A high-resolution map of the vicinity of the R1 locus on chromosome V of potato based on RFLP and AFLP markers

The R1 allele confers on potato a race-specific resistance to Phytophthora infestans. The corresponding genetic locus maps on chromosome V in a region in which several other resistance genes are also located. As part of a strategy for cloning R1, a high-resolution genetic map was constructed for the segment of chromosome V that is bordered by the RFLP loci GP21 and GP179 and includes the R1 locus. Bulked segregant analysis and markers based on amplified fragment length polymorphisms (AFLP markers) were used to select molecular markers closely linked to R1. Twenty-nine of approximately 3200 informative AFLP loci displayed linkage to the R1 locus. Based on the genotypic analysis of 461 gametes, eight loci mapped within the GP21–GP179 interval. Two of those could not be seperated from R1 by recombination. For genotyping large numbers of plants with respect to the flanking markers GP21 and GP179 PCR based assays were also developed which allowed marker-assisted selection of plants with genotypes Rr and rr and of recombinant plants.     

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).     

Using SNP markers to dissect linkage disequilibrium at a major quantitative trait locus for resistance to the potato cyst nematode Globodera pallida on potato chromosome V

The damage caused by the parasitic root cyst nematode Globodera pallida is a major yield-limiting factor in potato cultivation . Breeding for resistance is facilitated by the PCR-based marker ‘HC’, which is diagnostic for an allele conferring high resistance against G. pallida pathotype Pa2/3 that has been introgressed from the wild potato species Solanum vernei into the Solanum tuberosum tetraploid breeding pool. The major quantitative trait locus (QTL) controlling this nematode resistance maps on potato chromosome V in a hot spot for resistance to various pathogens including nematodes and the oomycete Phytophthora infestans. An unstructured sample of 79 tetraploid, highly heterozygous varieties and breeding clones was selected based on presence (41 genotypes) or absence (38 genotypes) of the HC marker. Testing the clones for resistance to G. pallida confirmed the diagnostic power of the HC marker. The 79 individuals were genotyped for 100 single nucleotide polymorphisms (SNPs) at 10 loci distributed over 38 cM on chromosome V. Forty-five SNPs at six loci spanning 2 cM in the interval between markers GP21-GP179 were associated with resistance to G. pallida. Based on linkage disequilibrium (LD) between SNP markers, six LD groups comprising between 2 and 18 SNPs were identified. The LD groups indicated the existence of multiple alleles at a single resistance locus or at several, physically linked resistance loci. LD group C comprising 18 SNPs corresponded to the ‘HC’ marker. LD group E included 16 SNPs and showed an association peak, which positioned one nematode resistance locus physically close to the R1 gene family. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

A QTL for broad-spectrum resistance to cyst nematode species (Globodera spp.) maps to a resistance gene cluster in potato

 Broad-spectrum resistance in potato to the potato cyst nematode (PCN) species Globodera rostochiensis and G. pallida is commonly regarded as a polygenically inherited trait. Yet, by use of QTL analysis and a selected set of PCN populations, resistance to both PCN species could be ascribed to the action of locus Grp1. Grp1 confers major resistance to G. rostochiensis line Ro₅-22 and G. pallida population Pa₂-D383 and partial resistance to G. pallida population Pa₃-Rookmaker. Grp1 was mapped on chromosome 5 using previously characterized AFLP markers. Cleaved amplified polymorphic sequence (CAPS) markers available for RFLP loci GP21 and GP179 revealed that Grp1 maps on a genomic region harboring other resistance factors to viral, fungal and nematodal pathogens. The present data indicate that Grp1 is a compound locus which contains multiple genes involved in PCN resistance. Received: 10 September 1997 / Accepted: 6 October 1997     

The R1 gene conferring race-specific resistance to Phytophthora infestans in potato is located on potato chromosome V.

Summary Late blight in potato is caused by the fungusPhytophthora infestans and can inflict severe damage on the potato crop. Resistance toP. infestans is either based on major dominantR genes conferring vertical, race-specific resistance or on minor genes inducing horizontal, unspecific resistance. A dihaploid potato line was identified which carried theR1 gene, conferring vertical resistance to allP. infestans races, with the exception of those homozygous for the recessive virulence allele of the locusV1. The F₁ progeny of a cross between this resistant parent P(R1) and P(r), a line susceptible to all races, was analysed for segregation ofR1 and of restriction fragment length polymorphism (RFLP) markers distributed on the potato RFLP map comprising more than 300 loci. TheR1 locus was mapped to chromosome V in the interval between RFLP markers GP21 and GP179. The map position ofR1 was found to be very similar to the one ofRx2, a dominant locus inducing extreme resistance to potato virus X.     

