Physical organisation of simple sequence repeats (SSRs) in Triticeae: structural, functional and evolutionary implications

Are scattered microsatellites weak chromosomal markers? Guided mapping reveals new insights into Trachelyopterus (Siluriformes: Auchenipteridae) diversity

The scattered distribution pattern of microsatellites is a challenging problem in fish cytogenetics. This type of array hinders the identification of useful patterns and the comparison between species, often resulting in over-limited interpretations that only label it as "scattered" or "widely distributed". However, several studies have shown that the distribution pattern of microsatellites is non-random. Thus, here we tested whether a scattered microsatellite could have distinct distribution patterns on homeologous chromosomes of closely related species. The clustered sites of 18S and 5S rDNA, U2 snRNA and H3/H4 histone genes were used as a guide to compare the (GATA)n microsatellite distribution pattern on the homeologous chromosomes of six Trachelyopterus species: T. coriaceus and Trachelyopterus aff. galeatus from the Araguaia River basin; T. striatulus, T. galeatus and T. porosus from the Amazonas River basin; and Trachelyopterus aff. coriaceus from the Paraguay River basin. Most species had similar patterns of the (GATA)n microsatellite in the histone genes and 5S rDNA carriers. However, we have found a chromosomal polymorphism of the (GATA)n sequence in the 18S rDNA carriers of Trachelyopterus galeatus, which is in Hard-Weinberg equilibrium and possibly originated through amplification events; and a chromosome polymorphism in Trachelyopterus aff. galeatus, which combined with an inversion polymorphism of the U2 snRNA in the same chromosome pair resulted in six possible cytotypes, which are in Hardy-Weinberg disequilibrium. Therefore, comparing the distribution pattern on homeologous chromosomes across the species, using gene clusters as a guide to identify it, seems to be an effective way to further the analysis of scattered microsatellites in fish cytogenetics.

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.

Karyotype conservation and genomic organization of repetitive sequences in the leaf-cutting ant Atta cephalotes (Linnaeus, 1758) (Formicidae: Myrmicinae)

Leaf-cutting ants are among the New World's most conspicuous and studied ant species due to their notable ecological and economic role. Cytogenetic studies carried out in Atta show remarkable karyotype conservation among the species. We performed classical cytogenetics and physical mapping of repetitive sequences in the leaf-cutting ant Atta cephalotes, the type species of the genus. Our goal was to test the karyotype conservation in Atta and to start to understand the genomic organization and diversity regarding repetitive sequences in leaf-cutting ants. Atta cephalotes showed 2n=22 (18m+2sm+2st) chromosomes. The heterochromatin followed a centromeric pattern, and the GC-rich regions and 18S rDNA clusters were co-located interstitially in the 4th metacentric pair. These cytogenetic characteristics were observed in other Atta species that had previously been studied, confirming the karyotype conservation in Atta. Evolutionary implications regarding the conservation of the chromosome number in leaf-cutting ants are discussed. Telomeric motif (TTAGG)n was detected in A. cephalotes as observed in other ants. Five out of the 11 microsatellites showed a scattered distribution exclusively on euchromatic areas of the chromosomes. Repetitive sequences mapped on the chromosomes of A. cephalotes are the first insights into genomic organization and diversity in leaf-cutting ants, useful in further comparative studies.

Migration of repetitive DNAs during evolution of the permanent translocation heterozygosity in the oyster plant (Tradescantia section Rhoeo)

Due to translocation heterozygosity for all chromosomes in the cell complement, the oyster plant (Tradescantia spathacea) forms a complete meiotic ring. It also shows Rabl-arrangement at interphase, featured by polar centromere clustering. We demonstrate that the pericentromeric regions of the oyster plant are homogenized in concert by three subtelomeric sequences: 45S rDNA, (TTTAGGG)n motif, and TSrepI repeat. The Rabl-based clustering of pericentromeric regions may have been an excellent device to combine the subtelomere-pericentromere sequence migration (via inversions) with the pericentromere-pericentromere DNA movement (via whole arm translocations) that altogether led to the concerted homogenization of all the pericentromeric domains by the subtelomeric sequences. We also show that the repetitive sequence landscape of interstitial chromosome regions contains many loci consisting of Arabidopsis-type telomeric sequence or of TSrepI repeat, and it is extensively heterozygous. However, the sequence arrangement on some chromosomal arms suggest segmental inversions that are fully or partially homozygous, a fact that could be explained if the inversions started to create linkages already in a bivalent-forming ancestor. Remarkably, the subterminal TSrepI loci reside exclusively on the longer arms that could be due to sharing sequences between similarly-sized chromosomal arms in the interphase nucleus. Altogether, our study spotlights the supergene system of the oyster plant as an excellent model to link complex chromosome rearrangements, evolution of repetitive sequences, and nuclear architecture.

Repeatome Analyses and Satellite DNA Chromosome Patterns in Deschampsia sukatschewii, D. cespitosa, and D. antarctica (Poaceae)

Subpolar and polar ecotypes of Deschampsia sukatschewii (Popl.) Roshev, D. cespitosa (L.) P. Beauv, and D. antarctica E. Desv. are well adapted to stressful environmental conditions, which make them useful model plants for genetic research and breeding. For the first time, the comparative repeatome analyses of subpolar and polar D. sukatschewii, D. cespitosa, and D. antarctica was performed using RepeatExplorer/TAREAN pipelines and FISH-based chromosomal mapping of the identified satellite DNA families (satDNAs). In the studied species, mobile genetic elements of class 1 made up the majority of their repetitive DNA; interspecific variations in the total amount of Ty3/Gypsy and Ty1/Copia retroelements, DNA transposons, ribosomal, and satellite DNA were revealed; 12–18 high confident and 7–9 low confident putative satDNAs were identified. According to BLAST, most D. sukatschewii satDNAs demonstrated sequence similarity with satDNAs of D. antarctica and D. cespitosa indicating their common origin. Chromosomal mapping of 45S rDNA, 5S rDNA, and satDNAs of D. sukatschewii allowed us to construct the species karyograms and detect new molecular chromosome markers important for Deschampsia species. Our findings confirmed that genomes of D. sukatschewii and D. cespitosa were more closely related compared to D. antarctica according to repeatome composition and patterns of satDNA chromosomal distribution.

First record of the spatial organization of the nucleosome-less chromatin of dinoflagellates: The nonrandom distribution of microsatellites and bipolar arrangement of telomeres in the nucleus of Gambierdiscus australes (Dinophyceae)

Dinoflagellates are a group of protists whose exceptionally large genome is organized in permanently condensed nucleosome-less chromosomes. In this study, we examined the potential role of repetitive DNAs in both the structure of dinoflagellate chromosomes and the architecture of the dinoflagellate nucleus. Non-denaturing fluorescent in situ hybridization (ND-FSH) was used to determine the abundance and physical distribution of telomeric DNA and 16 microsatellites (1- to 4-bp repeats) in the nucleus of Gambierdiscus australes. The results showed an increased relative abundance of the different microsatellite motifs with increasing GC content. Two ND-FISH probes, (A)₂₀ and (AAT)₅ , did not yield signals whereas the remainder revealed a dispersed but nonrandom distribution of the microsatellites, mostly in clusters. The bean-shaped interphase nucleus of G. australes contained a region with a high density of trinucleotides. This nuclear compartment was located between the nucleolar organizer region (NOR), located on the concave side of the nucleus, and the convex side. Telomeric DNA was grouped in multiple foci and distributed in two polarized compartments: one associated with the NOR and the other peripherally located along the convex side of the nucleus. Changes in the position of the telomeres during cell division evidenced their dynamic distribution and thus that of the chromosomes during dinomitosis. These insights into the spatial organization of microsatellites and telomeres and thus into the nuclear architecture of G. australes will open up new lines of research into the structure and function of the nucleosome-less chromatin of dinoflagellates.

Cytogenetic evidence supports Avena insularis being closely related to hexaploid oats

Cytogenetic observations, phylogenetic studies and genome analysis using high-density genetic markers have suggested a tetraploid Avena species carrying the C and D genomes (formerly C and A) to be the donor of all hexaploid oats (AACCDD). However, controversy surrounds which of the three extant CCDD tetraploid species—A. insularis, A. magna and A. murphyi—is most closely related to hexaploid oats. The present work describes a comparative karyotype analysis of these three CCDD tetraploid species and two hexaploid species, A. sativa and A. byzantina. This involved the use of FISH with six simple sequence repeats (SSRs) with the motifs CT, AAC, AAG, ACG, ATC and ACT, two repeated ribosomal sequences, and C genome-specific repetitive DNA. The hybridization pattern of A. insularis with oligonucleotide (AC)₁₀ was also determined and compared with those previously published for A. sativa and A. byzantina. Significant differences in the 5S sites and SSR hybridization patterns of A. murphyi compared to the other CCDD species rule out its being directly involved in the origin of the hexaploids. In contrast, the repetitive and SSR hybridization patterns shown by the D genome chromosomes, and by most of the C genome chromosomes of A. magna and A. insularis, can be equated with the corresponding chromosomes of the hexaploids. Several chromosome hybridization signals seen for A. insularis, but not for A. magna, were shared with the hexaploid oats species, especially with A. byzantina. These diagnostic signals add weight to the idea that the extant A. insularis, or a direct ancestor of it, is the most closely related progenitor of hexaploid oats. The similarity of the chromosome hybridization patterns of the hexaploids and CCDD tetraploids was taken as being indicative of homology. A common chromosome nomenclature for CCDD species based on that of the hexaploid species is proposed.

Subgenome Discrimination in Brassica and Raphanus Allopolyploids Using Microsatellites

Intergeneric crosses between Brassica species and Raphanus sativus have produced crops with prominent shoot and root systems of Brassica and R. sativus, respectively. It is necessary to discriminate donor genomes when studying cytogenetic stability in distant crosses to identify homologous chromosome pairing, and microsatellite repeats have been used to discriminate subgenomes in allopolyploids. To identify genome-specific microsatellites, we explored the microsatellite content in three Brassica species (B. rapa, AA, B. oleracea, CC, and B. nigra, BB) and R. sativus (RR) genomes, and validated their genome specificity by fluorescence in situ hybridization. We identified three microsatellites showing A, C, and B/R genome specificity. ACBR_msat14 and ACBR_msat20 were detected in the A and C chromosomes, respectively, and ACBR_msat01 was detected in B and R genomes. However, we did not find a microsatellite that discriminated the B and R genomes. The localization of ACBR_msat20 in the 45S rDNA array in ×Brassicoraphanus 977 corroborated the association of the 45S rDNA array with genome rearrangement. Along with the rDNA and telomeric repeat probes, these microsatellites enabled the easy identification of homologous chromosomes. These data demonstrate the utility of microsatellites as probes in identifying subgenomes within closely related Brassica and Raphanus species for the analysis of genetic stability of new synthetic polyploids of these genomes.

Identification and characterization of satellite DNAs in Poa L

BACKGROUND

Poa L. is a large genus of grass in Gramineae, among which P. pratensis is widely cultivated as turf and forage. Satellite DNA is the main components of the plant genome. Information of satellites will helpful for dissection the genome composition and definition of the phylogeny relationship of these species. However, the knowledge about the satellites in genus Poa is still limited.

RESULTS

Four satellite DNAs were identified using the Repeat Explorer pipeline in HiSeq Illumina reads from diploid plants in P. malaca (2n = 26). Two satellites showed high similarity with the previously identified PpTr-1 and PpTr-3, whereas two others are newly identified with the monomer of 326 bp (Poa-326) and 353 bp (Poa-353) respectively. The clone DNAs of PpTr-1 and PpTr-3, and oligonucleotides designed representing satellites Poa-326 and Poa-353 were probed to test on chromosomes across 13 Poa speceis with different polyploidy level by fluorescent in situ hybridization (FISH). PpTr-1, PpTr-3, and Poa-362 were stably positioned in the subtelomeric regions in nearly all species with the variation of hybridization sites number. However, Poa-353 showed different FISH patterns of multiple regions with the variation of hybridization intensity and distribution sites across species. In addition, 5S rDNA and 45S rDNA were used to characterize the genome of the Poa species. Four rDNA FISH patterns were revealed in the tested species.

CONCLUSION

Four identified satellite were high conservable across Poa species. Genome distribution of these satellites can be characterized by FISH. The variation of satellite DNAs and rDNA chromosomal distributions between species provide useful information for phylogenetic analysis in genus Poa.

A Cross between Bread Wheat and a 2D(2R) Disomic Substitution Triticale Line Leads to the Formation of a Novel Disomic Addition Line and Provides Information of the Role of Rye Secalins on Breadmaking Characteristics

A bread wheat line (N11) and a disomic 2D(2R) substitution triticale line were crossed and backrossed four times. At each step electrophoretic selection for the seeds that possessed, simultaneously, the complete set of high molecular weight glutenin subunits of N11 and the two high molecular weight secalins of rye, present in the 2D(2R) line, was carried out. Molecular cytogenetic analyses of the BC₄F₈ generation revealed that the selection carried out produced a disomic addition line (2n = 44). The pair of additional chromosomes consisted of the long arm of chromosome 1R (1RL) from rye fused with the satellite body of the wheat chromosome 6B. Rheological analyses revealed that the dough obtained by the new addition line had higher quality characteristics when compared with the two parents. The role of the two additional high molecular weight secalins, present in the disomic addition line, in influencing improved dough characteristics is discussed.

FISH karyotype comparison between Ab- and A-genome chromosomes using oligonucleotide probes

Triticum boeoticum (2n = 2x = 14, A^bA^b) contains beneficial traits for common wheat improvement. The discrimination of A^b-genome chromosomes from A-genome chromosomes is an important step in gene transfer from T. boeoticum to common wheat. In this study, fluorescence in situ hybridization (FISH) analysis using nine oligonucleotide probes revealed high divergence between chromosomes of the common wheat germplasm Crocus and T. boeoticum accession G52. The combination of Oligo-pTa535-HM and Oligo-pSc119.2-HM can differentiate A^b and A chromosomes within homologous groups 2, 4, 5, and 6; chromosomes 2A^b and 6A^b can be identified by using (ACT)₇, (CTT)₇, and (GAA)₇. The probes Oligo-pTa713 and (ACT)₇ can be utilized for the identification of chromosomes 1A^b and 3A^b, respectively. Probes (CAG)₇ and (CAC)₇ can be applied in the identification of 7A^b. Moreover, probe combinations consisting of Oligo-pTa535-HM and (AAC)₇ with (ACT)₇ or (CTT)₇ and of Oligo-pTa535-HM and Oligo-pTa713 with (CAC)₇ or (CTT)₇ will help discriminate the A^b-genome chromosomes of T. boeoticum. These probes are being used as potential markers to select common wheat Crocus-T. boeoticum G52 alien chromosome lines. Moreover, FISH patterns are highly divergent between A^b- and A-genome chromosomes, indicating that obvious chromosome structural variations arose during wheat evolution.

De Novo Assembled Transcriptome Analysis and Identification of Genic SSR Markers in Red-Flowered Strawberry

Red-flowered strawberry is a new ornamental flower derived from intergeneric hybridization (Fragaria × Potentilla). To date, few molecular markers have been reported for this plant. RNA sequencing provides a relatively fast and low-cost approach for large-scale detection of simple sequence repeats (SSRs). In the present study, we profiled the transcriptome of red-flowered strawberry by Illumina HiSeq 2500 to identify SSRs related to petal color. Based on 2 million clean reads of red and white flowers from red-flowered strawberry hybrids, we assembled 91,835 unigenes with an average length of 717 bp. After functional annotation and prediction, there were 47,058 unigenes; of these, 26,861 had a gene ontology annotation, with 14,264 SSR loci. Mononucleotide SSRs were the predominant repeat type (47.20%, n = 6724), followed by di- (32.50%, n = 4641), tri- (19.10%, n = 2729), tetra- (0.90%, n = 132), hexa- (0.2%, n = 21), and penta- (0.10%, n = 16) nucleotide repeats. The most frequent di-, tri-, and tetra-nucleotide repeats were AG/CT, AAG/CTT, and AAAG/CTTT, respectively. PCR amplification with 105 SSR primer pairs yielded four bands specific to red flowers, namely UgRFsr57622, UgRFsr94149, UgRFsr40142, and UgRFsr54608; corresponding 4 trait-specific markers were found to co-segregate with white and red flower color in hybrid population, demonstrating that the genic SSR marker is useful to discriminate between white and red flowers in strawberry. Markers to discriminate flower color in red-flowered strawberry will be useful for early selection of progeny and for breeding management.

Chromosomal markers in the genus Karenia: Towards an understanding of the evolution of the chromosomes, life cycle patterns and phylogenetic relationships in dinoflagellates

Dinoflagellates are a group of protists whose genome is unique among eukaryotes in terms of base composition, chromosomal structure and gene expression. Even after decades of research, the structure and behavior of their amazing chromosomes-which without nucleosomes exist in a liquid crystalline state-are still poorly understood. We used flow cytometry and fluorescence in situ hybridization (FISH) to analyze the genome size of three species of the toxic dinoflagellate genus Karenia as well the organization and behavior of the chromosomes in different cell-cycle stages. FISH was also used to study the distribution patterns of ribosomal DNA (45S rDNA), telomeric and microsatellites repeats in order to develop chromosomal markers. The results revealed several novel and important features regarding dinoflagellate chromosomes during mitosis, including their telocentric behavior and radial arrangement along the nuclear envelope. Additionally, using the (AG)10 probe we identified an unusual chromosome in K. selliformis and especially in K. mikimotoi that is characterized by AG repeats along its entire length. This feature was employed to easily differentiate morphologically indistinguishable life-cycle stages. The evolutionary relationship between Karenia species is discussed with respect to differences in both DNA content and the chromosomal distribution patterns of the DNA sequences analyzed.

Comparative cytogenetics and derived phylogenic relationship among Sitophilus grain weevils (Coleoptera, Curculionidae, Dryophthorinae)

Cytogenetic characteristics and genome size are powerful tools for species characterization and identification of cryptic species, providing critical insights into phylogenetic and evolutionary relationships. Sitophilus Linnaeus, 1758 grain weevils can benefit from such tools as key pest species of stored products and also as sources of archeological information on human history and past urban environments. Moreover, the phylogenetic relationship among these weevil species remains controversial and is largely based on single DNA fragment analyses. Therefore, cytogenetic analyses and genome size determinations were performed for four Sitophilus grain weevil species, namely the granary weevil Sitophilus granarius (Linnaeus, 1758), the tamarind weevil S. linearis (Herbst, 1797), the rice weevil S. oryzae (Linnaeus, 1763), and the maize weevil S. zeamais Motschulsky, 1855. Both maize and rice weevils exhibited the same chromosome number (2n=22; 10 A + Xyp). In contrast, the granary and tamarind weevils exhibited higher chromosome number (2n=24; 11 A + Xyp and 11 A + neo-XY, respectively). The nuclear DNA content of these species was not proportionally related to either chromosome number or heterochromatin amount. Maize and rice weevils exhibited similar and larger genome sizes (0.730±0.003 pg and 0.786±0.003 pg, respectively), followed by the granary weevil (0.553±0.003 pg), and the tamarind weevil (0.440±0.001 pg). Parsimony phylogenetic analysis of the insect karyotypes indicate that S. zeamais and S. oryzae were phylogenetically closer than S. granarius and S. linearis, which were more closely related and share a more recent ancestral relationship.

Diversification of the P genome among Agropyron Gaertn. (Poaceae) species detected by FISH

The genomes of five Agropyron Gaertner, 1770 species were characterized using all potential di- or trinucleotide simple sequence repeat (SSR) motifs and four satellite DNA repeats as fluorescence in situ hybridization (FISH) probes. The sites of 5S and 45S rDNA were relatively conserved among the diploid and tetraploid species. A number of sites for the dinucleotide SSRs AC, AG, and pSc119.2 was detected in all investigated species except A. mongolicum Keng, 1938. Several different trinucleotide SSRs were identified in different tetraploid species. All Agropyron species were suggested to include the basic P genome, although genome differentiation was still observed. The P genome of A. mongolicum was distinct from that of the diploid A. cristatum (Linnaeus, 1753) Gaertner, 1770. and other tetraploid species, with no hybridizations for AC, AG, or pSc119.2 observed. This finding supports designation of the P genomes of A. cristatum and A. mongolicum as P^c and P^m, respectively. An exceptional 5S rDNA site revealed in one set of homoeologous chromosomes strongly supports the allopolyploid origin of A. desertorum (Fischer ex Link, 1821) Schultes, 1824. However, the diploid donors to A. desertorum need further investigation. Similarly, the unique FISH pattern of a pair of 5S rDNA-carrying chromosomes was indicative of a potential allopolyploid origin for A. fragile (Roth, 1800) Candargy, 1984. The conserved distribution of 5S and 45S rDNA suggests A. cristatum (4x) and A. michnoi Roshevitz, 1929 are closely related. Two forms of B chromosomes were identified among individuals A. mongolicum and A. desertorum plants.

A simple and efficient non-denaturing FISH method for maize chromosome differentiation using single-strand oligonucleotide probes

Single-strand oligonucleotides (SSONs hereafter) as probes are becoming a powerful method of chromosome painting in many species. In this study, nine SSONs ((ACT)₁₀, (ACT)₁₉, Knob-1, Knob-2, Knob-3, CentC69-1, MR68-3, K10-72-1, and TR1-357-2) were developed and used for chromosome identification in 16 maize (Zea mays L., 2n = 20) inbred lines and hybrids by non-denaturing fluorescence in situ hybridization (ND-FISH). Each SSON produced clear signals on 2-10 chromosomes of inbred lines B73 and Mo17. A multiplex probe set containing four SSONs ((ACT)₁₀, Knob-2, CentC69-1, and MR68-3) clearly characterized all maize chromosomes in the 16 lines by a single round of ND-FISH and revealed genetic variation at a chromosome level. For example, unique signals on chromosome 6 clearly distinguished all 16 genotypes. The SSONs and multiplex probe developed in this research will facilitate genotype identification and chromosome research in maize.

St2-80: a new FISH marker for St genome and genome analysis in Triticeae

The St genome is one of the most fundamental genomes in Triticeae. Repetitive sequences are widely used to distinguish different genomes or species. The primary objectives of this study were to (i) screen a new sequence that could easily distinguish the chromosome of the St genome from those of other genomes by fluorescence in situ hybridization (FISH) and (ii) investigate the genome constitution of some species that remain uncertain and controversial. We used degenerated oligonucleotide primer PCR (Dop-PCR), Dot-blot, and FISH to screen for a new marker of the St genome and to test the efficiency of this marker in the detection of the St chromosome at different ploidy levels. Signals produced by a new FISH marker (denoted St₂-80) were present on the entire arm of chromosomes of the St genome, except in the centromeric region. On the contrary, St₂-80 signals were present in the terminal region of chromosomes of the E, H, P, and Y genomes. No signal was detected in the A and B genomes, and only weak signals were detected in the terminal region of chromosomes of the D genome. St₂-80 signals were obvious and stable in chromosomes of different genomes, whether diploid or polyploid. Therefore, St₂-80 is a potential and useful FISH marker that can be used to distinguish the St genome from those of other genomes in Triticeae.

Sequencing of long stretches of repetitive DNA

Repetitive DNA is widespread in eukaryotic genomes, in some cases making up more than 80% of the total. SSRs are a type of repetitive DNA formed by short motifs repeated in tandem arrays. In some species, SSRs may be organized into long stretches, usually associated with the constitutive heterochromatin. Variation in repeats can alter the expression of genes, and changes in the number of repeats have been linked to certain human diseases. Unfortunately, the molecular characterization of these repeats has been hampered by technical limitations related to cloning and sequencing. Indeed, most sequenced genomes contain gaps owing to repetitive DNA-related assembly difficulties. This paper reports an alternative method for sequencing of long stretches of repetitive DNA based on the combined use of 1) a linear vector to stabilize the cloning process, and 2) the use of exonuclease III for obtaining progressive deletions of SSR-rich fragments. This strategy allowed the sequencing of a fragment containing a stretch of 6.2 kb of continuous SSRs. To demonstrate that this procedure can sequence other kinds of repetitive DNA, it was used to examine a 4.5 kb fragment containing a cluster of 15 repeats of the 5S rRNA gene of barley.

Chromosomal distribution patterns of the (AC)10 microsatellite and other repetitive sequences, and their use in chromosome rearrangement analysis of species of the genus Avena

Fluorescence in situ hybridization (FISH) was used to determine the physical location of the (AC)₁₀ microsatellite in metaphase chromosomes of six diploid species (AA or CC genomes), two tetraploid species (AACC genome), and five cultivars of two hexaploid species (AACCDD genome) of the genus Avena, a genus in which genomic relationships remain obscure. A preferential distribution of the (AC)₁₀ microsatellite in the pericentromeric and interstitial regions was seen in both the A- and D-genome chromosomes, while in C-genome chromosomes the majority of signals were located in the pericentromeric heterochromatic regions. New large chromosome rearrangements were detected in two polyploid species: an intergenomic translocation involving chromosomes 17AL and 21DS in Avena sativa 'Araceli' and another involving chromosomes 4CL and 21DS in the analyzed cultivars of Avena byzantina. The latter 4CL-21DS intergenomic translocation differentiates clearly between A. sativa and A. byzantina. Searches for common hybridization patterns on the chromosomes of different species revealed chromosome 10A of Avena magna and 21D of hexaploid oats to be very similar in terms of the distribution of 45S and Am1 sequences. This suggests a common origin for these chromosomes and supports a CCDD rather than an AACC genomic designation for this species.

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.

Chromosomal organization of repetitive DNAs in Hordeum bogdanii and H. brevisubulatum (Poaceae)

Molecular karyotypes of Hordeum bogdanii Wilensky, 1918 (2n = 14), and Hordeum brevisubulatum Link, 1844 ssp. brevisubulatum (2n = 28), were characterized by physical mapping of several repetitive sequences. A total of 18 repeats, including all possible di- or trinucleotide SSR (simple sequence repeat) motifs and satellite DNAs, such as pAs1, 5S rDNA, 45S rDNA, and pSc119.2, were used as probes for fluorescence in situ hybridization on root-tip metaphase chromosomes. Except for the SSR motifs AG, AT and GC, all the repeats we examined produced detectable hybridization signals on chromosomes of both species. A detailed molecular karyotype of the I genome of Hordeum bogdanii is described for the first time, and each repetitive sequence is physically mapped. A high degree of chromosome variation, including aneuploidy and structural changes, was observed in Hordeum brevisubulatum. Although the distribution of repeats in the chromosomes of Hordeum brevisubulatum is different from that of Hordeum bogdanii, similar patterns between the two species imply that the autopolyploid origin of Hordeum brevisubulatum is from a Hordeum species with an I genome. A comparison of the I genome and the other Hordeum genomes, H, Xa and Xu, shows that colocalization of motifs AAC, ACT and CAT and colocalization of motifs AAG and AGG are characteristic of the I genome. In addition, we discuss the evolutionary significance of repeats in the genome during genome differentiation.

Cytomolecular discrimination of the Am chromosomes of Triticum monococcum and the A chromosomes of Triticum aestivum using microsatellite DNA repeats

The cytomolecular discrimination of the A^m- and A-genome chromosomes facilitates the selection of wheat-Triticum monococcum introgression lines. Fluorescence in situ hybridisation (FISH) with the commonly used DNA probes Afa family, 18S rDNA and pSc119.2 showed that the more complex hybridisation pattern obtained in T. monococcum relative to bread wheat made it possible to differentiate the A^m and A chromosomes within homoeologous groups 1, 4 and 5. In order to provide additional chromosomal landmarks to discriminate the A^m and A chromosomes, the microsatellite repeats (GAA)_n, (CAG)_n, (CAC)_n, (AAC)_n, (AGG)_n and (ACT)_n were tested as FISH probes. These showed that T. monococcum chromosomes have fewer, generally weaker, simple sequence repeat (SSR) signals than the A-genome chromosomes of hexaploid wheat. A differential hybridisation pattern was observed on 6A^m and 6A chromosomes with all the SSR probes tested except for the (ACT)_n probe. The 2A^m and 2A chromosomes were differentiated by the signals given by the (GAA)_n, (CAG)_n and (AAC)_n repeats, while only (GAA)_n discriminated the chromosomes 3A^m and 3A. Chromosomes 7A^m and 7A could be differentiated by the lack of (GAA)_n and (AGG)_n signals on 7A. As potential landmarks for identifying the A^m chromosomes, SSR repeats will facilitate the introgression of T. monococcum chromatin into wheat.

Cytomolecular discrimination of the Am chromosomes of Triticum monococcum and the A chromosomes of Triticum aestivum using microsatellite DNA repeats

The cytomolecular discrimination of the A^m- and A-genome chromosomes facilitates the selection of wheat-Triticum monococcum introgression lines. Fluorescence in situ hybridisation (FISH) with the commonly used DNA probes Afa family, 18S rDNA and pSc119.2 showed that the more complex hybridisation pattern obtained in T. monococcum relative to bread wheat made it possible to differentiate the A^m and A chromosomes within homoeologous groups 1, 4 and 5. In order to provide additional chromosomal landmarks to discriminate the A^m and A chromosomes, the microsatellite repeats (GAA)_n, (CAG)_n, (CAC)_n, (AAC)_n, (AGG)_n and (ACT)_n were tested as FISH probes. These showed that T. monococcum chromosomes have fewer, generally weaker, simple sequence repeat (SSR) signals than the A-genome chromosomes of hexaploid wheat. A differential hybridisation pattern was observed on 6A^m and 6A chromosomes with all the SSR probes tested except for the (ACT)_n probe. The 2A^m and 2A chromosomes were differentiated by the signals given by the (GAA)_n, (CAG)_n and (AAC)_n repeats, while only (GAA)_n discriminated the chromosomes 3A^m and 3A. Chromosomes 7A^m and 7A could be differentiated by the lack of (GAA)_n and (AGG)_n signals on 7A. As potential landmarks for identifying the A^m chromosomes, SSR repeats will facilitate the introgression of T. monococcum chromatin into wheat.

Cytogenetic Diversity of Simple Sequences Repeats in Morphotypes of Brassica rapa ssp. chinensis

A significant fraction of the nuclear DNA of all eukaryotes is comprised of simple sequence repeats (SSRs). Although these sequences are widely used for studying genetic variation, linkage mapping and evolution, little attention had been paid to the chromosomal distribution and cytogenetic diversity of these sequences. In this paper, we report the distribution characterization of mono-, di-, and tri-nucleotide SSRs in Brassica rapa ssp. chinensis. Fluorescence in situ hybridization was used to characterize the cytogenetic diversity of SSRs among morphotypes of B. rapa ssp. chinensis. The proportion of different SSR motifs varied among morphotypes of B. rapa ssp. chinensis, with tri-nucleotide SSRs being more prevalent in the genome of B. rapa ssp. chinensis. We determined the chromosomal locations of mono-, di-, and tri-nucleotide repeat loci. The results showed that the chromosomal distribution of SSRs in the different morphotypes is non-random and motif-dependent, and allowed us to characterize the relative variability in terms of SSR numbers and similar chromosomal distributions in centromeric/peri-centromeric heterochromatin. The differences between SSR repeats with respect to abundance and distribution indicate that SSRs are a driving force in the genomic evolution of B. rapa species. Our results provide a comprehensive view of the SSR sequence distribution and evolution for comparison among morphotypes B. rapa ssp. chinensis.

Chromosomal detection of simple sequence repeats (SSRs) using nondenaturing FISH (ND-FISH)

Simple Sequence Repeats (SSRs) are known to be scattered and present in high number in eukaryotic genomes. We demonstrate that dye-labeled oligodeoxyribonucleotides with repeated mono-, di-, tri, or tetranucleotide motifs (15-20 nucleotides in length) have an unexpected ability to recognize SSR target sequences in non-denatured chromosomes. The results show that all these probes are able to invade chromosomes, independent of the size of the repeat motif, their nucleotide sequence, or their ability to form alternative B-DNA structures such as triplex DNA. This novel and remarkable property of binding SSR oligonucleotides to duplex DNA targets permitted the development of a non-denaturing fluorescence in situ hybridization method that quickly and efficiently detects SSR-enriched chromosome regions in mitotic, meiotic, and polytene chromosome spreads of different model organisms. These results have implications for genome analysis and for investigating the roles of SSRs in chromosome structure and function.

Distribution of new satellites and simple sequence repeats in annual and perennial Glycine species

The repeat sequences occupied more than 50 % of soybean genome. In order to understand where these repeat sequences distributed in soybean genome and its related Glycine species, we examined three new repeat sequences-soybean repeat sequence (SBRS1, SBRS2 and SBRS3), some nonspecific repeat sequences and 45S rDNA on several Glycine species, including annual and perennial accessions in this study. In the annual species, G. soja, signals for SBRS1 and ATT repeat can be found on each chromosome in GG genome, but those for SBRS2 and SBRS3 were located at three specific loci. In perennial Glycine species, these three SBR repeat frequently co-localized with 45S rDNA, two major 45S rDNA loci were found in all tetraploid species. However, an extra minor locus was found in one accession of the G. pescadrensis (Tab074), but not in another accession (Tab004). We demonstrate that some repetitive sequences are present in all Glycine species used in the study, but the abundancy is different in annual or perennial species. We suggest this study may provide additional information in investigations of the phylogeny in the Glycine species.

Exploring the tertiary gene pool of bread wheat: sequence assembly and analysis of chromosome 5M(g) of Aegilops geniculata

Next-generation sequencing (NGS) provides a powerful tool for the discovery of important genes and alleles in crop plants and their wild relatives. Despite great advances in NGS technologies, whole-genome shotgun sequencing is cost-prohibitive for species with complex genomes. An attractive option is to reduce genome complexity to a single chromosome prior to sequencing. This work describes a strategy for studying the genomes of distant wild relatives of wheat by isolating single chromosomes from addition or substitution lines, followed by chromosome sorting using flow cytometry and sequencing of chromosomal DNA by NGS technology. We flow-sorted chromosome 5M(g) from a wheat/Aegilops geniculata disomic substitution line [DS5M(g) (5D)] and sequenced it using an Illumina HiSeq 2000 system at approximately 50 × coverage. Paired-end sequences were assembled and used for structural and functional annotation. A total of 4236 genes were annotated on 5M(g) , in close agreement with the predicted number of genes on wheat chromosome 5D (4286). Single-gene FISH indicated no major chromosomal rearrangements between chromosomes 5M(g) and 5D. Comparing chromosome 5M(g) with model grass genomes identified synteny blocks in Brachypodium distachyon, rice (Oryza sativa), sorghum (Sorghum bicolor) and barley (Hordeum vulgare). Chromosome 5M(g) -specific SNPs and cytogenetic probe-based resources were developed and validated. Deletion bin-mapped and ordered 5M(g) SNP markers will be useful to track 5M-specific introgressions and translocations. This study provides a detailed sequence-based analysis of the composition of a chromosome from a distant wild relative of bread wheat, and opens up opportunities to develop genomic resources for wild germplasm to facilitate crop improvement.

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.

Physical mapping of chromosome 4J of Thinopyrum bessarabicum using gamma radiation-induced aberrations

Gamma radiation induced a series of structural aberrations involving Thinopyrum bessarabicum chromosome 4J. The aberrations allowed for deletion mapping of 101 4J-specific markers and fine mapping of blue-grained gene BaThb. Irradiation can induce translocations and deletions to assist physically locating genes and markers on chromosomes. In this study, a 12-Gy dosage of (60)Co-γ was applied to pollen and eggs of a wheat (Triticum aestivum) landrace Chinese Spring (CS)-Thinopyrum bessarabicum chromosome 4J disomic addition line (DA4J), and the gametes from irradiated plants were fertilized with normal CS eggs or pollen to produce M1 seeds. Based on genomic in situ hybridization analysis of 261 M1 plants, we identified 74 lines carrying structural aberrations involving chromosome 4J with the higher aberration rate in treated pollen (31.2 %) than in the treated eggs (21.3 %). We further identified 43 (53.8 %) lines with structural aberrations on chromosome 4J by analyzing another 80 M1 plants with 74 4J-specific markers, indicating that combining molecular and cytological methods was more efficient for detecting chromosome aberrations. Marker analysis thus was performed prior to cytogenetic identification on M2-M4 seeds to detect chromosome structural aberrations. Sixty-eight M3 lines with structural aberrations on chromosome 4J and six previously obtained chromosome 4J alien lines were then analyzed using 101 chromosome 4J-specific markers. After combining marker results with chromosome aberrations in each line, chromosome 4J was physically divided into 24 segmental blocks with 7 in the short arm and 17 in the long arm. The blue-grained gene BaThb was further mapped into the region corresponding to block 4JL-11. The chromosome aberrations and the physical map developed in this research provide useful stocks and tools for introgression of genes on chromosome 4J into wheat.

Characterization of repetitive DNA landscape in wheat homeologous group 4 chromosomes

Background

The number and complexity of repetitive elements varies between species, being in general most represented in those with larger genomes. Combining the flow-sorted chromosome arms approach to genome analysis with second generation DNA sequencing technologies provides a unique opportunity to study the repetitive portion of each chromosome, enabling comparisons among them. Additionally, different sequencing approaches may produce different depth of insight to repeatome content and structure. In this work we analyze and characterize the repetitive sequences of Triticum aestivum cv. Chinese Spring homeologous group 4 chromosome arms, obtained through Roche 454 and Illumina sequencing technologies, hereinafter marked by subscripts 454 and I, respectively.

Repetitive sequences were identified with the RepeatMasker software using the interspersed repeat database mips-REdat_v9.0p. The input sequences consisted of our 4DS₄₅₄ and 4DL₄₅₄ scaffolds and 4AS_I, 4AL_I, 4BS_I, 4BL_I, 4DS_I and 4DL_I contigs, downloaded from the International Wheat Genome Sequencing Consortium (IWGSC).

Results

Repetitive sequences content varied from 55% to 63% for all chromosome arm assemblies except for 4DL_I, in which the repeat content was 38%. Transposable elements, small RNA, satellites, simple repeats and low complexity sequences were analyzed. SSR frequency was found one per 24 to 27 kb for all chromosome assemblies except 4DL_I, where it was three times higher. Dinucleotides and trinucleotides were the most abundant SSR repeat units. (GA)n/(TC)n was the most abundant SSR except for 4DL_I where the most frequently identified SSR was (CCG/CGG)n. Retrotransposons followed by DNA transposons were the most highly represented sequence repeats, mainly composed of CACTA/En-Spm and Gypsy superfamilies, respectively. This whole chromosome sequence analysis allowed identification of three new LTR retrotransposon families belonging to the Copia superfamily, one belonging to the Gypsy superfamily and two TRIM retrotransposon families. Their physical distribution in wheat genome was analyzed by fluorescent in situ hybridization (FISH) and one of them, the Carmen retrotransposon, was found specific for centromeric regions of all wheat chromosomes.

Conclusion

The presented work is the first deep report of wheat repetitive sequences analyzed at the chromosome arm level, revealing the first insight into the repeatome of T. aestivum chromosomes of homeologous group 4.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1579-0) contains supplementary material, which is available to authorized users.

De novo assembly of transcriptome sequencing in Caragana korshinskii Kom. and characterization of EST-SSR markers

Caragana korshinskii Kom. is widely distributed in various habitats, including gravel desert, clay desert, fixed and semi-fixed sand, and saline land in the Asian and African deserts. To date, no previous genomic information or EST-SSR marker has been reported in Caragana Fabr. genus. In this study, more than two billion bases of high-quality sequence of C. korshinskii were generated by using illumina sequencing technology and demonstrated the de novo assembly and annotation of genes without prior genome information. These reads were assembled into 86,265 unigenes (mean length = 709 bp). The similarity search indicated that 33,955 and 21,978 unigenes showed significant similarities to known proteins from NCBI non-redundant and Swissprot protein databases, respectively. Among these annotated unigenes, 26,232 a unigenes were separately assigned to Gene Ontology (GO) database. When 22,756 unigenes searched against the Kyoto Encyclopedia of Genes and Genomes Pathway (KEGG) database, 5,598 unigenes were assigned to 5 main categories including 32 KEGG pathways. Among the main KEGG categories, metabolism was the biggest category (2,862, 43.7%), suggesting the active metabolic processes in the desert tree. In addition, a total of 19,150 EST-SSRs were identified from 15,484 unigenes, and the characterizations of EST-SSRs were further compared with other four species in Fabraceae. 126 potential marker sites were randomly selected to validate the assembly quality and develop EST-SSR markers. Among the 9 germplasms in Caranaga Fabr. genus, PCR success rate were 93.7% and the phylogenic tree was constructed based on the genotypic data. This research generated a substantial fraction of transcriptome sequences, which were very useful resources for gene annotation and discovery, molecular markers development, genome assembly and annotation. The EST-SSR markers identified and developed in this study will facilitate marker-assisted selection breeding.

Assessment of genetic diversity among barley cultivars and breeding lines adapted to the US Pacific Northwest, and its implications in breeding barley for imidazolinone-resistance

Extensive application of imidazolinone (IMI) herbicides had a significant impact on barley productivity contributing to a continuous decline in its acreage over the last two decades. A possible solution to this problem is to transfer IMI-resistance from a recently characterized mutation in the 'Bob' barley AHAS (acetohydroxy acid synthase) gene to other food, feed and malting barley cultivars. We focused our efforts on transferring IMI-resistance to barley varieties adapted to the US Pacific Northwest (PNW), since it comprises ∼23% (335,000 ha) of the US agricultural land under barley production. To effectively breed for IMI-resistance, we studied the genetic diversity among 13 two-rowed spring barley cultivars/breeding-lines from the PNW using 61 microsatellite markers, and selected six barley genotypes that showed medium to high genetic dissimilarity with the 'Bob' AHAS mutant. The six selected genotypes were used to make 29-53 crosses with the AHAS mutant and a range of 358-471 F1 seeds were obtained. To make informed selection for the recovery of the recipient parent genome, the genetic location of the AHAS gene was determined and its genetic nature assessed. Large F2 populations ranging in size from 2158-2846 individuals were evaluated for herbicide resistance and seedling vigor. Based on the results, F3 lines from the six most vigorous F2 genotypes per cross combination were evaluated for their genetic background. A range of 20%-90% recovery of the recipient parent genome for the carrier chromosome was observed. An effort was made to determine the critical dose of herbicide to distinguish between heterozygotes and homozygotes for the mutant allele. Results suggested that the mutant can survive up to the 10× field recommended dose of herbicide, and the 8× and 10× herbicide doses can distinguish between the two AHAS mutant genotypes. Finally, implications of this research in sustaining barley productivity in the PNW are discussed.

Microsatellite organization in the grasshopper Abracris flavolineata (Orthoptera: Acrididae) revealed by FISH mapping: remarkable spreading in the A and B chromosomes

With the aim of acquiring deeper knowledge about repetitive DNAs chromosomal organization in grasshoppers, we used fluorescent in situ hybridization (FISH) to map the distribution of 16 microsatellite repeats, including mono-, di-, tri- and tetra-nucleotides, in the chromosomes of the species Abracris flavolineata (Acrididae), which harbors B chromosome. FISH revealed two main patterns: (i) exclusively scattered signals, and (ii) scattered and specific signals, forming evident blocks. The enrichment was observed in both euchromatic and heterochromatic areas and only the motif (C)30 was absent in heterochromatin. The A and B chromosomes were enriched with all the elements that were mapped, being observed in the B chromosome more distinctive blocks for (GA)15 and (GAG)10. For A complement distinctive blocks were noticed for (A)30, (CA)15, (CG)15, (GA)15, (CAC)10, (CAA)10, (CGG)10, (GAA)10, (GAC)10 and (GATA)8. These results revealed an intense spreading of microsatellites in the A. flavolineata genome that was independent of the A+T or G+C enrichment in the repeats. The data indicate that the microsatellites compose the B chromosome and could be involved in the evolution of this element in this species, although no specific relationship with any A chromosome was observed to discuss about its origin. The systematic analysis presented here contributes to the knowledge of repetitive DNA chromosomal organization among grasshoppers including the B chromosomes.

Development of a wheat single gene FISH map for analyzing homoeologous relationship and chromosomal rearrangements within the Triticeae

A cytogenetic map of wheat was constructed using FISH with cDNA probes. FISH markers detected homoeology and chromosomal rearrangements of wild relatives, an important source of genes for wheat improvement. To transfer agronomically important genes from wild relatives to bread wheat (Triticum aestivum L., 2n = 6 x = 42, AABBDD) by induced homoeologous recombination, it is important to know the chromosomal relationships of the species involved. Fluorescence in situ hybridization (FISH) can be used to study chromosome structure. The genomes of allohexaploid bread wheat and other species from the Triticeae tribe are colinear to some extent, i.e., composed of homoeoloci at similar positions along the chromosomes, and with genic regions being highly conserved. To develop cytogenetic markers specific for genic regions of wheat homoeologs, we selected more than 60 full-length wheat cDNAs using BLAST against mapped expressed sequence tags and used them as FISH probes. Most probes produced signals on all three homoeologous chromosomes at the expected positions. We developed a wheat physical map with several cDNA markers located on each of the 14 homoeologous chromosome arms. The FISH markers confirmed chromosome rearrangements within wheat genomes and were successfully used to study chromosome structure and homoeology in wild Triticeae species. FISH analysis detected 1 U-6 U chromosome translocation in the genome of Aegilops umbellulata, showed colinearity between chromosome A of Ae. caudata and group-1 wheat chromosomes, and between chromosome arm 7S#3 L of Thinopyrum intermedium and the long arm of the group-7 wheat chromosomes.

Development of a wheat single gene FISH map for analyzing homoeologous relationship and chromosomal rearrangements within the Triticeae

A cytogenetic map of wheat was constructed using FISH with cDNA probes. FISH markers detected homoeology and chromosomal rearrangements of wild relatives, an important source of genes for wheat improvement. To transfer agronomically important genes from wild relatives to bread wheat (Triticum aestivum L., 2n = 6 x = 42, AABBDD) by induced homoeologous recombination, it is important to know the chromosomal relationships of the species involved. Fluorescence in situ hybridization (FISH) can be used to study chromosome structure. The genomes of allohexaploid bread wheat and other species from the Triticeae tribe are colinear to some extent, i.e., composed of homoeoloci at similar positions along the chromosomes, and with genic regions being highly conserved. To develop cytogenetic markers specific for genic regions of wheat homoeologs, we selected more than 60 full-length wheat cDNAs using BLAST against mapped expressed sequence tags and used them as FISH probes. Most probes produced signals on all three homoeologous chromosomes at the expected positions. We developed a wheat physical map with several cDNA markers located on each of the 14 homoeologous chromosome arms. The FISH markers confirmed chromosome rearrangements within wheat genomes and were successfully used to study chromosome structure and homoeology in wild Triticeae species. FISH analysis detected 1 U-6 U chromosome translocation in the genome of Aegilops umbellulata, showed colinearity between chromosome A of Ae. caudata and group-1 wheat chromosomes, and between chromosome arm 7S#3 L of Thinopyrum intermedium and the long arm of the group-7 wheat chromosomes.

The evolutionary history of sea barley (Hordeum marinum) revealed by comparative physical mapping of repetitive DNA

BACKGROUND AND AIMS

Hordeum marinum is a species complex that includes the diploid subspecies marinum and both diploid and tetraploid forms of gussoneanum. Their relationships, the rank of the taxa and the origin of the polyploid forms remain points of debate. The present work reports a comparative karyotype analysis of six H. marinum accessions representing all taxa and cytotypes.

METHODS

Karyotypes were determined by analysing the chromosomal distribution of several tandemly repeated sequences, including the Triticeae cloned probes pTa71, pTa794, pAs1 and pSc119·2 and the simple sequence repeats (SSRs) (AG)10, (AAC)5, (AAG)5, (ACT)5 and (ATC)5.

KEY RESULTS

The identification of each chromosome pair in all subspecies and cytotypes is reported for the first time. Homologous relationships are also established. Wide karyotypic differences were detected within marinum accessions. Specific chromosomal markers characterized and differentiated the genomes of marinum and diploid gussoneanum. Two subgenomes were detected in the tetraploids. One of these had the same chromosome complement as diploid gussoneanum; the second subgenome, although similar to the chromosome complement of diploid H. marinum sensu lato, appeared to have no counterpart in the marinum accessions analysed here.

CONCLUSIONS

The tetraploid forms of gussoneanum appear to have come about through a cross between a diploid gussoneanum progenitor and a second, related-but unidentified-diploid ancestor. The results reveal the genome structure of the different H. marinum taxa and demonstrate the allopolyploid origin of the tetraploid forms of gussoneanum.

Cytogenetic diversity of SSR motifs within and between Hordeum species carrying the H genome: H. vulgare L. and H. bulbosum L

Non-denaturing FISH (ND-FISH) was used to compare the distribution of four simple sequence repeats (SSRs)-(AG) n , (AAG) n , (ACT) n and (ATC) n -in somatic root tip metaphase spreads of 12 barley (H. vulgare ssp. vulgare) cultivars, seven lines of their wild progenitor H. vulgare ssp. spontaneum, and four lines of their close relative H. bulbosum, to determine whether the range of molecular diversity shown by these highly polymorphic sequences is reflected at the chromosome level. In both, the cultivated and wild barleys, clusters of AG and ATC repeats were invariant. In contrast, clusters of AAG and ACT showed polymorphism. Karyotypes were prepared after the identification of their seven pairs of homologous chromosomes. Variation between these homologues was only observed in one wild accession that showed the segregation of a reciprocal translocation involving chromosomes 5H and 7H. The two subspecies of H. vulgare analysed were no different in terms of their SSRs. Only AAG repeats were found clustered strongly on the chromosomes of all lines of H. bulbosum examined. Wide variation was seen between homologous chromosomes within and across these lines. These results are the first to provide insight into the cytogenetic diversity of SSRs in barley and its closest relatives. Differences in the abundance and distribution of each SSR analysed, between H. vulgare and H. bulbosum, suggest that these species do not share the same H genome, and support the idea that these species are not very closely related. Southern blotting experiments revealed the complex organization of these SSRs, supporting the findings made with ND-FISH.

FISHIS: fluorescence in situ hybridization in suspension and chromosome flow sorting made easy

The large size and complex polyploid nature of many genomes has often hampered genomics development, as is the case for several plants of high agronomic value. Isolating single chromosomes or chromosome arms via flow sorting offers a clue to resolve such complexity by focusing sequencing to a discrete and self-consistent part of the whole genome. The occurrence of sufficient differences in the size and or base-pair composition of the individual chromosomes, which is uncommon in plants, is critical for the success of flow sorting. We overcome this limitation by developing a robust method for labeling isolated chromosomes, named Fluorescent In situ Hybridization In suspension (FISHIS). FISHIS employs fluorescently labeled synthetic repetitive DNA probes, which are hybridized, in a wash-less procedure, to chromosomes in suspension following DNA alkaline denaturation. All typical A, B and D genomes of wheat, as well as individual chromosomes from pasta (T. durum L.) and bread (T. aestivum L.) wheat, were flow-sorted, after FISHIS, at high purity. For the first time in eukaryotes, each individual chromosome of a diploid organism, Dasypyrum villosum (L.) Candargy, was flow-sorted regardless of its size or base-pair related content. FISHIS-based chromosome sorting is a powerful and innovative flow cytogenetic tool which can develop new genomic resources from each plant species, where microsatellite DNA probes are available and high quality chromosome suspensions could be produced. The joining of FISHIS labeling and flow sorting with the Next Generation Sequencing methodology will enforce genomics for more species, and by this mightier chromosome approach it will be possible to increase our knowledge about structure, evolution and function of plant genome to be used for crop improvement. It is also anticipated that this technique could contribute to analyze and sort animal chromosomes with peculiar cytogenetic abnormalities, such as copy number variations or cytogenetic aberrations.

Predicting polymorphic EST-SSRs in silico

The public availability of large quantities of gene sequence data provides a valuable resource of the mining of Simple Sequence Repeat (SSR) molecular genetic markers for genetic analysis. These markers are inexpensive, require minimal labour to produce and can frequently be associated with functionally annotated genes. This study presents the characterization of barley EST-SSRs and the identification of putative polymorphic SSRs from EST data. Polymorphic SSRs are distinguished from monomorphic SSRs by the representation of varying motif lengths within an alignment of sequence reads. Two measures of confidence are calculated, redundancy of a polymorphism and co-segregation with accessions. The utility of this method is demonstrated through the discovery of 597 candidate polymorphic SSRs, from a total of 452 642 consensus expressed sequences. PCR amplification primers were designed for the identified SSRs. Ten primer pairs were validated for polymorphism in barley and for transferability across species. Analysis of the polymorphisms in relation to SSR motif, length, position and annotation is discussed.

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.

Flow sorting and molecular cytogenetic identification of individual chromosomes of Dasypyrum villosum L. (H. villosa) by a single DNA probe

Dasypyrum villosum (L.) Candargy (sin. Haynaldia villosa) is an annual wild diploid grass species (2n = 2x = 14; genome VV) belonging to the Poaceae family, which is considered to be an important source of biotic and abiotic stress resistance genes for wheat breeding. Enhanced characterization of D. villosum chromosomes can facilitate exploitation of its gene pool and its use in wheat breeding programs. Here we present the cytogenetic identification of D. villosum chromosomes on slide by fluorescent in situ hybridization (FISH), with the GAA simple sequence repeat (SSR) as a probe. We also describe the isolation and the flow cytometric analysis of D. villosum chromosomes in suspension, resulting in a distinguished flow karyotype. Chromosomes were flow sorted into three fractions, according their DNA content, one of which was composed of a single type of chromosome, namely 6 V, sorted with over 85% purity. Chromosome 6 V is known to carry genes to code for important resistance and seed storage characteristics, and its isolation represents a new source of genetic traits and specific markers useful for wheat improvement.

Identification, characterization, and utilization of genome-wide simple sequence repeats to identify a QTL for acidity in apple

Background

Apple is an economically important fruit crop worldwide. Developing a genetic linkage map is a critical step towards mapping and cloning of genes responsible for important horticultural traits in apple. To facilitate linkage map construction, we surveyed and characterized the distribution and frequency of perfect microsatellites in assembled contig sequences of the apple genome.

Results

A total of 28,538 SSRs have been identified in the apple genome, with an overall density of 40.8 SSRs per Mb. Di-nucleotide repeats are the most frequent microsatellites in the apple genome, accounting for 71.9% of all microsatellites. AT/TA repeats are the most frequent in genomic regions, accounting for 38.3% of all the G-SSRs, while AG/GA dimers prevail in transcribed sequences, and account for 59.4% of all EST-SSRs. A total set of 310 SSRs is selected to amplify eight apple genotypes. Of these, 245 (79.0%) are found to be polymorphic among cultivars and wild species tested. AG/GA motifs in genomic regions have detected more alleles and higher PIC values than AT/TA or AC/CA motifs. Moreover, AG/GA repeats are more variable than any other dimers in apple, and should be preferentially selected for studies, such as genetic diversity and linkage map construction. A total of 54 newly developed apple SSRs have been genetically mapped. Interestingly, clustering of markers with distorted segregation is observed on linkage groups 1, 2, 10, 15, and 16. A QTL responsible for malic acid content of apple fruits is detected on linkage group 8, and accounts for ~13.5% of the observed phenotypic variation.

Conclusions

This study demonstrates that di-nucleotide repeats are prevalent in the apple genome and that AT/TA and AG/GA repeats are the most frequent in genomic and transcribed sequences of apple, respectively. All SSR motifs identified in this study as well as those newly mapped SSRs will serve as valuable resources for pursuing apple genetic studies, aiding the apple breeding community in marker-assisted breeding, and for performing comparative genomic studies in Rosaceae.

Characterization of 15 polymorphic microsatellite loci for Cephalotaxus oliveri (Cephalotaxaceae), a conifer of medicinal importance

Cephalotaxus oliveri is a scarce medicinal conifer endemic to the south central region of China and Vietnam. A small fragmented population presently exists due to anthropogenic disturbance. C. oliveri has been used for its alkaloids harringtonine and homoharringtonine, which are effective against leucocythemia and lymphadenosarcoma. Monoecious plants have been detected in nature, although they were understood to be dioecious. In order to study the mating system, population genetics and the genetic effects of habitat fragmentation on C. oliveri, 15 polymorphic and 12 monomorphic microsatellite loci were developed for C. oliveri by using the Fast Isolation by AFLP of Sequences Containing repeats (FIASCO) protocol. The polymorphisms were assessed in 96 individuals from three natural populations (32 individuals per population). The number of alleles per locus ranged from two to 33, the observed and expected heterozygosity per locus ranged from 0.000 to 1.000 and from 0.000 to 0.923, respectively. These loci would facilitate a comprehensive understanding of the genetic dynamics on C. oliveri, which will be useful for establishing effective conservation strategies for this species.

Genomic organization and dynamics of repetitive DNA sequences in representatives of three Fagaceae genera

Oaks, chestnuts, and beeches are economically important species of the Fagaceae. To understand the relationship between these members of this family, a deep knowledge of their genome composition and organization is needed. In this work, we have isolated and characterized several AFLP fragments obtained from Quercus rotundifolia Lam. through homology searches in available databases. Genomic polymorphisms involving some of these sequences were evaluated in two species of Quercus , one of Castanea , and one of Fagus with specific primers. Comparative FISH analysis with generated sequences was performed in interphase nuclei of the four species, and the co-immunolocalization of 5-methylcytosine was also studied. Some of the sequences isolated proved to be genus-specific, while others were present in all the genera. Retroelements, either gypsy-like of the Tat/Athila clade or copia-like, are well represented, and most are dispersed in euchromatic regions of these species with no DNA methylation associated, pointing to an interspersed arrangement of these retroelements with potential gene-rich regions. A particular gypsy-sequence is dispersed in oaks and chestnut nuclei, but its confinement to chromocenters in beech evidences genome restructuring events during evolution of Fagaceae. Several sequences generated in this study proved to be good tools to comparatively study Fagaceae genome organization.

Use of methylation filtration and C(0)t fractionation for analysis of genome composition and comparative genomics in bread wheat

We investigated the compositional and structural differences in sequences derived from different fractions of wheat genomic DNA obtained using methylation filtration and C(0)t fractionation. Comparative analysis of these sequences revealed large compositional and structural variations in terms of GC content, different structural elements including repeat sequences (e.g., transposable elements and simple sequence repeats), protein coding genes, and non-coding RNA genes. A correlation between methylation status [determined on the basis of selective inclusion/exclusion in methylation-filtered (MF) library] of different repeat elements and expression level was observed. The expression levels were determined by comparing MF sequences with expressed sequence tags (ESTs) available in the public domain. Only a limited overlap among MF, high C(0)t (HC), and ESTs was observed, suggesting that these sequences may largely either represent the low-copy non-transcribed sequences or include genes with low expression levels. Thus, these results indicated a need to study MF and HC sequences along with ESTs to fully appreciate complexity of wheat gene space.

Identification of drought-responsive universal stress proteins in viridiplantae

Genes encoding proteins that contain the universal stress protein (USP) domain are known to provide bacteria, archaea, fungi, protozoa, and plants with the ability to respond to a plethora of environmental stresses. Specifically in plants, drought tolerance is a desirable phenotype. However, limited focused and organized functional genomic datasets exist on drought-responsive plant USP genes to facilitate their characterization. The overall objective of the investigation was to identify diverse plant universal stress proteins and Expressed Sequence Tags (ESTs) responsive to water-deficit stress. We hypothesize that cross-database mining of functional annotations in protein and gene transcript bioinformatics resources would help identify candidate drought-responsive universal stress proteins and transcripts from multiple plant species. Our bioinformatics approach retrieved, mined and integrated comprehensive functional annotation data on 511 protein and 1561 ESTs sequences from 161 viridiplantae taxa. A total of 32 drought-responsive ESTs from 7 plant genera Glycine, Hordeum, Manihot, Medicago, Oryza, Pinus and Triticum were identified. Two Arabidopsis USP genes At3g62550 and At3g53990 that encode ATP-binding motif were up-regulated in a drought microarray dataset. Further, a dataset of 80 simple sequence repeats (SSRs) linked to 20 singletons and 47 transcript assembles was constructed. Integrating the datasets on SSRs and drought-responsive ESTs identified three drought-responsive ESTs from bread wheat (BE604157), soybean (BM887317) and maritime pine (BX682209). The SSR sequence types were CAG, ATA and AT respectively. The datasets from cross-database mining provide organized resources for the characterization of USP genes as useful targets for engineering plant varieties tolerant to unfavorable environmental conditions.