Oligo-pTa535-1, and Oligo-(GAA)₈. After establishing A. bicornis 3S^b and 7S^b chromosome-specific FISH markers, Holdfast (common wheat)-A. bicornis 3S^b addition, 7S^b addition, 3S^b(3A) substitution, 3S^b(3B) substitution, 3S^b(3D) substitution, 7S^b(7A) substitution, and 7S^b(7B) substitution lines were identified by the molecular and cytological markers. Stripe rust and powdery mildew resistance, along with agronomic traits, were investigated to evaluate the breeding potential of these lines. Holdfast and Holdfast-A. bicornis progenies were all highly resistant to stripe rust, indicating that the stripe rust resistance might derive from Holdfast. However, Holdfast-A. bicornis 3S^b addition, 3S^b(3A) substitution, 3S^b(3B) substitution, and 3S^b(3D) substitution lines showed high resistance to powdery mildew while Holdfast was highly susceptible, indicating that chromosome 3S^b of A. bicornis carries previously unknown powdery mildew resistance gene(s). Additionally, the transfer of the 3S^b chromosome from A. bicornis to wheat significantly increased tiller number, but chromosome 7S^b has a negative effect on agronomic traits. Therefore, wheat germplasm containing A. bicornis chromosome 3S^b has potential to contribute to improving powdery mildew resistance and tiller number during wheat breeding.

Advances in the Molecular Cytogenetics of Bananas, Family Musaceae

The banana is a staple food crop and represents an important trade commodity for millions of people living in tropical and subtropical countries. The most important edible banana clones originated from natural crosses between diploid Musa balbisiana and various subspecies of M. acuminata. It is worth mentioning that evolution and speciation in the Musaceae family were accompanied by large-scale chromosome structural changes, indicating possible reasons for lower fertility or complete sterility of these vegetatively propagated clones. Chromosomal changes, often accompanied by changes in genome size, are one of the driving forces underlying speciation in plants. They can clarify the genomic constitution of edible bananas and shed light on their origin and on diversification processes in members of the Musaceae family. This article reviews the development of molecular cytogenetic approaches, ranging from classical fluorescence in situ hybridization (FISH) using common cytogenetic markers to oligo painting FISH. We discuss differences in genome size and chromosome number across the Musaceae family in addition to the development of new chromosome-specific cytogenetic probes and their use in genome structure and comparative karyotype analysis. The impact of these methodological advances on our knowledge of Musa genome evolution at the chromosomal level is demonstrated. In addition to citing published results, we include our own new unpublished results and outline future applications of molecular cytogenetics in banana research.

Genetic diversity of ribosomal loci (5S and 45S rDNA) and pSc119.2 repetitive DNA sequence among four species of Aegilops (Poaceae) from Algeria

In continuation of our previous research we carried out the karyological investigation of 53 populations of four Aegilops species (A. geniculata, A. triuncialis, A. ventricosa, and A. neglecta) sampled in different eco-geographical habitats in Algeria. The genetic variability of the chromosomal DNA loci of the same collection of Aegilops is highlighted by the Fluorescence In Situ Hybridization technique (FISH) using three probes: 5S rDNA, 45S rDNA, and repetitive DNA (pSc119.2). We found that the two rDNA loci (5S and 45S) hybridized with some chromosomes and showed a large genetic polymorphism within and between the four Aegilops species, while the repetitive DNA sequences (pSc119.2) hybridized with all chromosomes and differentiated the populations of the mountains with a humid bioclimate from the populations of the steppe regions with an arid bioclimate. However, the transposition of the physical maps of the studied loci (5S rDNA, 45S rDNA, and pSc119.2) with those of other collections revealed the existence of new loci in Aegilops from Algeria.

Physical localization of 45S rDNA in Cymbopogon and the analysis of differential distribution of rDNA in homologous chromosomes of Cymbopogon winterianus

Cymbopogon, commonly known as lemon grass, is one of the most important aromatic grasses having therapeutic and medicinal values. FISH signals on somatic chromosome spreads off Cymbopogon species indicated the localization of 45S rDNA on the terminal region of short arms of a chromosome pair. A considerable interspecific variation in the intensity of 45S rDNA hybridization signals was observed in the cultivars of Cymbopogon winterianus and Cymbopogon flexuosus. Furthermore, in all the varieties of C. winterianus namely Bio-13, Manjari and Medini, a differential distribution of 45S rDNA was observed in a heterologous pair of chromosomes 1. The development of C. winterianus var. Manjari through gamma radiation may be responsible for breakage of fragile rDNA site from one of the chromosomes of this heterologous chromosome pair. While, in other two varieties of C. winterianus (Bio-13 and Medini), this variability may be because of evolutionary speciation due to natural cross among two species of Cymbopogon which was fixed through clonal propagation. However, in both the situations these changes were fixed by vegetative method of propagation which is general mode of reproduction in the case of C. winterianus.

Chorus2: design of genome-scale oligonucleotide-based probes for fluorescence in situ hybridization

Oligonucleotide (oligo)-fluorescence in situ hybridization (FISH) has rapidly becoming the new generation of FISH technique in plant molecular cytogenetics research. Genome-scale identification of single-copy oligos is the foundation of successful oligo-FISH experiments. Here, we introduce Chorus2, a software that is developed specifically for oligo selection. We demonstrate that Chorus2 is highly effective to remove all repetitive elements in selection of single-copy oligos, which is critical for the development of successful FISH probes. Chorus2 is more effective than Chorus, the original version of the pipeline, and OligoMiner for repeat removal. Chorus2 allows to select oligos that are conserved among related species, which extends the usage of oligo-FISH probes among phylogenetically related plant species. We also implemented a new function in Chorus2 that allows development of FISH probes from plant species without an assembled genome. We anticipate that Chorus2 can be used in plants as well as in mammalian and other non-plant species. Chorus2 will broadly facilitate the design of FISH probes for various types of application in molecular cytogenetics research.

Single Copy Oligonucleotide Fluorescence In Situ Hybridization Probe Design Platforms: Development, Application and Evaluation

Oligonucleotides fluorescence in situ hybridization (Oligo-FISH) is an emerging technology and is an important tool in research areas such as detection of chromosome variation, identification of allopolyploid, and deciphering of three-dimensional (3D) genome structures. Based on the demand for highly efficient oligo probes for oligo-FISH experiments, increasing numbers of tools have been developed for probe design in recent years. Obsolete oligonucleotide design tools have been adapted for oligo-FISH probe design because of their similar considerations. With the development of DNA sequencing and large-scale synthesis, novel tools have been designed to increase the specificity of designed oligo probes and enable genome-scale oligo probe design, which has greatly improved the application of single copy oligo-FISH. Despite this, few studies have introduced the development of the oligo-FISH probe design tools and their application in FISH experiments systematically. Besides, a comprehensive comparison and evaluation is lacking for the available tools. In this review, we provide an overview of the oligo-FISH probe design process, summarize the development and application of the available tools, evaluate several state-of-art tools, and eventually provide guidance for single copy oligo-FISH probe design.

Development of DNA Markers From Physically Mapped Loci in Aegilops comosa and Aegilops umbellulata Using Single-Gene FISH and Chromosome Sequences

Breeding of agricultural crops adapted to climate change and resistant to diseases and pests is hindered by a limited gene pool because of domestication and thousands of years of human selection. One way to increase genetic variation is chromosome-mediated gene transfer from wild relatives by cross hybridization. In the case of wheat (Triticum aestivum), the species of genus Aegilops are a particularly attractive source of new genes and alleles. However, during the evolution of the Aegilops and Triticum genera, diversification of the D-genome lineage resulted in the formation of diploid C, M, and U genomes of Aegilops. The extent of structural genome alterations, which accompanied their evolution and speciation, and the shortage of molecular tools to detect Aegilops chromatin hamper gene transfer into wheat. To investigate the chromosome structure and help develop molecular markers with a known physical position that could improve the efficiency of the selection of desired introgressions, we developed single-gene fluorescence in situ hybridization (FISH) maps for M- and U-genome progenitors, Aegilops comosa and Aegilops umbellulata, respectively. Forty-three ortholog genes were located on 47 loci in Ae. comosa and on 52 loci in Ae. umbellulata using wheat cDNA probes. The results obtained showed that M-genome chromosomes preserved collinearity with those of wheat, excluding 2 and 6M containing an intrachromosomal rearrangement and paracentric inversion of 6ML, respectively. While Ae. umbellulata chromosomes 1, 3, and 5U maintained collinearity with wheat, structural reorganizations in 2, 4, 6, and 7U suggested a similarity with the C genome of Aegilops markgrafii. To develop molecular markers with exact physical positions on chromosomes of Aegilops, the single-gene FISH data were validated in silico using DNA sequence assemblies from flow-sorted M- and U-genome chromosomes. The sequence similarity search of cDNA sequences confirmed 44 out of the 47 single-gene loci in Ae. comosa and 40 of the 52 map positions in Ae. umbellulata. Polymorphic regions, thus, identified enabled the development of molecular markers, which were PCR validated using wheat-Aegilops disomic chromosome addition lines. The single-gene FISH-based approach allowed the development of PCR markers specific for cytogenetically mapped positions on Aegilops chromosomes, substituting as yet unavailable segregating map. The new knowledge and resources will support the efforts for the introgression of Aegilops genes into wheat and their cloning.

A universal karyotypic system for hexaploid and diploid Avena species brings oat cytogenetics into the genomics era

BACKGROUND

The identification of chromosomes among Avena species have been studied by C-banding and in situ hybridization. However, the complicated results from several cytogenetic nomenclatures for identifying oat chromosomes are often contradictory. A universal karyotyping nomenclature system for precise chromosome identification and comparative evolutionary studies would be essential for genus Avena based on the recently released genome sequences of hexaploid and diploid Avena species.

RESULTS

Tandem repetitive sequences were predicted and physically located on chromosomal regions of the released Avena sativa OT3098 genome assembly v1. Eight new oligonucleotide (oligo) probes for sequential fluorescence in situ hybridization (FISH) were designed and then applied for chromosome karyotyping on mitotic metaphase spreads of A. brevis, A. nuda, A. wiestii, A. ventricosa, A. fatua, and A. sativa species. We established a high-resolution standard karyotype of A. sativa based on the distinct FISH signals of multiple oligo probes. FISH painting with bulked oligos, based on wheat-barley collinear regions, was used to validate the linkage group assignment for individual A. sativa chromosomes. We integrated our new Oligo-FISH based karyotype system with earlier karyotype nomenclatures through sequential C-banding and FISH methods, then subsequently determined the precise breakage points of some chromosome translocations in A. sativa.

CONCLUSIONS

This new universal chromosome identification system will be a powerful tool for describing the genetic diversity, chromosomal rearrangements and evolutionary relationships among Avena species by comparative cytogenetic and genomic approaches.

Chromosome and Molecular Analyses Reveal Significant Karyotype Diversity and Provide New Evidence on the Origin of Aegilops columnaris

Aegilops columnaris Zhuk. is tetraploid grass species (2n = 4x = 28, UcUcXcXc) closely related to Ae. neglecta and growing in Western Asia and a western part of the Fertile Crescent. Genetic diversity of Ae. columnaris was assessed using C-banding, FISH, nuclear and chloroplast (cp) DNA analyses, and gliadin electrophoresis. Cytogenetically Ae. columnaris was subdivided into two groups, C-I and C-II, showing different karyotype structure, C-banding, and FISH patterns. C-I group was more similar to Ae. neglecta. All types of markers revealed significant heterogeneity in C-II group, although group C-I was also polymorphic. Two chromosomal groups were consistent with plastogroups identified in a current study based on sequencing of three chloroplast intergenic spacer regions. The similarity of group C-I of Ae. columnaris with Ae. neglecta and their distinctness from C-II indicate that divergence of the C-I group was associated with minor genome modifications. Group C-II could emerge from C-I relatively recently, probably due to introgression from another Aegilops species followed by a reorganization of the parental genomes. Most C-II accessions were collected from a very narrow geographic region, and they might originate from a common ancestor. We suggest that the C-II group is at the initial stage of species divergence and undergoing an extensive speciation process.

Analysis of Structural Genomic Diversity in Aegilops umbellulata, Ae. markgrafii, Ae. comosa, and Ae. uniaristata by Fluorescence In Situ Hybridization Karyotyping

Fluorescence in situ hybridization karyotypes have been widely used for evolutionary analysis on chromosome organization and genetic/genomic diversity in the wheat alliance (tribe Triticeae of Poaceae). The karyotpic diversity of Aegilops umbellulata, Ae. markgrafii, Ae. comosa subsp. comosa and subsp. subventricosa, and Ae. uniaristata was evaluated by the fluorescence in situ hybridization (FISH) probes oligo-pSc119.2 and pTa71 in combination with (AAC)5, (ACT)7, and (CTT)12, respectively. Abundant intra- and interspecific genetic variation was discovered in Ae. umbellulata, Ae. markgrafii, and Ae. comosa, but not Ae. uniaristata. Chromosome 7 of Ae. umbellulata had more variants (six variants) than the other six U chromosomes (2-3 variants) as revealed by probes oligo-pSc119.2 and (AAC)5. Intraspecific variation in Ae. markgrafii and Ae. comosa was revealed by oligo-pSc119.2 in combination with (ACT)7 and (CTT)12, respectively. At least five variants were found in every chromosome of Ae. markgrafii and Ae. comosa, and up to 18, 10, and 15 variants were identified for chromosomes 2 of Ae. markgrafii, 4 of Ae. comosa subsp. comosa, and 6 of Ae. comosa subsp. subventricosa. The six Ae. uniaristata accessions showed identical FISH signal patterns. A large number of intra-specific polymorphic FISH signals were observed between the homologous chromosomes of Ae. markgrafii and Ae. comosa, especially chromosomes 1, 2, 4, and 7 of Ae. markgrafii, chromosome 4 of Ae. comosa subsp. comosa, and chromosome 6 of Ae. comosa subsp. subventricosa. Twelve Ae. comosa and 24 Ae. markgrafii accessions showed heteromorphism between homologous chromosomes. Additionally, a translocation between the short arms of chromosomes 1 and 7 of Ae. comosa PI 551038 was identified. The FISH karyotypes can be used to clearly identify the chromosome variations of each chromosome in these Aegilops species and also provide valuable information for understanding the evolutionary relationships and structural genomic variation among Aegilops species.

Chromosome restructuring among hybridizing wild wheats

The wheat group offers an outstanding system to address the interplay between hybridization, chromosomal evolution and biological diversification. Most diploid wild wheats originated following hybridization between the A-genome lineage and the B-genome lineage some 4 Myr ago, resulting in an admixed D-genome lineage that presented dramatic radiation accompanied by considerable changes in genome size and chromosomal rearrangements. Comparative profiling of low-copy genes, repeated sequences and transposable elements among those divergent species characterized by different karyotypes highlights high genome dynamics and sheds new light on the processes underlying chromosomal evolution in wild wheats. One of the hybrid clades presents upsizing of metacentric chromosomes going along with the proliferation of specific repeats (i.e. 'genomic obesity'), whereas other species show stable genome size associated with increasing chromosomal asymmetry. Genetic and ecological variation in those specialized species suggest that genome restructuring was coupled with adaptive processes to support the evolution of a majority of acrocentric chromosomes. This synthesis of current knowledge on genome restructuring across the diversity of wild wheats paves the way towards surveys based on latest sequencing technologies to characterize valuable resources and address the significance of chromosomal evolution in species with complex genomes.

Molecular-cytogenetic analysis of diploid wheatgrass Thinopyrum bessarabicum (Savul. and Rayss) A. Löve

Thinopyrum bessarabicum (T. Săvulescu & T. Rayss, 1923) A. Löve, 1980 is diploid (2n=2x=14, JJ or E^bE^b), perennial self-fertilizing rhizomatous maritime beach grass, which is phylogenetically close to another diploid wheatgrass species, Agropyron elongatum (N. Host, 1797) P. de Beauvois, 1812. The detailed karyotype of Th. bessarabicum was constructed based on FISH with six DNA probes representing 5S and 45S rRNA gene families and four tandem repeats. We found that the combination of pAesp_SAT86 (= pTa-713) probe with pSc119.2 or pAs1/ pTa-535 allows the precise identification of all J-genome chromosomes. Comparison of our data with the results of other authors showed that karyotypically Th. bessarabicum is distinct from A. elongatum. On the other hand, differences between the J-genome chromosomes of Th. bessarabicum and the chromosomes of hexaploid Th. intermedium (N. Host, 1797) M. Barkworth & D.R. Dewey, 1985 and decaploid Th. ponticum (J. Podpěra, 1902) Z.-W. Liu & R.-C. Wang, 1993 in the distribution of rDNA loci and hybridization patterns of pSc119.2 and pAs1 probes could be an indicative of (1) this diploid species was probably not involved in the origin of these polyploids or (2) it could has contributed the J-genome to Th. intermedium and Th. ponticum, but it was substantially modified over the course of speciation.

Cytological markers used for identification and transfer of Aegilops spp. chromatin carrying valuable genes into cultivated forms of Triticum

There are many reports describing chromosome structure, organization and evolution within goatgrasses (Aegilops spp.). Chromosome banding and fluorescence in situ hybridization techniques are main methods used to identify Aegilops Linnaeus, 1753 chromosomes. These data have essential value considering the close genetic and genomic relationship of goatgrasses with wheat (Triticumaestivum Linnaeus, 1753) and triticale (× Triticosecale Wittmack, 1899). A key question is whether those protocols are useful and effective for tracking Aegilops chromosomes or chromosome segments in genetic background of cultivated cereals. This article is a review of scientific reports describing chromosome identification methods, which were applied for development of prebreeding plant material and for transfer of desirable traits into Triticum Linnaeus, 1753 cultivated species. Moreover, this paper is a resume of the most efficient cytomolecular markers, which can be used to follow the introgression of Aegilops chromatin during the breeding process.

Characterization of Chromosomal Rearrangement in New Wheat—Thinopyrum intermedium Addition Lines Carrying Thinopyrum—Specific Grain Hardness Genes

The wild species, Thinopyrum intermedium. (Genome StStJSJSJJ), serves as a valuable germplasm resource providing novel genes for wheat improvement. In the current study, non-denaturing fluorescence in situ hybridization (ND-FISH) with multiple probes and comparative molecular markers were applied to characterize two wheat-Th. intermedium chromosome additions. Sequential ND-FISH with new labeled Th. intermedium specific oligo-probes were used to precisely determine the chromosomal constitution of Th. intermedium, wheat—Th. intermedium partial amphiploids and addition lines Hy36 and Hy37. The ND-FISH results showed that the added JS-St translocated chromosomes in Hy36 had minor Oligo-5S ribosomal DNA (rDNA) signals at the short arm, while a pair of J-St chromosomes in Hy37 had major Oligo-pTa71 and minor Oligo-5S rDNA signals. The 90K SNP array and PCR-based molecular markers that mapped on wheat linkage group 5 and 3 facilitated the identification of Thinopyrum chromosome introgressions in the addition lines, and confirmed that added chromosomes in Hy36 and Hy37 were 5JSS.3StS and 5JS.3StS, respectively. Complete coding sequences at the paralogous puroindoline-a (Pina) loci from Th. intermedium were cloned and localized on the short arm of chromosome 5JS of Hy36. Line Hy36 showed a reduction in the hardness index, which suggested that Th. intermedium-specific Pina gene sequences may be associated with the softness trait in wheat background. The molecular cytogenetic identification of novel wheat—Th. intermedium derivatives indicated that the frequent chromosome rearrangement occurred in the progenies of wheat-Thinopyrum hybridization. The new wheat-Thinopyrum derived lines may increase the genetic diversity for wheat breeding.

FISH-Based Markers Enable Identification of Chromosomes Derived From Tetraploid Thinopyrum elongatum in Hybrid Lines

Tetraploid Thinopyrum elongatum, which has superior abiotic stress tolerance characteristics, and exhibits resistance to stripe rust, powdery mildew, and Fusarium head blight, is a wild relative of wheat and a promising source of novel genes for wheat improvement. Currently, a high-resolution Fluorescence in situ hybridization (FISH) karyotype of tetraploid Th. elongatum is not available. To develop chromosome-specific FISH-based markers, the hexaploid Trititrigia 8801 and two accessions of tetraploid Th. elongatum were characterized by different repetitive sequences probes. We found that all E-genome chromosomes could be unambiguously identified using a combination of pSc119.2, pTa535, pTa71, and pTa713 repeats, and the E-genome chromosomes of the wild accessions and the partial amphiploid failed to exhibit any significant variation in the probe hybridization patterns. To verify the validation of these markers, the chromosome constitution of eight wheat- Th. elongatum hybrid derivatives were analyzed. We revealed that these probes could quickly detect wheat and tetraploid Th. elongatum chromosomes in hybrid lines. K16-712-1-2 was a 1E (1D) chromosome substitution line, K16-681-4 was a 2E disomic chromosome addition line, K16-562-3 was a 3E, 4E (3D, 4D) chromosome substitution line, K15-1033-8-2 contained one 4E, two 5E, and one 4ES⋅1DL Robertsonian translocation chromosome, and four other lines carried monosomic 4E, 5E, 6E, and 7E chromosome, respectively. Furthermore, the E-genome specific molecular markers analysis corresponded perfectly with the FISH results. The developed FISH markers will facilitate rapid identification of tetraploid Th. elongatum chromosomes in wheat improvement programs and allow appropriate alien chromosome transfer.

Evolution of the S-Genomes in Triticum-Aegilops Alliance: Evidences From Chromosome Analysis

Five diploid Aegilops species of the Sitopsis section: Ae. speltoides, Ae. longissima, Ae. sharonensis, Ae. searsii, and Ae. bicornis, two tetraploid species Ae. peregrina (= Ae. variabilis) and Ae. kotschyi (Aegilops section) and hexaploid Ae. vavilovii (Vertebrata section) carry the S-genomes. The B- and G-genomes of polyploid wheat are also the derivatives of the S-genome. Evolution of the S-genome species was studied using Giemsa C-banding and fluorescence in situ hybridization (FISH) with DNA probes representing 5S (pTa794) and 18S-5.8S-26S (pTa71) rDNAs as well as nine tandem repeats: pSc119.2, pAesp_SAT86, Spelt-1, Spelt-52, pAs1, pTa-535, and pTa-s53. To correlate the C-banding and FISH patterns we used the microsatellites (CTT)₁₀ and (GTT)₉, which are major components of the C-banding positive heterochromatin in wheat. According to the results obtained, diploid species split into two groups corresponding to Emarginata and Truncata sub-sections, which differ in the C-banding patterns, distribution of rDNA and other repeats. The B- and G-genomes of polyploid wheat are most closely related to the S-genome of Ae. speltoides. The genomes of allopolyploid wheat have been evolved as a result of different species-specific chromosome translocations, sequence amplification, elimination and re-patterning of repetitive DNA sequences. These events occurred independently in different wheat species and in Ae. speltoides _. The 5S rDNA locus of chromosome 1S was probably lost in ancient Ae. speltoides prior to formation of Timopheevii wheat, but after the emergence of ancient emmer. Evolution of Emarginata species was associated with an increase of C-banding and (CTT)₁₀-positive heterochromatin, amplification of Spelt-52, re-pattering of the pAesp_SAT86, and a gradual decrease in the amount of the D-genome-specific repeats pAs1, pTa-535, and pTa-s53. The emergence of Ae. peregrina and Ae. kotschyi did not lead to significant changes of the S^*-genomes. However, partial elimination of 45S rDNA repeats from 5S^* and 6S^* chromosomes and alterations of C-banding and FISH-patterns have been detected. Similarity of the S^v-genome of Ae. vavilovii with the S^s genome of diploid Ae. searsii confirmed the origin of this hexaploid. A model of the S-genome evolution is suggested.

Genetic classification of Aegilops columnaris Zhuk. (2n=4x=28, UcUcXcXc) chromosomes based on FISH analysis and substitution patterns in common wheat × Ae. columnaris introgressive lines

Aegilops columnaris is a tetraploid species originated from Ae. umbellulata (2n=2x=14, UU) and a yet unknown diploid grass species. Although Ae. columnaris possesses some agronomically valuable traits, such as heat and drought tolerance and resistance to pests, it has never been used in wheat breeding because of difficulties in producing hybrids and a lack of information on the relationships between Ae. columnaris and common wheat chromosomes. In this paper, we report the development of 57 wheat - Ae. columnaris introgressive lines covering 8 of the14 chromosomes of Aegilops. Based on substitution spectra of hybrids and the results of FISH analysis of the parental Ae. columnaris line with seven DNA probes, we have developed the genetic nomenclature of the U^c and X^c chromosomes. Genetic groups and genome affinities were established for 11 of 14 chromosomes; the classification of the remaining three chromosomes remains unsolved. Each Ae. columnaris chromosome was characterized on the basis of C-banding pattern and the distribution of seven DNA sequences. Introgression processes were shown to depend on the parental wheat genotype and the level of divergence of homoeologous chromosomes. We found that lines carrying chromosome 5X^c are resistant to leaf rust; therefore, this chromosome could possess novel resistance genes that have never been utilized in wheat breeding.

Fine organization of genomic regions tagged to the 5S rDNA locus of the bread wheat 5B chromosome

BACKGROUND

The multigene family encoding the 5S rRNA, one of the most important structurally-functional part of the large ribosomal subunit, is an obligate component of all eukaryotic genomes. 5S rDNA has long been a favored target for cytological and phylogenetic studies due to the inherent peculiarities of its structural organization, such as the tandem arrays of repetitive units and their high interspecific divergence. The complex polyploid nature of the genome of bread wheat, Triticum aestivum, and the technically difficult task of sequencing clusters of tandem repeats mean that the detailed organization of extended genomic regions containing 5S rRNA genes remains unclear. This is despite the recent progress made in wheat genomic sequencing. Using pyrosequencing of BAC clones, in this work we studied the organization of two distinct 5S rDNA-tagged regions of the 5BS chromosome of bread wheat.

RESULTS

Three BAC-clones containing 5S rDNA were identified in the 5BS chromosome-specific BAC-library of Triticum aestivum. Using the results of pyrosequencing and assembling, we obtained six 5S rDNA- containing contigs with a total length of 140,417 bp, and two sets (pools) of individual 5S rDNA sequences belonging to separate, but closely located genomic regions on the 5BS chromosome. Both regions are characterized by the presence of approximately 70-80 copies of 5S rDNA, however, they are completely different in their structural organization. The first region contained highly diverged short-type 5S rDNA units that were disrupted by multiple insertions of transposable elements. The second region contained the more conserved long-type 5S rDNA, organized as a single tandem array. FISH using probes specific to both 5S rDNA unit types showed differences in the distribution and intensity of signals on the chromosomes of polyploid wheat species and their diploid progenitors.

CONCLUSION

A detailed structural organization of two closely located 5S rDNA-tagged genomic regions on the 5BS chromosome of bread wheat has been established. These two regions differ in the organization of both 5S rDNA and the neighboring sequences comprised of transposable elements, implying different modes of evolution for these regions.

Major structural genomic alterations can be associated with hybrid speciation in Aegilops markgrafii (Triticeae)

During evolutionary history many grasses from the tribe Triticeae have undergone interspecific hybridization, resulting in allopolyploidy; whereas homoploid hybrid speciation was found only in rye. Homoeologous chromosomes within the Triticeae preserved cross-species macrocolinearity, except for a few species with rearranged genomes. Aegilops markgrafii, a diploid wild relative of wheat (2n = 2x = 14), has a highly asymmetrical karyotype that is indicative of chromosome rearrangements. Molecular cytogenetics and next-generation sequencing were used to explore the genome organization. Fluorescence in situ hybridization with a set of wheat cDNAs allowed the macrostructure and cross-genome homoeology of the Ae. markgrafii chromosomes to be established. Two chromosomes maintained colinearity, whereas the remaining were highly rearranged as a result of inversions and inter- and intrachromosomal translocations. We used sets of barley and wheat orthologous gene sequences to compare discrete parts of the Ae. markgrafii genome involved in the rearrangements. Analysis of sequence identity profiles and phylogenic relationships grouped chromosome blocks into two distinct clusters. Chromosome painting revealed the distribution of transposable elements and differentiated chromosome blocks into two groups consistent with the sequence analyses. These data suggest that introgressive hybridization accompanied by gross chromosome rearrangements might have had an impact on karyotype evolution and homoploid speciation in Ae. markgrafii.

Aegilops tauschii Accessions with Geographically Diverse Origin Show Differences in Chromosome Organization and Polymorphism of Molecular Markers Linked to Leaf Rust and Powdery Mildew Resistance Genes

Aegilops tauschii (2n = 2x = 14) is a diploid wild species which is reported as a donor of the D-genome of cultivated bread wheat. The main goal of this study was to examine the differences and similarities in chromosomes organization among accessions of Ae. tauschii with geographically diversed origin, which is believed as a potential source of genes, especially determining resistance to fungal diseases (i.e., leaf rust and powdery mildew) for breeding of cereals. We established and compared the fluorescence in situ hybridization patterns of 21 accessions of Ae. tauschii using various repetitive sequences mainly from the BAC library of wheat cultivar Chinese Spring. Results obtained for Ae. tauschii chromosomes revealed many similarities between analyzed accessions, however, some hybridization patterns were specific for accessions, which become from cognate regions of the World. The most noticeable differences were observed for accessions from China which were characterized by presence of distinct signals of pTa-535 in the interstitial region of chromosome 3D, less intensity of pTa-86 signals in chromosome 2D, as well as lack of additional signals of pTa-86 in chromosomes 1D, 5D, or 6D. Ae. tauschii of Chinese origin appeared homogeneous and separate from landraces that originated in western Asia. Ae. tauschii chromosomes showed similar hybridization patterns to wheat D-genome chromosomes, but some differences were also observed among both species. What is more, we identified reciprocal translocation between short arm of chromosome 1D and long arm of chromosome 7D in accession with Iranian origin. High polymorphism between analyzed accessions and extensive allelic variation were revealed using molecular markers associated with resistance genes. Majority of the markers localized in chromosomes 1D and 2D showed the diversity of banding patterns between accessions. Obtained results imply, that there is a moderate or high level of polymorphism in the genome of Ae. tauschii determined by a geographical origin, which we proved by cytogenetic and molecular markers analysis. Therefore, selected accessions might constitute an accessible source of variation for improvement of Triticeae species like wheat and triticale.

Molecular cytogenetic (FISH) and genome analysis of diploid wheatgrasses and their phylogenetic relationship

This paper reports detailed FISH-based karyotypes for three diploid wheatgrass species Agropyron cristatum (L.) Beauv., Thinopyrum bessarabicum (Savul.&Rayss) A. Löve, Pseudoroegneria spicata (Pursh) A. Löve, the supposed ancestors of hexaploid Thinopyrum intermedium (Host) Barkworth & D.R.Dewey, compiled using DNA repeats and comparative genome analysis based on COS markers. Fluorescence in situ hybridization (FISH) with repetitive DNA probes proved suitable for the identification of individual chromosomes in the diploid JJ, StSt and PP genomes. Of the seven microsatellite markers tested only the (GAA)_n trinucleotide sequence was appropriate for use as a single chromosome marker for the P. spicata A^S chromosome. Based on COS marker analysis, the phylogenetic relationship between diploid wheatgrasses and the hexaploid bread wheat genomes was established. These findings confirmed that the J and E genomes are in neighbouring clusters.

Dissecting the U, M, S and C genomes of wild relatives of bread wheat (Aegilops spp.) into chromosomes and exploring their synteny with wheat

Goat grasses (Aegilops spp.) contributed to the evolution of bread wheat and are important sources of genes and alleles for modern wheat improvement. However, their use in alien introgression breeding is hindered by poor knowledge of their genome structure and a lack of molecular tools. The analysis of large and complex genomes may be simplified by dissecting them into single chromosomes via flow cytometric sorting. In some species this is not possible due to similarities in relative DNA content among chromosomes within a karyotype. This work describes the distribution of GAA and ACG microsatellite repeats on chromosomes of the U, M, S and C genomes of Aegilops, and the use of microsatellite probes to label the chromosomes in suspension by fluorescence in situ hybridization (FISHIS). Bivariate flow cytometric analysis of chromosome DAPI fluorescence and fluorescence of FITC-labelled microsatellites made it possible to discriminate all chromosomes and sort them with negligible contamination by other chromosomes. DNA of purified chromosomes was used as a template for polymerase chain reation (PCR) using Conserved Orthologous Set (COS) markers with known positions on wheat A, B and D genomes. Wheat-Aegilops macrosyntenic comparisons using COS markers revealed significant rearrangements in the U and C genomes, while the M and S genomes exhibited structure similar to wheat. Purified chromosome fractions provided an attractive resource to investigate the structure and evolution of the Aegilops genomes, and the COS markers assigned to Aegilops chromosomes will facilitate alien gene introgression into wheat.

Intraspecific Polymorphisms of Cytogenetic Markers Mapped on Chromosomes of Triticum polonicum L

Triticum genus encloses several tetraploid species that are used as genetic stocks for expanding the genetic variability of wheat (Triticum aestivum L.). Although the T. aestivum (2n = 6x = 42, AABBDD) and T. durum (2n = 4x = 28, AABB) karyotypes were well examined by chromosome staining, Giemsa C-banding and FISH markers, other tetraploids are still poorly characterized. Here, we established and compared the fluorescence in situ hybridization (FISH) patterns on chromosomes of 20 accessions of T. polonicum species using different repetitive sequences from BAC library of wheat ‘Chinese Spring’. The chromosome patterns of Polish wheat were compared to tetraploid (2n = 4x = 28, AABB) Triticum species: T. durum, T. diccocon and T. turanicum, as well. A combination of pTa-86, pTa-535 and pTa-713 probes was the most informative among 6 DNA probes tested. Probe pTa-k374, which is similar to 28S rDNA sequence enabled to distinguish signal size and location differences, as well as rDNA loci elimination. Furthermore, pTa-465 and pTa-k566 probes are helpful for the detection of similar organized chromosomes. The polymorphisms of signals distribution were observed in 2A, 2B, 3B, 5B, 6A and 7B chromosomes. Telomeric region of the short arm of 6B chromosome was the most polymorphic. Our work is novel and contributes to the understanding of T. polonicum genome organization which is essential to develop successful advanced breeding strategies for wheat. Collection and characterization of this germplasm can contribute to the wheat biodiversity safeguard.

Allocation of the S-genome chromosomes of Aegilops variabilis Eig. carrying powdery mildew resistance in triticale (× Triticosecale Wittmack)

It has been hypothesized that the powdery mildew adult plant resistance (APR) controlled by the Pm13 gene in Aegilops longissima Schweinf. & Muschl. (S(l)S(l)) has been evolutionary transferred to Aegilops variabilis Eig. (UUSS). The molecular marker analysis and the visual evaluation of powdery mildew symptoms in Ae. variabilis and the Ae. variabilis × Secale cereale amphiploid forms (2n = 6x = 42, UUSSRR) showed the presence of product that corresponded to Pm13 marker and the lower infection level compared to susceptible model, respectively. This study also describes the transfer of Ae. variabilis Eig. (2n = 4x = 28, U(v)U(v)S(v)S(v)) chromosomes, carrying powdery mildew resistance, into triticale (× Triticosecale Wittm., 2n = 6x = 42, AABBRR) using Ae. variabilis × S. cereale amphiploid forms. The individual chromosomes of Ae. variabilis, triticale 'Lamberto' and hybrids were characterized by genomic and fluorescence in situ hybridization (GISH/FISH). The chromosome configurations of obtained hybrid forms were studied at first metaphase of meiosis of pollen mother cells (PMCs) using GISH. The statistical analysis showed that the way of S-genome chromosome pairing and transmission to subsequent hybrid generations was diploid-like and had no influence on chromosome pairing of triticale chromosomes. The cytogenetic study of hybrid forms were supported by the marker-assisted selection using Pm13 marker and visual evaluation of natural infection by Blumeria graminis, that allowed to select the addition or substitution lines of hybrids carrying chromosome 3S(v) which were tolerant to the powdery mildew infection.

Haplotype Detection from Next-Generation Sequencing in High-Ploidy-Level Species: 45S rDNA Gene Copies in the Hexaploid Spartina maritima

Gene and whole-genome duplications are widespread in plant nuclear genomes, resulting in sequence heterogeneity. Identification of duplicated genes may be particularly challenging in highly redundant genomes, especially when there are no diploid parents as a reference. Here, we developed a pipeline to detect the different copies in the ribosomal RNA gene family in the hexaploid grass Spartina maritima from next-generation sequencing (Roche-454) reads. The heterogeneity of the different domains of the highly repeated 45S unit was explored by identifying single nucleotide polymorphisms (SNPs) and assembling reads based on shared polymorphisms. SNPs were validated using comparisons with Illumina sequence data sets and by cloning and Sanger (re)sequencing. Using this approach, 29 validated polymorphisms and 11 validated haplotypes were reported (out of 34 and 20, respectively, that were initially predicted by our program). The rDNA domains of S. maritima have similar lengths as those found in other Poaceae, apart from the 5′-ETS, which is approximately two-times longer in S. maritima. Sequence homogeneity was encountered in coding regions and both internal transcribed spacers (ITS), whereas high intragenomic variability was detected in the intergenic spacer (IGS) and the external transcribed spacer (ETS). Molecular cytogenetic analysis by fluorescent in situ hybridization (FISH) revealed the presence of one pair of 45S rDNA signals on the chromosomes of S. maritima instead of three expected pairs for a hexaploid genome, indicating loss of duplicated homeologous loci through the diploidization process. The procedure developed here may be used at any ploidy level and using different sequencing technologies.

Flow sorting of C-genome chromosomes from wild relatives of wheat Aegilops markgrafii, Ae. triuncialis and Ae. cylindrica, and their molecular organization

BACKGROUND AND AIMS

Aegilops markgrafii (CC) and its natural hybrids Ae. triuncialis (U(t)U(t)C(t)C(t)) and Ae. cylindrica (D(c)D(c)C(c)C(c)) represent a rich reservoir of useful genes for improvement of bread wheat (Triticum aestivum), but the limited information available on their genome structure and the shortage of molecular (cyto-) genetic tools hamper the utilization of the extant genetic diversity. This study provides the complete karyotypes in the three species obtained after fluorescent in situ hybridization (FISH) with repetitive DNA probes, and evaluates the potential of flow cytometric chromosome sorting.

METHODS

The flow karyotypes obtained after the analysis of 4',6-diamidino-2-phenylindole (DAPI)-stained chromosomes were characterized and the chromosome content of the peaks on the flow karyotypes was determined by FISH. Twenty-nine conserved orthologous set (COS) markers covering all seven wheat homoeologous chromosome groups were used for PCR with DNA amplified from flow-sorted chromosomes and genomic DNA.

KEY RESULTS

FISH with repetitive DNA probes revealed that chromosomes 4C, 5C, 7C(t), T6U(t)S.6U(t)L-5C(t)L, 1C(c) and 5D(c) could be sorted with purities ranging from 66 to 91 %, while the remaining chromosomes could be sorted in groups of 2-5. This identified a partial wheat-C-genome homology for group 4 and 5 chromosomes. In addition, 1C chromosomes were homologous with group 1 of wheat; a small segment from group 2 indicated 1C-2C rearrangement. An extensively rearranged structure of chromosome 7C relative to wheat was also detected.

CONCLUSIONS

The possibility of purifying Aegilops chromosomes provides an attractive opportunity to investigate the structure and evolution of the Aegilops C genome and to develop molecular tools to facilitate the identification of alien chromatin and support alien introgression breeding in bread wheat.

A Set of Cytogenetic Markers Allows the Precise Identification of All A-Genome Chromosomes in Diploid and Polyploid Wheat

Karyotypes of 3 diploid wheat species containing different variants of the A-genome, Triticum boeoticum (A(b)), T. monococcum (A(b)), and T. urartu (A(u)), were examined using C-banding and FISH with DNA probes representing 5S and 45S rDNA families, the microsatellite sequences GAAn and GTTn, the already known satellite sequences pSc119.2, Spelt52, Fat, pAs1, and pTa535, and a newly identified repeat called Aesp_SAT86. The C-banding patterns of the 3 species in general were similar; differences were observed in chromosomes 4A and 6A. Besides 2 major 45S rDNA loci on chromosomes 1A and 5A, 2 minor polymorphic NORs were observed in the terminal part of 5AL and in the distal part of 6AS in all species. An additional minor locus was found in the distal part of 7A(b)L of T. boeoticum and T. monococcum, but not in T. urartu. Two 5S rDNA loci were observed in 1AS and 5AS. The pTa535 probe displayed species- and chromosome-specific hybridization patterns, allowing complete chromosome identification and species discrimination. The distribution of pTa535 on the A(u)-genome chromosomes was more similar to that on the A-genome chromosomes of T. dicoccoides and T. araraticum, thus confirming the origin of these genomes from T. urartu. The probe pAs1 allowed the identification of 4 chromosomes of T. urartu and 2 of T. boeoticum or T. monococcum. The Aesp_SAT86-derived patterns were polymorphic; main clusters were observed on chromosomes 1A(u )and 3A(u) of T. urartu and chromosomes 3A(b) and 6A(b) of T. boeoticum. Thus, a set of probes, pTa535, pAs1, GAAn and GTTn, pTa71, pTa794, and Aesp_SAT86, proved to be most informative for the analysis of A-genomes in diploid and polyploid wheat species.

The chloroplast view of the evolution of polyploid wheat

Polyploid wheats comprise four species: Triticum turgidum (AABB genomes) and T. aestivum (AABBDD) in the Emmer lineage, and T. timopheevii (AAGG) and T. zhukovskyi (AAGGA(m) A(m) ) in the Timopheevi lineage. Genetic relationships between chloroplast genomes were studied to trace the evolutionary history of the species. Twenty-five chloroplast genomes were sequenced, and 1127 plant accessions were genotyped, representing 13 Triticum and Aegilops species. The A. speltoides (SS genome) diverged before the divergence of T. urartu (AA), A. tauschii (DD) and the Aegilops species of the Sitopsis section. Aegilops speltoides forms a monophyletic clade with the polyploid Emmer and Timopheevi wheats, which originated within the last 0.7 and 0.4 Myr, respectively. The geographic distribution of chloroplast haplotypes of the wild tetraploid wheats and A. speltoides illustrates the possible geographic origin of the Emmer lineage in the southern Levant and the Timopheevi lineage in northern Iraq. Aegilops speltoides is the closest relative of the diploid donor of the chloroplast (cytoplasm), as well as the B and G genomes to Timopheevi and Emmer lineages. Chloroplast haplotypes were often shared by species or subspecies within major lineages and between the lineages, indicating the contribution of introgression to the evolution and domestication of polyploid wheats.

Flow cytometric chromosome sorting from diploid progenitors of bread wheat, T. urartu, Ae. speltoides and Ae. tauschii

Chromosomes 5A (u) , 5S and 5D can be isolated from wild progenitors, providing a chromosome-based approach to develop tools for breeding and to study the genome evolution of wheat. The three subgenomes of hexaploid bread wheat originated from Triticum urartu (A(u)A(u)), from a species similar to Aegilops speltoides (SS) (progenitor of the B genome), and from Ae. tauschii (DD). Earlier studies indicated the potential of chromosome genomics to assist gene transfer from wild relatives of wheat and discover novel genes for wheat improvement. This study evaluates the potential of flow cytometric chromosome sorting in the diploid progenitors of bread wheat. Flow karyotypes obtained by analysing DAPI-stained chromosomes were characterized and the contents of the chromosome peaks were determined. FISH analysis with repetitive DNA probes proved that chromosomes 5A(u), 5S and 5D could be sorted with purities of 78-90 %, while the remaining chromosomes could be sorted in groups of three. Twenty-five conserved orthologous set (COS) markers covering wheat homoeologous chromosome groups 1-7 were used for PCR with DNA amplified from flow-sorted chromosomes and genomic DNA. These assays validated the cytomolecular results as follows: peak I on flow karyotypes contained chromosome groups 1, 4 and 6, peak II represented homoeologous group 5, while peak III consisted of groups 2, 3 and 7. The isolation of individual chromosomes of wild progenitors provides an attractive opportunity to investigate the structure and evolution of the polyploid genome and to deliver tools for wheat improvement.

Cytogenetic analysis of Aegilops chromosomes, potentially usable in triticale (X Triticosecale Witt.) breeding

Chromosome identification using fluorescence in situ hybridization (FISH) is widely used in cytogenetic research. It is a diagnostic tool helpful in chromosome identification. It can also be used to characterize alien introgressions, when exercised in a combination with genomic in situ hybridization (GISH). This work aims to find chromosome identification of Aegilops species and Aegilops × Secale amphiploids, which can be used in cereal breeding as a source of favourable agronomic traits. Four diploid and two tetraploid Aegilops species and three Aegilops × Secale hybrids were analysed using FISH with pSc119.2, pAs1, 5S rDNA and 25S rDNA clones to differentiate the U-, M-, S^sh- and D-subgenome chromosomes of Aegilops genus. Additionally, GISH for chromosome categorization was carried out. Differences in the hybridization patterns allowed to identify all U-, M-, S^sh- and D-subgenome chromosomes. Some differences in localization of the rDNA, pSc119.2 and pAs1 sequences between analogue subgenomes in diploid and tetraploid species and Aegilops × Secale hybrids were detected. The hybridization pattern of the M and S genome was more variable than that of the U and D genome. An importance of the cytogenetic markers in plant breeding and their possible role in chromosome structure, function and evolution is discussed.

Karyotyping and in situ chromosomal localization of rDNA sites in black cumin Bunium persicum (Boiss) B. Fedtsch,1915 (Apiaceae)

The fluorescent in situ hybridization (FISH) technique has been applied to somatic chromosomes in the medicinally important species, Bunium persicum, to elucidate its karyotypes. The bicolour FISH technique involving 18S-5.8S-26S and 5S ribosomal RNA genes as probes was used to assign physical localization and measurement of rDNA sites on homologous pairs of chromosomes. The two 18S-5.8S-26S rRNA gene sites were at the terminal regions of the short arms of the chromosomes 1 and 2 involving NOR region of chromosome 1. The 5S rDNA sites were found on subtelomeric region of the long arm of the chromosome number 5 and at interstitial regions of the short arm of chromosome 7. Based on direct visual analysis of chromosome length, morphology and position of FISH signals, a pioneer attempt has been made to construct metaphase karyotype in Bunium persicum, an endangered medicinal plant of North Western Himalayas.

Microsatellite mapping of Ae. speltoides and map-based comparative analysis of the S, G, and B genomes of Triticeae species

The first microsatellite linkage map of Ae. speltoides Tausch (2n = 2x = 14, SS), which is a wild species with a genome closely related to the B and G genomes of polyploid wheats, was developed based on two F(2) mapping populations using microsatellite (SSR) markers from Ae. speltoides, wheat genomic SSRs (g-SSRs) and EST-derived SSRs. A total of 144 different microsatellite loci were mapped in the Ae. speltoides genome. The transferability of the SSRs markers between the related S, B, and G genomes allowed possible integration of new markers into the T. timopheevii G genome chromosomal maps and map-based comparisons. Thirty-one new microsatellite loci assigned to the genetic framework of the T. timopheevii G genome maps were composed of wheat g-SSR (genomic SSR) markers. Most of the used Ae. speltoides SSRs were mapped onto chromosomes of the G genome supporting a close relationship between the G and S genomes. Comparative microsatellite mapping of the S, B, and G genomes demonstrated colinearity between the chromosomes within homoeologous groups, except for intergenomic T6A(t)S.1G, T4AL.5AL.7BS translocations. A translocation between chromosomes 2 and 6 that is present in the T. aestivum B genome was found in neither Ae. speltoides nor in T. timopheevii. Although the marker order was generally conserved among the B, S, and G genomes, the total length of the Ae. speltoides chromosomal maps and the genetic distances between homoeologous loci located in the proximal regions of the S genome chromosomes were reduced compared with the B, and G genome chromosomes.

Tandem repeats on an eco-geographical scale: outcomes from the genome of Aegilops speltoides

The chromosomal pattern of tandem repeat fractions of repetitive DNA is one of the most important characteristics of a species. In the present research, we aimed to detect and evaluate the level of intraspecific variability in the chromosomal distribution of species-specific Spelt 1 and Aegilops-Triticum-specific Spelt 52 tandem repeats in Aegilops speltoides and in closely related diploid and polyploid species. There is a distinct eco-geographical gradient in Spelt 1 and Spelt 52 blocks abundance in Ae. speltoides. In marginal populations, the number of Spelt 1 chromosomal blocks could be 12-14 times lower than in the center of the species distribution. Also, in related diploid species, the abundance of Spelt 52 correlates with evolutionary proximity to Ae. speltoides. Finally, the B- and G-genomes of allopolyploid wheats have Spelt 1 chromosomal distribution patterns similar to those of the types of Ae. speltoides with poor and rich contents of Spelt 1, respectively. The observed changes in numbers of blocks of Spelt 1 and Spelt 52 tandem repeats along the eco-geographical gradient may due to their depletion in the marginal populations as a result of increased recombination frequency under stressful conditions. Alternatively, it may be accumulation of tandem repeats in conducive climatic/edaphic environments in the center of the species' geographical distribution. Anyway, we observe a bidirectional shift of repetitive DNA genomic patterns on the population level leading to the formation of population-specific chromosomal patterns of tandem repeats. The appearance of a new chromosomal pattern is considered an important factor in promoting the emergence of interbreeding barriers.

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.

Phylogenetic relationship of a new class of LMW-GS genes in the M genome of Aegilops comosa

A new class of low molecular weight glutenin subunit (LMW-GS) genes was isolated and characterized from Aegilops comosa (2n = 2x = 14, MM). Although their DNA structure displayed high similarity to LMW-i type genes, there are some key differences. The deduced amino acid sequences of their mature proteins showed that the first amino acid residue of each gene was leucine and therefore they were designated as LMW-l type subunits. An extra cysteine residue was present in the signal peptide and the first cysteine residue of mature proteins located at the end of repetitive domain. Additionally, a long insertion of 10-22 residues (LGQQPQ(5-17)) occurred in the end of the C-terminal II. Comparative analysis demonstrated that LMW-l type glutenin genes possessed a great number of single-nucleotide polymorphisms and insertions/deletions. A new classification system was proposed according to the gene structure and phylogenetic analysis. In this new system, LMW-GS is classified into two major classes, LMW-M and LMW-I, with each including two subclasses. The former included LMW-m and LMW-s types while the latter contained LMW-l and LMW-i types. Analysis of their evolutionary origin showed that the LMW-l genes diverged from the group 2 of LMW-m type genes at about 12-14 million years ago (MYA) while LMW-i type evolved from LMW-l type at approximately 8-12 MYA. The LMW-s type was a variant form of group 1 of LMW-m type and their divergence occurred about 4-6 MYA. In addition to homologous recombination, non-homologous illegitimate recombination could be an important molecular mechanism for the origin and evolution of LMW-GS gene family. The secondary structure prediction suggested that the novel LMW-l type subunits, such as AcLMW-L1 and AcLMW-L2, may have positive effects on dough properties.

Variability of ribosomal DNA sites in Festuca pratensis, Lolium perenne, and their intergeneric hybrids, revealed by FISH and GISH

This study focuses on the variability of chromosomal location and number of ribosomal DNA (rDNA) sites in some diploid and autotetraploid Festuca pratensis and Lolium perenne cultivars, as well as on identification of rDNA-bearing chromosomes in their triploid and tetraploid F. pratensis × L. perenne hybrids. The rDNA loci were mapped using fluorescence in situ hybridization (FISH) with 5S and 25S rDNA probes, and the origin of parental genomes was verified by genomic in situ hybridization (GISH) with L. perenne genomic DNA as a probe, and F. pratensis genomic DNA as a block. FISH detected variation in the number and chromosomal location of both 5S and 45S rDNA sites. In F. pratensis mostly additional signals of 5S rDNA loci occurred, as compared with standard F. pratensis karyotypes. Losses of 45S rDNA loci were more frequent in L. perenne cultivars and intergeneric hybrids. Comparison of the F. pratensis and L. perenne genomes approved a higher number of rDNA sites as well as variation in chromosomal rDNA location in L. perenne. A greater instability of F. pratensis-genome-like and L. perenne-genome-like chromosomes in tetraploid hybrids was revealed, indicating gains and losses of rDNA loci, respectively. Our data indicate that the rDNA loci physically mapped on chromosomes 2 and 3 in F. pratensis and on chromosome 3 in L. perenne are useful markers for these chromosomes in intergeneric Festuca × Lolium hybrids.

Phylogenetic Relationships of Tetraploid AB-Genome Avena Species Evaluated by Means of Cytogenetic (C-Banding and FISH) and RAPD Analyses

Tetraploid oat species Avena abyssinica, A. vaviloviana, A. barbata, and A. agadiriana were studied using C-banding technique, in situ hybridization with the 45S and 5S rDNA probes, and RAPD analysis in comparison with the diploid species carrying different types of the A-genome (A. wiestii, As; A. longiglumis, Al; A. canariensis, Ac; A. damascena, Ad, A. prostrata, Ap). The investigation confirmed that all four tetraploids belong to the same AB-genome group; however A. agadiriana occupies distinct position among others. The C-banding, FISH, and RAPD analyses showed that Avena abyssinica, A. vaviloviana, and A. barbata are very similar; most probably they originated from a common tetraploid ancestor as a result of minor translocations and alterations of C-banding polymorphism system. AB-genome species are closely related with the A-genome diploids, and an As-genome species may be regarded as the most probable donor of their A-genome. Although their second diploid progenitor has not been identified, it seems unlikely that it belongs to the As-genome group. The exact diploid progenitors of A. agadiriana have not been determined; however our results suggest that at least one of them could be related to A. damascena.