STS-PCR markers appropriate for wheat-barley introgression

Rapid and cost-effective molecular karyotyping in wheat, barley, and their cross-progeny by chromosome-specific multiplex PCR

BACKGROUND

Interspecific hybridisation is a powerful tool for increasing genetic diversity in plant breeding programmes. Hexaploid wheat (Triticum aestivum, 2n = 42) × barley (Hordeum vulgare, 2n = 14) intergeneric hybrids can contribute to the transfer of agronomically useful traits by creating chromosome addition or translocation lines as well as full hybrids. Information on the karyotype of hybrid progenies possessing various combinations of wheat and barley chromosomes is thus essential for the subsequent breeding steps. Since the standard technique of chromosome in situ hybridisation is labour-intensive and requires specific skills. a routine, cost-efficient, and technically less demanding approach is beneficial both for research and breeding.

RESULTS

We developed a Multiplex Polymerase Chain Reaction (MPCR) method to identify individual wheat and barley chromosomes. Chromosome-specific primer pairs were designed based on the whole genome sequences of 'Chinese Spring' wheat and 'Golden Promise' barley as reference cultivars. A pool of potential primers was generated by applying a 20-nucleotide sliding window with consecutive one-nucleotide shifts on the reference genomes. After filtering for optimal primer properties and defined amplicon sizes to produce an ordered ladder-like pattern, the primer pool was manually curated and sorted into four MPCR primer sets for the wheat A, B, and D sub-genomes, and for the barley genome. The designed MPCR primer sets showed high chromosome specificity in silico for the genome sequences of all 18 wheat and barley cultivars tested. The MPCR primers proved experimentally also chromosome-specific for the reference cultivars as well as for 13 additional wheat and four barley genotypes. Analyses of 16 wheat × barley F1 hybrid plants demonstrated that the MPCR primer sets enable the fast and one-step detection of all wheat and barley chromosomes. Finally, the established genotyping system was fully corroborated with the standard genomic in situ hybridisation (GISH) technique.

CONCLUSIONS

Wheat and barley chromosome-specific MPCR offers a fast, labour-friendly, and versatile alternative to molecular cytogenetic detection of individual chromosomes. This method is also suitable for the high-throughput analysis of distinct (sub)genomes, and, in contrast to GISH, can be performed with any tissue type. The designed primer sets proved to be highly chromosome-specific over a wide range of wheat and barley genotypes as well as in wheat × barley hybrids. The described primer design strategy can be extended to many species with precise genome sequence information.

[Molecular marker analysis of wheat-Roegneria ciliaris additions lines.]

A total of 135 EST, 27 STS, and 253 SSR primer pairs located in 7 homoeologous groups of wheat and barly were used for amplification of 24 possible Wheat-R. ciliaris disomic addition lines. Fifty-five primer pairs could amplified polymorphic bands between common wheat variety "Inayama Komugi" and the Inayama Komugi- R. ciliaris amphiploid, and 31 of the 55 could amplify the specific bands of R. ciliaris in the addition lines. According to the PCR (polymerase Chain Reaction) results, the added R. ciliaris chromosomes in lines 07K02, 07K06, 07K39, 07K201, 07K202, 07K255, and 07K256 belong to homoeologous group 1 of wheat; the added R. ciliaris chromosomes in lines 07K07, 07K08, 07K09, 07K11, 07K14, and 07K17 belong to homoeologous group 2 of wheat; the added R. ciliaris chromosomes in lines 07K15, 07K16, 07K21, and 07K47 belong to homoeologous group 6 of wheat.

Expressed sequence tag-PCR markers for identification of alien barley chromosome 2H in wheat

We developed EST-PCR markers specific to barley chromosome 2H, for the purpose of effectively tracing alien chromosomes or chromosome parts in the wheat genetic background. The target alien chromosome 2H confers high resistance to pre-harvest sprouting, which is a worldwide natural disaster in wheat. A total of 120 primer pairs were selected by combining the wheat group 2 chromosomes of the EST database and the genome sequences of the new model plant Brachypodium distachyon. Seventy-seven of 120 primer pairs were polymorphic and 31 of 120 primer pairs were monomorphic between a set of wheat-barley chromosome 2H disomic addition/substitution lines and their parents by agarose gel electrophoresis and polyacrylamide gel electrophoresis. Thirty of 77 polymorphic primer pairs including primer pair P120 derived from the basi gene were chromosome 2H-specific. These markers are expected to be valuable in screening of wheat-barley chromosome 2H recombination lines and pre-harvest sprouting resistant varieties.