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).     

Mapping of the cyst nematode resistance locus Gpa2 in potato using a strategy based on comigrating AFLP markers

 The nematode resistance locus Gpa2 was mapped on chromosome 12 of potato using information on the genomic positions of 733 known AFLP markers. The minimum number of AFLP primer combinations required to map Gpa2 was three. This demonstrates that a reference collection of potato AFLP markers may be a valuable tool for mapping studies in potato. By use of RFLP probes, Gpa2 was more precisely mapped at the distal end of chromosome 12. Gpa2 confers resistance to a distinct group of populations of the potato cyst nematode Globodera pallida and originates from the same potato accession as locus H1, conferring resistance to pathotype Ro₁ of G. rostochiensis. This study shows that these two nematode resistance loci are unlinked and that Gpa2 is linked to the Rx1 locus conferring resistance to potato virus X. The efficiency of AFLPs for genetic mapping of a highly heterozygous crop like potato is discussed and compared with the RFLP technique. Received: 24 February 1997/Accepted: 2 May 1997     

A high-resolution map of the <Emphasis Type="Italic">Grp1</Emphasis> locus on chromosome V of potato harbouring broad-spectrum resistance to the cyst nematode species <Emphasis Type="Italic">Globodera pallida</Emphasis> and <Emphasis Type="Italic">Globodera rostochiensis</Emphasis>

The Grp1 locus confers broad-spectrum resistance to the potato cyst nematode species Globodera pallida and Globodera rostochiensis and is located in the GP21-GP179 interval on the short arm of chromosome V of potato. A high-resolution map has been developed using the diploid mapping population RHAM026, comprising 1,536 genotypes. The flanking markers GP21 and GP179 have been used to screen the 1,536 genotypes for recombination events. Interval mapping of the resistances to G. pallida Pa2 and G. rostochiensis Ro5 resulted in two nearly identical LOD graphs with the highest LOD score just north of marker TG432. Detailed analysis of the 44 recombinant genotypes showed that G. pallida and G. rostochiensis resistance could not be separated and map to the same location between marker SPUD838 and TG432. It is suggested that the quantitative resistance to both nematode species at the Grp1 locus is mediated by one or more tightly linked R genes that might belong to the NBS-LRR class. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. A. Finkers-Tomczak and S. Danan contributed equally to this research.     

Exploitation of a marker dense linkage map of potato for positional cloning of a wart disease resistance gene

A marker-saturated linkage map of potato was used to genetically map a locus involved in the resistance against wart disease Synchytrium endobioticum race 1. The locus mapped on the long arm of chromosome 4 and is named Sen1-4 in contrast to a Sen1 locus on chromosome 11. The AFLP markers from the Sen1-4 interval enabled the isolation of BAC clones from an 11 genome equivalent BAC library. This was achieved via fingerprinting of BAC pools with the AFLP primer pairs that resemble the genetic marker loci. With non-selective AFLP primers, fingerprints of individual BAC clones were generated to analyse the overlap between BAC clones using FPC. This resulted in a complete contig and a minimal tiling path of 14 BAC clones enclosing the Sen1-4 locus. The BAC contig has a genetic length of ~6 cM and a physical length of ~1 Mb. Our results demonstrate that map-based cloning of Sen1-4 can be pursued on the basis of a strategy of marker saturation alone. Genetic resolution achieved by screening large numbers of offspring for recombination events may not be required. Together with the construction of the BAC contig, a physical map with the position of the markers is accomplished in one step. This provides proof of concept for the utility of the marker saturation that is offered by the ultra dense AFLP map of potato for gene cloning.     

Two additive QTLs conferring broad-spectrum resistance in potato to Globodera pallida are localized on resistance gene clusters

Broad-spectrum resistance in potato to the potato cyst nematode (PCN) is commonly regarded as a complex inherited trait. Yet, in this paper we show that, by use of a selected set of PCN test populations, broad-spectrum resistance to the species Globodera pallida can be fully ascribed to the action of two loci: Gpa5 and Gpa6. These loci were readily mapped by means of a strategy based on two steps. Firstly, the chromosomal localization of both loci was assessed by use of an online catalogue of AFLP markers covering a substantial part of the potato genome (http://www.spg.wau.nl/pv/aflp/catalog.htm). Subsequently the chromosomal regions of both loci were identified by means of CAPS markers based on RFLP insert sequences. Locus Gpa5 explains at least 61% of the genetic variation. This locus maps to chromosome 5 on a region which has previously been shown to harbor resistance factors to viral (Nb, Rx2), fungal (R1) and nematodal (Gpa, Grp1) pathogens. The Gpa6 locus exhibits a minor effect on the resistance (24%) and acts additively to Gpa5. Interestingly, the Gpa6 locus maps to a region on chromosome 9 where, in the homoeologous tomato genome, the virus resistance gene Sw-5 resides as part of a resistance gene cluster. In potato, resistance to potato virus X has been reported in the vicinity of this region. The map location of Gpa6 indicates the presence of a resistance gene cluster at the end of the long arm of chromosome 9 of potato. Received: 10 January 2000 / Accepted: 31 January 2000     

Segregation analysis and RFLP mapping of the R1 and R3 alleles conferring race-specific resistance to Phytophthora infestans in progeny of dihaploid potato parents

Phytophthora infestans (Mont.) de Bary is the most important fungal pathogen of the potato (Solanum tuberosum). The introduction of major genes for resistance from the wild species S. demissum into potato cultivars is the earliest example of breeding for resistance using wild germplasm in this crop. Eleven resistance alleles (R genes) are known, differing in the recognition of corresponding avirulence alleles of the fungus. The number of R loci, their positions on the genetic map and the allelic relationships between different R variants are not known, except that the R1 locus has been mapped to potato chromosome V The objective of this work was the further genetic analysis of different R alleles in potato. Tetraploid potato cultivars carrying R alleles were reduced to the diploid level by inducing haploid parthenogenetic development of 2n female gametes. Of the 157 isolated primary dihaploids, 7 set seeds and carried the resistance alleles R1, R3 and R10 either individually or in combinations. Independent segregation of the dominant R1 and R3 alleles was demonstrated in two F₁ populations of crosses among a dihaploid clone carrying R1 plus R3 and susceptible pollinators. Distorted segregation in favour of susceptibility was found for the R3 allele in 15 of 18 F₁ populations analysed, whereas the RI allele segregated with a 1:1 ratio as expected in five F₁ populations. The mode of inheritance of the R10 allele could not be deduced as only very few F₁ hybrids bearing R10 were obtained. Linkage analysis in two F₁ populations between R1, R3 and RFLP markers of known position on the potato RFLP maps confirmed the position of the R1 locus on chromosome V and localized the second locus, R3, to a distal position on chromdsome XI.     

Chromosome landing at the Arabidopsis TORNADO1 locus using an AFLP-based strategy

The Arabidopsis tornado1 (trn1) mutation causes severe dwarfism combined with twisted growth of all organs. We present a chromosome landing strategy, using amplified restriction fragment length polymorphism (AFLP) marker technology, for the isolation of the TRN1 gene. The recessive trn1 mutation was identified in a C24 transgenic line and is located 5?cM from a T-DNA insertion. We mapped the TRN1 locus to the bottom half of chromosome 5 relative to visible and restriction fragment length polymorphism (RFLP) markers. Recombinant classes within a 3-cM region around TRN1 were used to build a high-resolution map in this region, using the AFLP technique. Approximately 300 primer combinations have been used to test about 26?000 fragments for polymorphisms. Seventeen of these AFLP markers were identified in the 3-cM region around TRN1. These markers were mapped within this region using individual recombinants. Four of these AFLP markers co-segregate with TRN1 whereas one maps at one recombinant below TRN1. We isolated and cloned three of these AFLP markers. These markers identified two yeast artificial chromosome (YAC) clones, containing the RFLP marker above and the AFLP marker below TRN1, demonstrating that these YACs span the TRN1 locus and that chromosome landing has been achieved, using an AFLP-based strategy.     

High-resolution genetical and physical mapping of the Rx gene for extreme resistance to potato virus X in tetraploid potato

The Rx locus in potato confers extreme resistance to PVX. In the F₁ progeny of crosses between the PVX-susceptible cultivar Huinkel and the cultivar Cara (Rx genotype) there was a 1?:?1 segregation of PVX resistance, indicating that Rx in Cara is present in the simplex condition. Using potato and tomato RFLP markers, we mapped Rx in Cara to the distal end of chromosome XII at a different position to the previously mapped Rx1 locus. To generate a high-resolution linkage map in the vicinity of Rx a total 728 AFLP primer combinations were screened using DNA of bulked resistant and susceptible segregants. We also screened segregating populations for chromosomal recombination events linked to the Rx locus and identified 82 plants with recombination events close to Rx. Using these recombinant plants we have identified AFLPs that flank Rx and span an interval of 0.23 cM in a region of the genome where 1 cM corresponds to approximately 400?kb.     

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     

Autotetraploids and genetic mapping using common AFLP markers: the R2 allele conferring resistance to Phytophthora infestans mapped on potato chromosome 4

 Due to the complexity of tetrasomic inheritance, mapping studies in potato (Solanum tuberosum L.) are generally conducted at the diploid level. In the present study we tested the feasibility of Bulked Segregant Analysis (BSA) using a tetraploid offspring for the identification of AFLP markers linked to the R2 allele, which confers race-specific resistance to Phytophthora infestans. Eleven bulk-specific AFLP markers, detected in fingerprints of 205 AFLP primer combinations, could be mapped in a linkage group encompassing the R2 locus. The efficiency of BSA at the tetraploid level, determined by the frequency of single-dose restriction fragments (SDRF), was much higher than expected on the basis of overall genetic dissimilarity between the parental clones. The fortuitous detection of AFLPs with linkage to the R2 allele is explained on the basis of specific genetic dissimilarity between cultivated potato and the chromosomal segment introgressed from S. demissum carrying the resistant R2 allele. AFLP markers common to those with linkage to R2 were visually recognized by their electrophoretic mobility in the AFLP fingerprint in a parental clone of a reference mapping population. Using these common AFLP markers we anchored the linkage group comprising the R2 allele to potato chromosome 4. Received: 30 October 1997 / Accepted: 6 November 1997     

Genetic analysis of phenylpropanoid metabolites associated with resistance against Verticillium longisporum in Brassica napus

Verticillium longisporum is a major threat to production of oilseed rape (Brassica napus) in Europe. The aim of the study was to develop new markers and obtain insights into putative mechanisms and pathways involved in the resistance reaction. A genetic approach was used to identify quantitative trait loci (QTL) for V. longisporum resistance and metabolic traits potentially influencing resistance in a B. napus mapping population. Resistance to V. longisporum was mapped in a doubled haploid (DH) population from a cross between the partially resistant winter oilseed rape variety Express 617 and a resistant resynthesized B. napus line, R53. One major resistance QTL contributed by R53 was identified on chromosome C5, while a further, minor QTL contributed by Express 617 was detected on chromosome C1. Markers flanking the QTL also significantly correlated with V. longisporum resistance in four further DH populations derived from crosses between elite oilseed rape cultivars and other resynthesized B. napus lines originating from genetically and geographically diverse brassica A and C genome donors. The tightly-linked markers developed enable the combination of favorable alleles for novel resistance loci from resynthesized B. napus materials with existing resistance loci from commercial breeding lines. HPLC analysis of hypocotyls from infected DH lines revealed that concentrations of a number of phenylpropanoids were correlated with V. longisporum resistance. QTL for some of these phenylpropanoids were also found to co-localize with the QTL for V. longisporum resistance. Genes from the phenylpropanoid pathway are suggested as candidates for V. longisporum resistance.     

Genetic mapping of <Emphasis Type="Italic">psl</Emphasis> locus and quantitative trait loci for angular leaf spot resistance in cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis> L.)

One of the most important cucumber diseases is bacterial angular leaf spot (ALS), whose increased occurrence in open-field production has been observed over the last years. To map ALS resistance genes, a recombinant inbred line (RIL) mapping population was developed from a narrow cross of cucumber line Gy14 carrying psl resistance gene and susceptible B10 line. Parental lines and RILs were tested under growth chamber conditions as well as in the field for angular leaf spot symptoms. Based on simple sequence repeat and DArTseq, genotyping a genetic map was constructed, which contained 717 loci in seven linkage groups, spanning 599.7 cM with 0.84 cM on average between markers. Monogenic inheritance of the lack of chlorotic halo around the lesions, which is typical for ALS resistance and related with the presence of recessive psl resistance gene, was confirmed. The psl locus was mapped on cucumber chromosome 5. Two major quantitative trait loci (QTL) psl5.1 and psl5.2 related to disease severity were found and located next to each other on chromosome 5; moreover, psl5.1 was co-located with psl locus. Identified QTL were validated in the field experiment. Constructed genetic map and markers linked to ALS resistance loci are novel resources that can contribute to cucumber breeding programs.     

A molecular genetic map of the long arm of chromosome 6R of rye incorporating the cereal cyst nematode resistance gene, CreR

 A genetic map of the long arm of chromosome 6R of rye was constructed using eight homoeologous group-6 RFLP clones and five PCR markers derived from the rye-specific dispersed repetitive DNA family, R173. The map was developed using a novel test-cross F₁ (TC-F₁) population segregating for resistance to the cereal cyst nematode. Comparisons were made between the map generated with other rye and wheat group-6 chromosome maps by the inclusion of RFLP clones previously mapped in those species. Co-linearity was observed for common loci. This comparison confirmed a dramatic reduction in recombination for chromosome 6R in the TC-F₁ population. The CreR locus was included in the linkage map via progeny testing of informative TC-F₁ individuals. CreR mapped 3.7 cM distal from the RFLP locus, XksuF37. Comparative mapping should allow the identification of additional RFLP markers more closely linked to the CreR locus. Received: 14 April 1998 / Accepted: 29 April 1998     

Comparative next-generation mapping of the Phytophthora infestans resistance gene Rpi-dlc2 in a European accession of Solanum dulcamara

Phytophthora infestans, the causal agent of late blight, remains the main threat to potato production worldwide. Screening of 19 accessions of Solanum dulcamara with P. infestans isolate Ipo82001 in detached leaf assays revealed strong resistance in an individual belonging to accession A54750069-1. This plant was crossed with a susceptible genotype, and an F₁ population consisting of 63 individuals was obtained. This population segregated for resistance in 1:1 ratio, both in detached leaf assays and in an open-field experiment. Presence of the formerly mapped Rpi-dlc1 gene as the cause of the observed segregating resistance could be excluded. Subsequently, AFLP analyses using 128 primer combinations enabled identification of five markers linked to a novel resistance gene named Rpi-dlc2. AFLP markers did not show sequence similarity to the tomato and potato genomes, hampering comparative genetic positioning of the gene. For this reason we used next-generation mapping (NGM), an approach that exploits direct sequencing of DNA (in our case: cDNA) pools from bulked segregants to calculate the genetic distance between SNPs and the locus of interest. Plotting of these genetic distances on the tomato and potato genetic map and subsequent PCR-based marker analysis positioned the gene on chromosome 10, in a region overlapping with the Rpi-ber/ber1 and -ber2 loci from S. berthaultii. Pyramiding of Rpi-dlc2 and Rpi-dlc1 significantly increased resistance to P. infestans, compared with individuals containing only one of the genes, showing the usefulness of this strategy to enhance resistance against Phytophthora.     

Identification and physical mapping of three Haynaldia villosa chromosome-6V deletion lines

Three deletion lines (del6V?2S-1, del6V? 2L-1, and del6V?2L-2) of Haynaldia villosa chromosome 6V added to wheat were identified by C-banding and characterized by RFLP analyses. The breakpoints were located at fraction lengths (FL) 0.58 in del6V?2S-1 in the short arm, and FL 0.66 in del6V?2L-1 and FL 0.64 in del6V?2L-2 in the long arm. Thirty-one Triticeae homoeologous group-6 DNA probes were used to map RFLP loci in the deletion lines and the wheat-H. villosa disomic substitution (DS) line 6V?2(6A). Nine probes failed to detect polymorphism between Chinese Spring and DS6V?2(6A). Ten of sixteen polymorphic short-arm loci were not detected in del6V?2S-1. Thus, the loci are located in the deleted distal chromosome region. Six RFLP markers were mapped in the proximal 58% of 6VS. