FISH to meiotic pachytene chromosomes of tomato locates the root-knot nematode resistance gene Mi-1 and the acid phosphatase gene Aps-1 near the junction of euchromatin and pericentromeric heterochromatin of chromosome arms 6S and 6L, respectively

 The root-knot nematode resistance gene Mi-1 in tomato has long been thought to be located in the pericentromeric heterochromatin region of the long arm of chromosome 6 because of its very tight genetic linkage (approx. 1 cM) to the markers Aps-1 (Acid phosphatase 1) and yv (yellow virescent). Using Mi-BAC clones and an Aps-1 YAC clone in fluorescence in situ hybridisation (FISH) to pachytene chromosomes we now provide direct physical evidence showing that Mi-1 is located at the border of the euchromatin and heterochromatin regions in the short arm (6S) and Aps-1 in the pericentromeric heterochromatin of the long arm (6L) close to the euchromatin. Taking into account both the estimated DNA content of hetero- and euchromatin regions and the compactness of the tomato chromosomes at pachytene (2 Mb/μm), our data suggest that Mi-1 and Aps-1 are at least 40 Mb apart, a base pair-to-centiMorgan relationship that is more than 50-fold higher than the average value of 750 kb/cM of the tomato genome. An integrated cytogenetic-molecular map of chromosome 6 is presented that provides a framework for physical mapping. Received: 24 July 1998 / Accepted: 14 August 1998     

Localization of<Emphasis Type="Italic"> jointless-2</Emphasis> gene in the centromeric region of tomato chromosome 12 based on high resolution genetic and physical mapping

Abstract Abscission is a universal process whereby plants shed their organs, such as flowers, fruit and leaves. In tomato, the non-allelic mutations jointless and jointless-2 have been discovered as recessive mutations that completely suppress the formation of pedicel abscission zones. A high resolution genetic map of jointless-2 was constructed using 1,122 jointless F₂ plants. Restriction fragment length polymorphism (RFLP) marker RPD140 completely co-segregated with the jointless-2 locus and mapped in a 2.4 cM interval between RFLP markers CD22 and TG618. To chromosome walk to jointless-2, all three markers were used to screen a bacterial artificial chromosome (BAC) library and contigs were developed. Intensive efforts to expand and merge the BAC contigs were unsuccessful because of the highly repetitive sequence content on the distal ends of each contig. To determine the physical distance between and the orientation of the three contigs, we used high resolution pachytene fluorescence in situ hybridization (FISH) mapping. The RPD140 contig was positioned in the centromeric region of chromosome 12 between two large pericentric heterochromatin blocks, about 50 Mb from the TG618 contig on the short arm and 10 Mb from the CD22 contig on the long arm, respectively. Based on high resolution genetic and physical mapping, we conclude that the jointless-2 gene is located within or near the chromosome 12 centromere where 1 cM is approximately 25 Mb in length.Communicated by Q. ZhangM.A. Budiman, S-B. Chang and S. Lee contributed equally to the work.     

Molecular genetic analysis of theripening-inhibitor andnon-ripening loci of tomato: A first step in genetic map-based cloning of fruit ripening genes

Ripening represents a complex developmental process unique to plants. We are using tomato fruit ripening mutants as tools to understand the regulatory components that control and coordinate the physiological and biochemical changes which collectively confer the ripe phenotype. We have genetically characterized two loci which result in significant inhibition of the ripening process in tomato,ripening-inhibitor (rin), andnon-ripening (nor), as a first step toward isolating genes likely to encode key regulators of this developmental process. A combination of pooled-sample mapping as well as classical restriction fragment length polymorphism (RFLP) analysis has permitted the construction of high-density genetic maps for the regions of chromosomes 5 and 10 spanning therin andnor loci, respectively. To assess the feasibility of initiating a chromosome walk, physical mapping of high molecular weight genomic DNA has been employed to estimate the relationship between physical distance (in kb) and genetic distance (in cM) around the targeted loci. Based on this analysis, the relationship in the region spanning therin locus is estimated to be 200–300 kb/cM, while thenor locus region ratio is approximately 200 kb/1 cM. Using RFLP markers tightly linked torin andnor, chromosome walks have been initiated to both loci in a yeast artificial chromosome (YAC) library of tomato genomic DNA. We have isolated and characterized several YAC clones linked to each of the targeted ripening loci and present genetic evidence that at least one YAC clone contains thenot locus.     