Characterization of alien chromosomes in backcross derivatives of Triticum aestivum × Elymus rectisetus hybrids by using molecular markers and sequential multicolor FISH/GISH

Wild Triticeae grasses serve as important gene pools for forage and cereal crops. Based on DNA sequences of genome-specific RAPD markers, sequence-tagged site (STS) markers specific for W and Y genomes have been obtained. Coupling with the use of genomic in situ hybridization, these STS markers enabled the identification of the W- and Y-genome chromosomes in backcross derivatives from hybrids of bread wheat Triticum aestivum L. (2n = 42; AABBDD) and Elymus rectisetus (Nees in Lehm.) Á. Löve & Connor (2n = 42; StStWWYY). The detection of six different alien chromosomes in five of these derivatives was ascertained by quantitative PCR of STS markers, simple sequence repeat markers, rDNA genes, and (or) multicolor florescence in situ hybridization. Disomic addition line 4687 (2n = 44) has the full complement of 42 wheat chromosomes and a pair of 1Y chromosomes that carry genes for resistance to tan spot (caused by Pyrenophora tritici-repentis (Died.) Drechs.) and Stagonospora nodorum blotch (caused by Stagonospora nodorum (Berk.) Castellani and Germano). The disomic addition line 4162 has a pair of 1St chromosomes and 21 pairs of wheat chromosomes. Lines 4319 and 5899 are two triple substitution lines (2n = 42) having the same chromosome composition, with 2A, 4B, and 6D of wheat substituted by one pair of W- and two pairs of St-genome chromosomes. Line 4434 is a substitution–addition line (2n = 44) that has the same W- and St-genome chromosomes substituting 2A, 4B, and 6D of wheat as in lines 4319 and 5899 but differs by having an additional pair of Y-genome chromosome, which is not the 1Y as in line 4687. The production and identification of these alien cytogenetic stocks may help locate and isolate genes for useful agronomic traits.

Molecular mapping of Rym17, a dominant and rym18 a recessive barley yellow mosaic virus (BaYMV) resistance genes derived from Hordeum vulgare L

PK23-2, a line of six-rowed barley (Hordeum vulgare L.) originating from Pakistan, has resistance to Japanese strains I and III of the barley yellow mosaic virus (BaYMV). To identify the source of resistance in this line, reciprocal crosses were made between the susceptible cultivar Daisen-gold and PK23-2. Genetic analyses in the F(1) generation, F(2) generation, and a doubled haploid population (DH45) derived from the F(1) revealed that PK23-2 harbors one dominant and one recessive resistance genes. A linkage map was constructed using 61 lines of DH45 and 127 DNA markers; this map covered 1268.8 cM in 10 linkage groups. One QTL having a LOD score of 4.07 and explaining 26.8% of the phenotypic variance explained (PVE) for resistance to BaYMV was detected at DNA marker ABG070 on chromosome 3H. Another QTL having a LOD score of 3.53 and PVE of 27.2% was located at marker Bmag0490 on chromosome 4H. The resistance gene on chromosome 3H, here named Rym17, showed dominant inheritance, whereas the gene on chromosome 4H, here named rym18, showed recessive inheritance in F(1) populations derived from crosses between several resistant lines of DH45 and Daisen-gold. The BaYMV recessive resistance genes rym1, rym3, and rym5, found in Japanese barley germplasm, were not allelic to rym18. These results revealed that PK23-2 harbors two previously unidentified resistance genes, Rym17 on 3H and rym18 on 4H; Rym17 is the first dominant BaYMV resistance gene to be identified in primary gene pool. These new genes, particularly dominant Rym17, represent a potentially valuable genetic resource against BaYMV disease.

CAPS markers specific to Eb, Ee, and R genomes in the tribe Triticeae

Wild Triticeae grasses serve as important gene pools for forage and cereal crops. Understanding their genome compositions is pivotal for efficient use of this vast gene pool in germplasm-enhancement programs. Several cleaved amplified polymorphic sequence (CAPS) markers were developed to distinguish the Eb, Ee, and R genomes. With the aid of disomic addition lines of wheat, it was confirmed that all 7 chromosomes of Eb, Ee, and R genomes carry these genome-specific CAPS markers. Thus, the identified CAPS markers are useful in detecting and monitoring the chromosomes of these 3 genomes. This study also provides evidence suggesting that some Purdue and Chinese germplasm lines developed for barley yellow dwarf virus (BYDV) resistance are different from those developed in Australia. Furthermore, Thinopyrum intermedium and Thinopyrum ponticum were shown to have different genome constitutions. Sequence analyses of the 1272 bp sequences, containing Ty3/gypsy retrotransposons, from the Eb, Ee, and R genomes also shed light on the evolution of these 3 genomes.

Detection and mapping of homologous and homoeologous segments in homoeologous groups of allotetraploid cotton by BAC-FISH

Background

Cotton, as an allopolyploid species, contains homoeologous A and D subgenomes. The study of the homoeologous (duplicated) segments or chromosomes can facilitate insight into the evolutionary process of polyploidy and the development of genomic resources. Fluorescence in situ hybridization (FISH) using bacterial artificial chromosome (BAC) clones as probes has commonly been used to provide a reliable cytological technique for chromosome identification. In polyploids, it also presents a useful approach for identification and localization of duplicated segments. Here, two types of BACs that contained the duplicated segments were isolated and analyzed in tetraploid cotton by FISH.

Results

Homologous and homoeologous BACs were isolated by way of SSR marker-based selection and then used to develop BAC-FISH probes. Duplicated segments in homoeologous chromosomes were detected by FISH. The FISH and related linkage map results followed known reinforced the relationships of homoeologous chromosomes in allotetraploid cotton, and presented a useful approach for isolation of homoeologous loci or segments and for mapping of monomorphic loci. It is very important to find that the large duplicated segments (homologous BACs) do exist between homoeologous chromosomes, so the shot-gun approach for genome sequencing was unavailable for tetraploid cotton. However, without doubt, it will contain more information and promote the research for duplicated segments as well as the genome evolution in cotton.

Conclusion

These findings and the analysis method by BAC-FISH demonstrated the powerful nature and wide use for the genome and genome evolutionary researches in cotton and other polyploidy species.

TOPAAS, a tomato and potato assembly assistance system for selection and finishing of bacterial artificial chromosomes

We have developed the software package Tomato and Potato Assembly Assistance System (TOPAAS), which automates the assembly and scaffolding of contig sequences for low-coverage sequencing projects. The order of contigs predicted by TOPAAS is based on read pair information; alignments between genomic, expressed sequence tags, and bacterial artificial chromosome (BAC) end sequences; and annotated genes. The contig scaffold is used by TOPAAS for automated design of nonredundant sequence gap-flanking PCR primers. We show that TOPAAS builds reliable scaffolds for tomato (Solanum lycopersicum) and potato (Solanum tuberosum) BAC contigs that were assembled from shotgun sequences covering the target at 6- to 8-fold coverage. More than 90% of the gaps are closed by sequence PCR, based on the predicted ordering information. TOPAAS also assists the selection of large genomic insert clones from BAC libraries for walking. For this, tomato BACs are screened by automated BLAST analysis and in parallel, high-density nonselective amplified fragment length polymorphism fingerprinting is used for constructing a high-resolution BAC physical map. BLAST and amplified fragment length polymorphism analysis are then used together to determine the precise overlap. Assembly onto the seed BAC consensus confirms the BACs are properly selected for having an extremely short overlap and largest extending insert. This method will be particularly applicable where related or syntenic genomes are sequenced, as shown here for the Solanaceae, and potentially useful for the monocots Brassicaceae and Leguminosea.

Recent progress in barley improvement using wild species of Hordeum

In this review we describe recent progress in barley (Hordeum vulgare) improvement through hybridisation with its wild relatives. We have focused on one species in the secondary genepool of cultivated barley, namely H. bulbosum. This wild species has desirable traits, such as disease resistance, that are worthwhile transferring to its cultivated relative. Progress has been achieved through developing partially fertile interspecific hybrids that have been selfed or backcrossed to barley. We present the results of cytogenetic and molecular analyses that have enabled us to characterise and produce agronomically useful recombinant lines obtained from the hybrids.

Comparative high resolution map of the six-rowed spike locus 1 (vrs1) in several populations of barley, Hordeum vulgare L

Multiple alleles at the vrs1 locus control the development and fertility of the lateral spikelets of barley (Hordeum vulgare L.), which is a key character in the study of yield, utilization and domestication. In this study, six linkage maps of the vrs1 locus were constructed, using different mapping populations developed from nine different barley cultivars (H. vulgare subsp. vulgare) or mutant and wild barley (H. vulgare subsp. spontaneum). A total of 8387 chromosomes (gametes) were sampled for analysis based on a hypothesis that orders of marker loci were the same over the different parental lines. The results showed that four markers and the vrs1 locus in all cases were arranged in the same order, which was in a good agreement with the hypothesis. This makes the linkage maps suitable for the positional cloning of the alleles at the vrs1 locus.

Identification and mapping of cleistogamy genes in barley

Cleistogamy is a closed type of flowering with ensured self-pollination and an important trait to study evolutionary development in flower organs, reproduction systems, gene flow, and disease control. Still, very limited information is available about the genetic control and regulatory mechanism of this trait in barley. In this work, from the eight crosses between cleistogamous and chasmogamous accessions, five crosses generated chasmogamous F1 plants and their F2 plants segregated as 3 chasmogamous:1 cleistogamous, whereas three crosses generated cleistogamous F1 plants, and their F2 plants segregated as 1 chasmogamous:3 cleistogamous. Although a single gene was responsible for the control of cleistogamy in these two groups of crosses, the direction of dominance was opposite, suggesting two genes, cly1 and Cly2, for the genetic control of cleistogamy in barley. Epistatic type of gene interaction between the two loci was detected. In the analysis of 99 recombinant inbred lines of 'Azumamugi' x 'Kanto Nakate Gold' and doubled haploid lines of 'Harrington' x 'Mikamo Golden', where in both crosses F1 was chasmogamous, the cly1 locus has been mapped on chromosome 2HL. Using the analysis of the F2 population of 'Misato Golden' and 'Satsuki Nijo' where F1 was cleistogamous, the Cly2 locus was mapped in the same region of chromosome 2HL. Because the cly1 and Cly2 loci were mapped in the same region in these three different mapping populations, it was concluded that the expression of cleistogamy is under the control of two tightly linked genes or different alleles of the same gene.

Genetic diversity in wild Spanish populations of Thinopyrum junceum and Thinopyrum junceiforme using endosperm proteins and PCR-based markers

The genetic variation of sixteen wild, Spanish populations of Thinopyrum junceum and Thinopyrumjunceiforme and their interspecific relationships were analyzed. The relationships between these species and the diploids T. bessarabicum and T. elongatum were also investigated. The number of phenotypes and the composition of bands yielded by the electrophoretic separation of endosperm proteins were used to estimate intra- and interpopulational variability. DNA polymorphism generated by 24 arbitrary 10-mer primers and 14 specific 20-mer primers was used to determine interpopulational variability and interspecific relationships. Jaccard's coefficient of similarity was used to analyze presence and absence data in the DNA polymorphism and endosperm protein determinations of individual plants. Pearson's product-moment correlation coefficient was used to analyse interpopulational variation using endosperm protein band frequency data. Dendrograms were constructed using an unweighted pair group method with arithmetical average (UPGMA). The high level of intrapopulational variability found in T. junceum and T. junceiforme was inconsistent with the traditional classification of these species as self-pollinating. The level of interpopulational variation varied according to the degree of polymorphism of the corresponding markers. The endosperm proteins and random amplified polymorphic DNAs (RAPDs) proved to be the most polymorphic markers to those used although only the former were able to distinguish between the different populations. Interspecific relationships were consistently confirmed by all the PCR-based markers, and were also in agreement with the results of other authors.

Efficient construction of high-density linkage map and its application to QTL analysis in barley

Using a High Efficiency Genome Scanning (HEGS) system and recombinant inbred (RI) lines derived from the cross of Russia 6 and H.E.S. 4, a high-density genetic map was constructed in barley. The resulting 1,595.7-cM map encompassed 1,172 loci distributed on the seven linkage groups comprising 1,134 AFLP, 34 SSR, three STS and vrs1 (kernel row type) loci. Including PCR reactions, gel electrophoresis and data processing, 6 months of work by a single person was sufficient for the whole mapping procedure under a reasonable cost. To make an appraisal of the resolution of genetic analysis for the 95 RI lines based on the constructed linkage map, we measured three agronomic traits: plant height, spike exsertion length and 1,000-kernel weight, and the analyzed quantitative trait loci (QTLs) associated with these traits. The results were compared on the number of detected QTLs and their effects between a high-density map and a skeleton map constructed by selected AFLP and anchor markers. The composite interval mapping on the high-density map detected more QTLs than the other analyses. Closely linked markers with QTLs on the high-density map could be powerful tools for marker-assisted selection in barley breeding programs and further genetic analyses including an advanced backcross analysis or a map-based cloning of QTL.

The conversion of wheat RFLP probes into STS markers via the single-stranded conformation polymorphism technique

We describe a flexible and general strategy for converting a wheat RFLP-based assay into a PCR-based sequence-tagged site (STS), and have applied it to derive markers for a powdery mildew resistance gene present in a wheat-rye translocation. The concept is based on deriving PCR primers that amplify all of the homoeoloci defined by a single-copy cDNA sequence, and separating the resulting mixture of homoeoamplicons via single-stranded conformation polymorphism (SSCP) gels, which are able to detect minor differences between related DNA sequences. After their separation, the individual homoeoamplicons were sequenced and these were used to define nucleotide polymorphisms that could be exploited to design locus-specific PCR primers. In one case, we were able to demonstrate that the assay was allele specific.

Cross-species amplification of the Hordeum chilense genome using barley sequence-tagged-sites (STSs)

A selection of 51 barley Sequence-Tagged Sites (STSs) were studied for their utility in Hordeum chilense. They included four primer sets from wheat origin and six primer sets from oat origin. Forty-four primer pairs amplified H. chilense products consistently. Five primer pairs were suitable for studying the introgression of H. chilense in wheat because they amplified H. chilense products of distinct size. Six of the STSs showed polymorphism between different H. chilense accessions. The results showed that barley STSs could be useful for the genetic characterization of H. chilense, tritordeums and derived introgression lines.

Molecular and physical mapping of a barley gene on chromosome arm 1 HL that causes sterility in hybrids with wheat

Addition of the long arm of barley chromosome 1H (1HL) to wheat causes severe meiotic abnormalities and complete sterility of the plants. To map the barley gene responsible for the 1H-induced sterility of wheat, a series of addition lines of translocated 1H chromosomes were developed from the crosses between the wheat 'Shinchunaga' and five reciprocal translocation lines derived from the barley line St.13559. Examination of the seed fertility of the addition lines revealed that the sterility gene is located in the interstitial 25% region of the 1HL arm. The genetic location of the sterility gene was also estimated by physically mapping sequence-tagged site (STS) markers and simple-sequence repeat (SSR) markers with known map locations. The sterility gene is designated Shw (sterility in hybrids with wheat). Comparison of the present physical map of 1HL with two previously published genetic maps revealed a paucity of markers in the proximal 30% region and non-random distribution of SSR markers. Two inconsistencies in marker order were found between the present physical map and the consensus genetic map of group 1 chromosomes of Triticeae. On the basis of the effects on meiosis and chromosomal location, the relationship of the present sterility gene with other fertility-related genes of Triticeae is discussed.

Rapid verification of wheat-Hordeum introgressions by direct staining of SCAR, STS, and SSR amplicons

A range of single tagged site (STS), simple sequence repeat (SSR), and sequence-characterized amplified region (SCAR) markers were screened for their utility in detecting Hordeum vulgare and H. chilense chromosomes in a wheat background. PCR conditions were optimized for specific amplification of the targeted sequences and to avoid cross-species amplification. Two H. vulgare derived STSs, six H. vulgare derived SSRs, and nine H. chilense derived SCARs were usable for the detection of five H. vulgare and three H. chilense chromosomes by direct ethidium bromide staining of the PCR products in test tubes, avoiding the more costly and time-consuming DNA electrophoresis step. The practical application of the method is illustrated by the identification of a monotelosomic substitution of H. vulgare chromosome 6HS in tritordeum and a monosomic addition of H. chilense chromosome 6Hch in durum wheat.

Construction of a genetic map of barley (Hordeum vulgare L.) cross 'Azumamugi' × 'Kanto Nakate Gold' using a simple and efficient amplified fragment-length polymorphism system

We have devised a simple and efficient amplified fragment-length polymorphism (AFLP) system consisting of small slab gels, a discontinuous buffer system, and silver staining. Using this system, a single worker developed a barley map with 227 polymorphic fragments in 2 months. As a mapping population, 99 recombinant inbred lines of barley cultivars 'Azumamugi' × 'Kanto Nakate Gold' were used. Most of the 227 AFLP fragments showed a Mendelian segregation ratio of 1:1, and all were assigned to the seven barley chromosomes. Thus, these fragments are useful as molecular markers. They were integrated with 40 previously characterized sequence-tagged sites, 3 isozymes, and 2 morphological markers to construct an integrated map. The resulting map covered 925.6 cM with 272 markers (detecting 150 loci) at an average interval of 6.5 cM/locus. This system greatly simplifies map construction.Key words: amplified fragment length polymorphism (AFLP), Hordeum vulgare, linkage map, recombinant inbred line (RIL), sequence-tagged site (STS).

Sequence haplotypes revealed by sequence-tagged site fine mapping of the Ror1 gene in the centromeric region of barley chromosome 1H

We describe the development of polymerase chain reaction-based, sequence-tagged site (STS) markers for fine mapping of the barley (Hordeum vulgare) Ror1 gene required for broad-spectrum resistance to powdery mildew (Blumeria graminis f. sp. hordei). After locating Ror1 to the centromeric region of barley chromosome 1H using a combined amplified fragment length polymorphism/restriction fragment-length polymorphism (RFLP) approach, sequences of RFLP probes from this chromosome region of barley and corresponding genome regions from the related grass species oat (Avena spp.), wheat, and Triticum monococcum were used to develop STS markers. Primers based on the RFLP probe sequences were used to polymerase chain reaction-amplify and directly sequence homologous DNA stretches from each of four parents that were used for mapping. Over 28,000 bp from 22 markers were compared. In addition to one insertion/deletion of at least 2.0 kb, 79 small unique sequence polymorphisms were observed, including 65 single nucleotide substitutions, two dinucleotide substitutions, 11 insertion/deletions, and one 5-bp/10-bp exchange. The frequency of polymorphism between any two barley lines ranged from 0.9 to 3.0 kb, and was greatest for comparisons involving an Ethiopian landrace. Haplotype structure was observed in the marker sequences over distances of several hundred basepairs. Polymorphisms in 16 STSs were used to generate genetic markers, scored by restriction enzyme digestion or by direct sequencing. Over 2,300 segregants from three populations were used in Ror1 linkage analysis, mapping Ror1 to a 0.2- to 0.5-cM marker interval. We discuss the implications of sequence haplotypes and STS markers for the generation of high-density maps in cereals.

Identification of barley genome segments introgressed into wheat using PCR markers

Barley has several important traits that might be used in the genetic improvement of wheat. For this report, we have produced wheat-barley recombinants involving barley chromosomes 4 (4H) and 7 (5H). Wheat-barley disomic addition lines were crossed with 'Chinese Spring' wheat carrying the ph1b mutation to promote homoeologous pairing. Selection was performed using polymerase chain reaction (PCR) markers to identify lines with the barley chromosome in the ph1b background. These lines were self pollinated, and recombinants were identified using sequence-tagged-site (STS) primer sets that allowed differentiation between barley and wheat chromosomes. Several recombinant lines were isolated that involved different STS-PCR markers. Recombination was confirmed by allowing the lines to self pollinate and rescreening the progeny via STS-PCR. Progeny testing confirmed 9 recombinants involving barley chromosome 4 (4H) and 11 recombinants involving barley chromosome 7 (5H). Some recombinants were observed cytologically to eliminate the possibility of broken chromosomes. Since transmission of the recombinant chromosomes was lower than expected and since seed set was reduced in recombinant lines, the utility of producing recombinants with this method is uncertain.Key words: introgression, sequence-tagged-site, recombination.

Characterization of euploid backcross progenies derived from interspecific hybrids between Oryza sativa and O. eichingeri by restriction fragment length polymorphism (RFLP) analysis and genomic in situ hybridization (GISH)

Restriction fragment length polymorphism (RFLP) analysis and GISH (genomic in situ hybridization) were performed on euploid plants derived from crosses between Oryza sativa (2n = 24, AA) and two brown planthopper-resistant accessions of O. eichingeri (2n = 24, CC). After screening with 164 RFLP markers, 60 of the 67 euploid plants were identified as introgression lines, each carrying 1–6 small O. eichingeri segments integrated on chromosomes 1, 2, 6, or 10. In the somatic chromosome preparations of F1 hybrid, O. eichingeri chromosomes, fluorescing greenish-yellow in the sequential GISH, appeared to be longer and to contain more heterochromatin than O. sativa ones, and this karyotypic polymorphism can be used to detect some introgressed O. eichingeri segments in euploid plants. In addition, GISH identification presented direct evidence for the transfer of small segments from O. eichingeri to O. sativa chromosome(s) which were subsequently recognized according to their condensation pattern, arm ratio, and chromosome length. The present results would contribute to the molecular mapping and selection of O. eichingeri - derived brown planthopper-resistant gene and positive yield QTLs.Key words: Oryza sativa, Oryza eichingeri, introgression lines, RFLP, genomic in situ hybridization (GISH).