Map-based cloning in crop plants. Tomato as a model system: I. Genetic and physical mapping of jointless

A map-based cloning scheme is being used to isolate the jointless (j) gene of tomato. The jointless locus is defined by a single recessive mutation that completely suppresses the formation of the fruit and flower pedicel and peduncle abscission zone. jointless was mapped in an F₂ population of an interspecific cross between Lycopersicon esculentum and Lycopersicon pennellii to a 7.1 cM interval between two restriction fragment length polymorphism (RFLP) markers TG523 and TG194. Isogenic DNA pools were then constructed from a subset of the mapping population and screened with 800 random decamers for random amplification of polymorphic DNA (RAPD) polymorphisms. Five new RAPD markers were isolated and mapped to chromosome 11, two of which were mapped within the targeted interval. One marker, RPD158, was mapped 1.5 cM to the opposite side of jointless relative to TG523 and thus narrowed the interval between the closest flanking markers to 3.0 cM. Physical mapping by pulse-field gel electrophoresis using TG523 and RPD158 as probes demonstrated that both markers hybridize to a common 600 kb SmaI restriction fragment. This provided an estimate of 200 kb/cM for the relationship between physical and genetic distances in the region of chromosome 11 containing the j locus. The combined results provide evidence for the feasibility of the next step toward isolation of the jointless gene by map-based cloning — a chromosome walk or jump to jointless.     

Marker enrichment and high-resolution map of the segment of potato chromosome VII harbouring the nematode resistance gene Gro1

The dominant allele Gro1 confers on potato resistance to the root cyst nematode Globodera rostochiensis. The Gro1 locus has been mapped to chromosome VII on the genetic map of potato, using RFLP markers. This makes possible the cloning of Gro1 based on its map position. As part of this strategy we have constructed a high-resolution genetic map of the chromosome segment surrounding Gro1, based on RFLP, RAPD and AFLP markers. RAPD and RFLP markers closely linked to Gro1 were selected by bulked segregant analysis and mapped relative to the Gro1 locus in a segregating population of 1105 plants. Three RFLP and one RAPD marker were found to be inseparable from the Gro1 locus. Two AFLP markers were identified that flanked Gro1 at genetic distances of 0.6 cM and 0.8 cM, respectively. A genetic distance of 1 cM in the Gro1 region corresponds to a physical distance of ca. 100 kb as estimated by long-range restriction analysis. Marker-assisted selection for nematode resistance was accomplished in the course of constructing the high-resolution map. Plants carrying the resistance allele Gro1 could be distinguished from susceptible plants by marker assays based on the polymerase chain reaction (PCR).     

Molecular genetic analysis of theripening-inhibitor andnon-ripening loci of tomato: A first step in genetic map-based cloning of fruit ripening genes

Ripening represents a complex developmental process unique to plants. We are using tomato fruit ripening mutants as tools to understand the regulatory components that control and coordinate the physiological and biochemical changes which collectively confer the ripe phenotype. We have genetically characterized two loci which result in significant inhibition of the ripening process in tomato,ripening-inhibitor (rin), andnon-ripening (nor), as a first step toward isolating genes likely to encode key regulators of this developmental process. A combination of pooled-sample mapping as well as classical restriction fragment length polymorphism (RFLP) analysis has permitted the construction of high-density genetic maps for the regions of chromosomes 5 and 10 spanning therin andnor loci, respectively. To assess the feasibility of initiating a chromosome walk, physical mapping of high molecular weight genomic DNA has been employed to estimate the relationship between physical distance (in kb) and genetic distance (in cM) around the targeted loci. Based on this analysis, the relationship in the region spanning therin locus is estimated to be 200–300 kb/cM, while thenor locus region ratio is approximately 200 kb/1 cM. Using RFLP markers tightly linked torin andnor, chromosome walks have been initiated to both loci in a yeast artificial chromosome (YAC) library of tomato genomic DNA. We have isolated and characterized several YAC clones linked to each of the targeted ripening loci and present genetic evidence that at least one YAC clone contains thenot locus.     

High-resolution pachytene chromosome mapping of bacterial artificial chromosomes anchored by genetic markers reveals the centromere location and the distribution of genetic recombination along chromosome 10 of rice

Large-scale physical mapping has been a major challenge for plant geneticists due to the lack of techniques that are widely affordable and can be applied to different species. Here we present a physical map of rice chromosome 10 developed by fluorescence in situ hybridization (FISH) mapping of bacterial artificial chromosome (BAC) clones on meiotic pachytene chromosomes. This physical map is fully integrated with a genetic linkage map of rice chromosome 10 because each BAC clone is anchored by a genetically mapped restriction fragment length polymorphism marker. The pachytene chromosome-based FISH mapping shows a superior resolving power compared to the somatic metaphase chromosome-based methods. The telomere-centromere orientation of DNA clones separated by 40 kb can be resolved on early pachytene chromosomes. Genetic recombination is generally evenly distributed along rice chromosome 10. However, the highly heterochromatic short arm shows a lower recombination frequency than the largely euchromatic long arm. Suppression of recombination was found in the centromeric region, but the affected region is far smaller than those reported in wheat and barley. Our FISH mapping effort also revealed the precise genetic position of the centromere on chromosome 10.     

Chromosome landing at the tomato Bs4 locus

The tomato (Lycopersicon esculentum) Bs4 gene confers resistance to strains of Xanthomonas campestris pathovar vesicatoria that express the avirulence protein AvrBs4. As part of a map-based cloning strategy for the isolation of Bs4, we converted Bs4-linked amplified fragment length polymorphism (AFLP) and restriction fragment length polymorphism (RFLP) markers into locus-specific sequence-tagged-site (STS) markers. The use of these markers for the analysis of 1972 meiotic events allowed high-resolution genetic mapping within a 1.2-cM interval containing the target gene. Two tomato yeast artificial chromosome (YAC) clones, each harboring inserts of approximately 250 kb, were identified using the marker most closely linked to Bs4. YAC end-specific markers were established and employed to construct a local YAC contig. The ratio of physical to genetic distance at Bs4 was calculated to be 280 kb/cM, revealing that recombination rates in this region are about three times higher than the genome-wide average. Mapping of YAC end-derived markers demonstrated that the Bs4 locus maps within a region of 250 kb, corresponding to a genetic interval of 0.9 cM.     

Correlation of genetic and physical structure in the region surrounding the I2Fusarium oxysporum resistance locus in tomato

Summary The dominant gene I ₂ confers on tomato (Lycopersicon esculentum) resistance against the fungus Fusarium oxysporum f. sp. lycopersici race 2. A restriction fragment length polymorphism (RFLP) marker, TG105, has recently been found to be tightly linked to I ₂. The potential for cloning this gene by a reverse genetics approach prompted us to describe in both genetic and physical detail the region surrounding the I ₂ locus on chromosome 11. We have analyzed patterns of segregation of RFLP markers on chromosome 11 and Fusarium resistance in 140 F₂ plants from a cross between Fusarium-resistant and susceptible parental lines. Marker TG105 mapped 0.4 centi-Morgan (CM) from I ₂. Physical analysis of TG105 and its flanking RFLP markers, TG26 and TG36, by pulsed field gradient gel electrophoresis (PFGE) yielded a restriction map for this region encompassing at least 620 kb of the tomato genome. TG105 and TG26 hybridized to the same 175 kb MluI-NruI restriction fragment. We have therefore linked two genetically distinct RFLP markers. Based on the 4.1 cM distance between them, we have assigned a mean value of 43 kb for each cM recombination distance in the vicinity of I ₂. This local ratio between physical and genetic distances is more than 10-fold below the average for the tomato genome. It should therefore be possible to clone I ₂ by chromosome walking from TG105.     

A high-resolution map of the H1 locus harbouring resistance to the potato cyst nematode Globodera rostochiensis

The resistance gene H1 confers resistance to the potato cyst nematode Globodera rostochiensis and is located at the distal end of the long arm of chromosome V of potato. For marker enrichment of the H1 locus, a bulked segregant analysis (BSA) was carried out using 704 AFLP primer combinations. A second source of markers tightly linked to H1 is the ultra-high-density (UHD) genetic map of the potato cross SH × RH. This map has been produced with 387 AFLP primer combinations and consists of 10,365 AFLP markers in 1,118 bins (). Comparing these two methods revealed that BSA resulted in one marker/cM and the UHD map in four markers/cM in the H1 interval. Subsequently, a high-resolution genetic map of the H1 locus has been developed using a segregating F₁ SH × RH population consisting of 1,209 genotypes. Two PCR-based markers were designed at either side of the H1 gene to screen the 1,209 genotypes for recombination events. In the high-resolution genetic map, two of the four co-segregating AFLP markers could be separated from the H1 gene. Marker EM1 is located at a distance of 0.2 cM, and marker EM14 is located at a distance of 0.8 cM. The other two co-segregating markers CM1 (in coupling) and EM15 (in repulsion) could not be separated from the H1 gene.Communicated by J.G. Wenzel     

High-resolution genetic and physical mapping of the region containing the Bs2 resistance gene of pepper

The Bs2 resistance gene of pepper confers resistance against the bacterial pathogen Xanthomonas campestris pv. vesicatoria. As a first step toward isolation of the Bs2 gene, molecular markers tightly linked to the gene were identified by randomly amplified polymorphic DNA (RAPD) and amplified fragment length polymorphism (AFLP) analysis of near-isogenic lines. Markers flanking the locus were identified and a high-resolution linkage map of the region was developed. One AFLP marker, A2, was found to cosegregate with the locus, while two others, F1 and B3, flank the locus and are within 0.6 cM. Physical mapping of the A2 and F1 markers indicates that these markers may be within 150 kb of each other. Together, these results indicate that the Bs2 region may be cloned either by chromosome walker or landing. The linked markers were also used to characterize gamma-irradiation-induced mutants at the Bs2 locus. Received: 15 January 1999 / Accepted: 11 May 1999     

Marker enrichment and high-resolution map of the segment of potato chromosome VII harbouring the nematode resistance gene Gro1

The dominant allele Gro1 confers on potato resistance to the root cyst nematode Globodera rostochiensis. The Gro1 locus has been mapped to chromosome VII on the genetic map of potato, using RFLP markers. This makes possible the cloning of Gro1 based on its map position. As part of this strategy we have constructed a high-resolution genetic map of the chromosome segment surrounding Gro1, based on RFLP, RAPD and AFLP markers. RAPD and RFLP markers closely linked to Gro1 were selected by bulked segregant analysis and mapped relative to the Gro1 locus in a segregating population of 1105 plants. Three RFLP and one RAPD marker were found to be inseparable from the Gro1 locus. Two AFLP markers were identified that flanked Gro1 at genetic distances of 0.6 cM and 0.8 cM, respectively. A genetic distance of 1 cM in the Gro1 region corresponds to a physical distance of ca. 100 kb as estimated by long-range restriction analysis. Marker-assisted selection for nematode resistance was accomplished in the course of constructing the high-resolution map. Plants carrying the resistance allele Gro1 could be distinguished from susceptible plants by marker assays based on the polymerase chain reaction (PCR).     

Identification and fine mapping of <Emphasis Type="Italic">Pi39</Emphasis>(t), a major gene conferring the broad-spectrum resistance to <Emphasis Type="Italic">Magnaporthe oryzae</Emphasis>

Blast, caused by the ascomycete fungus Magnaporthe oryzae, is one of the most devastating diseases of rice worldwide. The Chinese native cultivar (cv.) Q15 expresses the broad-spectrum resistance to most of the isolates collected from China. To effectively utilize the resistance, three rounds of linkage analysis were performed in an F₂ population derived from a cross of Q15 and a susceptible cv. Tsuyuake, which segregated into 3:1 (resistant/susceptible) ratio. The first round of linkage analysis employing simple sequence repeat (SSR) markers was carried out in the F₂ population through bulked-segregant assay. A total of 180 SSR markers selected from each chromosome equally were surveyed. The results revealed that only two polymorphic markers, RM247 and RM463, located on chromosome 12, were linked to the resistance (R) gene. To further define the chromosomal location of the R gene locus, the second round of linkage analysis was performed using additional five SSR markers, which located in the region anchored by markers RM247 and RM463. The locus was further mapped to a 0.27 cM region bounded by markers RM27933 and RM27940 in the pericentromeric region towards the short arm. For fine mapping of the R locus, seven new markers were developed in the smaller region for the third round of linkage analysis, based on the reference sequences. The R locus was further mapped to a 0.18 cM region flanked by marker clusters 39M11 and 39M22, which is closest to, but away from the Pita/Pita ² locus by 0.09 cM. To physically map the locus, all the linked markers were landed on the respective bacterial artificial chromosome clones of the reference cv. Nipponbare. Sequence information of these clones was used to construct a physical map of the locus, in silico, by bioinformatics analysis. The locus was physically defined to an interval of ≈37 kb. To further characterize the R gene, five R genes mapped near the locus, as well as 10 main R genes those might be exploited in the resistance breeding programs, were selected for differential tests with 475 Chinese isolates. The R gene carrier Q15 conveys resistances distinct from those conditioned by the carriers of the 15 R genes. Together, this valuable R gene was, therefore, designated as Pi39(t). The sequence information of the R gene locus could be used for further marker-based selection and cloning. Xinqiong Liu and Qinzhong Yang contributed equally to this work.     

Genomic targeting and high-resolution mapping of the domestication gene Q in wheat.

The Q locus is largely responsible for the domestication of bread wheat. Q confers the free-threshing character of the spike and influences other important agronomic traits. Using chromosome deletion lines, Q was placed on the physical map within a submicroscopic segment of the long arm of chromosome 5A. We targeted markers to the segment by comparative mapping of anonymous RFLP clones, AFLP, and mRNA differential display analysis of deletion lines 5AL-7 and -23, which have deletion breakpoints that flank the Q locus. Differentially expressed sequences detected fragments at various loci on group 5 chromosomes suggesting that Q may be a regulatory gene. We identified 18 markers within the Q gene deletion interval and used them to construct a genetic linkage map of the region in F2 populations derived from chromosome 5A disomic substitution lines. The genetic map corresponding to the deletion segment was 20-cM long, and we identified markers as close as 0.7 cM to the Q gene. An estimate of base pairs per centimorgan within the region is 250 kb/cM, an 18-fold increase in recombination compared with the genomic average. Genomic targeting and high-density mapping provide a basis for the map-based cloning of the Q gene.     

Molecular mapping of the wheat powdery mildew resistance gene Pm24 and marker validation for molecular breeding

Molecular markers were identified in common wheat for the Pm24 locus conferring resistance to different isolates of the powdery mildew pathogen, Erysiphe graminis DM f. sp. tritici (Em. Marchal). Bulked segregant analysis was used to identify amplified fragment length polymorphism (AFLP) markers and microsatellite markers linked to the gene Pm24 in an F₂ progeny from the cross Chinese Spring (susceptible)× Chiyacao (resistant). Two AFLP markers XACA/CTA-407 and XACA/CCG-420, and three microsatellite markers Xgwm106, Xgwm337 and Xgwm458, were mapped in coupling phase to the Pm24 locus. The AFLP marker locus XACA/CTA-407 co-segregated with the Pm24 gene, and XACA/CCG-420 mapped 4.5 cM from this gene. Another AFLP marker locus XAAT/CCA-346 co- segregated in repulsion phase with the Pm24 locus. Pm24 was mapped close to the centromere on the short arm of chromosome 1D, contrary to the previously reported location on chromosome 6D. Pm24 segregated independently of gene Pm22, also located on chromosome 1D. An allele of microsatellite locus Xgwm337 located 2.4±1.2 cM from Pm24 was shown to be diagnostic and therefore potentially useful for pyramiding two or more genes for powdery mildew resistance in a single genotype. Received: 25 August 1999 / Accepted: 16 December 1999     

AFLP and PCR-based markers linked to Rf3, a fertility restorer gene for S cytoplasmic male sterility in maize

The Rf3 gene restores the pollen fertility disturbed by S male sterile cytoplasm. In order to develop molecular markers tightly linked to Rf3, we used amplified fragment length polymorphism (AFLP) technique with near isogenic lines (NILs) and bulk segregant analysis (BSA). A BC₁F₁ population from a pair of NILs with different Rf3 locus was constructed and 528 primer combinations was screened. A linkage map was constructed around the Rf3 locus, which was mapped on the distal region of chromosome 2 long arm with the help of SSR marker UMC2184. The closest marker E7P6 was 0.9 cM away from Rf3. Marker E3P1, 2.4 cM from Rf3, and E12M7, 1.8 cM from Rf3, were converted into a codominant CAPS and a dominant SCAR marker, and designated as CAPSE3P1 and SCARE12M7, respectively. These markers are useful for marker-assisted selection and map-based cloning of the Rf3 gene.     

Genetic characterization of the polycotyledon locus in tomato

Developmental mutants serve as a useful material to unravel the mechanisms necessary for organ development. The polycotyledon (poc) mutant of tomato, with multiple cotyledons in the seedling and varied phenotypic effects in the adult plant is one such mutant. Studies using physiological and anatomical methods in our lab suggest that POC is involved in the negative regulation of polar auxin transport, which is likely the reason for the pleiotropic phenotype in the mutant. Because of the physiological significance of the polycotyledon mutant described in this paper and also being first of its kind in tomato and also other plant species, we are using a map-based cloning approach to map the polycotyledon gene. Molecular mapping of this locus using segregating interspecific F₂ mapping population localized polycotyledon gene close to TG424 marker on the long arm of chromosome 9. The closest marker mapped was a PCR marker identified in this study, E8A2 at a distance of 7.4 cM from the poc locus. The absence of tightly linked RAPD markers and the non-availability of more mapped markers in this region led us to initiate chromosome walk to polycotyledon gene. Both the flanking markers TG248 and E8A2 were used to screen the BAC library and a contig was developed for TG248 marker. The BAC-end sequences were analyzed for their use as RFLP markers to enrich this region for markers. Analysis of the BAC-end sequences revealed that poc is localized in the region surrounded by copia-like retrotransposon elements explaining the absence of markers in the euchromatin region on long arm of chromosome 9. Further studies identified two BAC-end sequences which mapped around the poc locus and also indicated very low physical versus genetic distance ratio in this region. The double mutant analyses of poc with the other two known polycotyledon mutants of tomato, pct and dem revealed allelism with pct; therefore, the poc mutant was named as pct1-2, and also the original pct mutant was renamed as pct1-1.     

Locating the petunia Rf gene on a 650-kb DNA fragment

 A bulked segregant analysis was conducted in order to find RAPD and AFLP markers linked to the restorer of fertility (Rf ) gene in petunia. One RAPD marker, OP704, and one AFLP marker, ECCA/ MACT, were found to be closely linked to Rf (<1 cM) in our mapping population produced from an intraspecific Petunia hybrida cross. These two single-copy markers bracketing Rf were then mapped as RFLPs on the tomato map. Despite some rearrangement between the petunia and the tomato genomes, this synteny survey revealed two tomato markers, TG250 and CT24, closely linked to Rf. Physical mapping indicates that CT24, OP704 and ECCA/MACT lie on the same 650-kb MluI fragment. A physical to genetic distance ratio of 400 kb/cM around the Rf gene should make it feasible to identify markers physically very close to Rf. Received: 20 August 1997 / Accepted: 21 October 1997     

Construction of an 800-kb contig in the near-centromeric region of the rice blast resistance gene Pi-ta? 2 using a highly representative rice BAC library

We constructed a rice Bacterial Artificial Chromosome (BAC) library from green leaf protoplasts of the cultivar Shimokita harboring the rice blast resistance gene Pi-ta. The average insert size of 155 kb and the library size of seven genome equivalents make it one of the most comprehensive BAC libraries available, and larger than many plant YAC libraries. The library clones were plated on seven high density membranes of microplate size, enabling efficient colony identification in colony hybridization experiments. Seven percent of clones carried chloroplast DNA. By probing with markers close to the blast resistance genes Pi-ta ² (closely linked to Pi-ta) and Pi-b, respectively located in the centromeric region of chromosome 12 and near the telomeric end of chromosome 2, on average 2.2 ± 1.3 and 8.0 ± 2.6 BAC clones/marker were isolated. Differences in chromosomal structures may contribute to this wide variation in yield. A contig of about 800 kb, consisting of 19 clones, was constructed in the Pi-ta ² region. This region had a high frequency of repetitive sequences. To circumvent this difficulty, we devised a “two-step walking” method. The contig spanned a 300 kb region between markers located at 0 cM and 0.3 cM from Pi-ta ² . The ratio of physical to genetic distances (> 1,000 kb/cM) was more than three times larger than the average of rice (300 kb/cM). The low recombination rate and high frequency of repetitive sequences may also be related to the near centromeric character of this region. Fluorescent in situ hybridization (FISH) with a BAC clone from the Pi-b region yielded very clear signals on the long arm of chromosome 2, while a clone from the Pi-ta ² region showed various cross-hybridizing signals near the centromeric regions of all chromosomes. Received: 14 August 1996 / Accepted: 2 December 1996     

A map of barley chromosome 2 using isozymic and morphological markers

The F₂ progeny of a cross between a chromosome 2 multiple marker stock and an adapted cultivar of barley were analyzed for four morphological markers and electrophoretic patterns of eight leaf isozymes. TheIdh-2 locus was linked to thePer-5 locus (27.96±5.07 cM) and to thee locus (10.26±3.13 cM). Also, thePer-5 ande loci were located on the short arm of chromosome 2. In additionIdh-2 was also located on barley chromosome 2 and was linked to thev locus (13.18±3.56 cM), which is located on the long arm of chromosome 2. Two other marker genes,li andwst,,B, were linked (26.50±5.24 cM) on chromosome 2 but segregate independently of the other loci evaluated. This project was supported by funds from the U.S.-Spain Joint Committee for Scientific and Technological Cooperation.