Development of an efficient maintenance and screening system for large-insert genomic DNA libraries of hexaploid wheat in a transformation-competent artificial chromosome (TAC) vector

Three large-insert genomic DNA libraries of common wheat, Triticum aestivum cv. Chinese Spring, were constructed in a newly developed transformation-competent artificial chromosome (TAC) vector, pYLTAC17, which accepts and maintains large genomic DNA fragments stably in both Escherichia coli and Agrobacterium tumefaciens. The vector contains the cis sequence required for Agrobacterium-mediated gene transfer into grasses. The average insert sizes of the three genomic libraries were approximately 46, 65 and 120 kbp, covering three haploid genome equivalents. Genomic libraries were stored as frozen cultures in a 96-well format, each well containing approximately 300-600 colonies (12 plates for small library, four for medium-size library and four for large library). In each of the libraries, approximately 80% of the colonies harbored genomic DNA inserts of >50 kbp. TAC clones containing gene(s) of interest were identified by the pooled PCR technique. Once the target TAC clones were isolated, they could be immediately transferred into grass genomes with the Agrobacterium system. Five clones containing the thionin type I genes (single copy per genome), corresponding to each of the three genomes (A, B and D), were successfully selected by the pooled PCR method, in addition to an STS marker (aWG464; single copy per genome) and CAB (a multigene family). TAC libraries constructed as described here can be used to isolate genomic clones containing target genes, and to carry out genome walking for positional cloning.

Properties of sequence-tagged-site primer sets influencing repeatability

The polymerase chain reaction (PCR) has become a standard procedure in plant genetics, and is the basis for many emerging genomics approaches to mapping and gene identification. One advantage of PCR is that sequence information for primer sets can be exchanged between laboratories, obviating the need for exchange and maintenance of biological materials. Repeatability of primer sets, whereby the same products are amplified in different laboratories using the same primer set, is important to successful exchange and utilization. We have developed several hundred sequence-tagged site (STS) primer sets for wheat and barley. The ability of the primer sets to generate reproducible amplifications in other laboratories has been variable. We wished to empirically determine the properties of the primer sets that most influenced repeatability. A total of 96 primer sets were tested with four genomic DNA samples on each of four thermocyclers. All major bands were repeatable across all four thermocyclers for approximately 50% of the primer sets. Characteristics most often associated with differences in repeatability included primer GC content and 3'-end stability of the primers. The propensity for primer-dimer formation was not a factor in repeatability. Our results provide empirical direction for the development of repeatable primer sets. Key words: STS-PCR primers, wheat, barley.

Cytologically integrated physical restriction fragment length polymorphism maps for the barley genome based on translocation breakpoints

We have developed a new technique for the physical mapping of barley chromosomes using microdissected translocation chromosomes for PCR with sequence-tagged site primers derived from >300 genetically mapped RFLP probes. The positions of 240 translocation breakpoints were integrated as physical landmarks into linkage maps of the seven barley chromosomes. This strategy proved to be highly efficient in relating physical to genetic distances. A very heterogeneous distribution of recombination rates was found along individual chromosomes. Recombination is mainly confined to a few relatively small areas spaced by large segments in which recombination is severely suppressed. The regions of highest recombination frequency (</=1 Mb/cM) correspond to only 4.9% of the total barley genome and harbor 47.3% of the 429 markers of the studied RFLP map. The results for barley correspond well with those obtained by deletion mapping in wheat. This indicates that chromosomal regions characterized by similar recombination frequencies and marker densities are highly conserved between the genomes of barley and wheat. The findings for barley support the conclusions drawn from deletion mapping in wheat that for all plant genomes, notwithstanding their size, the marker-rich regions are all of similar gene density and recombination activity and, therefore, should be equally accessible to map-based cloning.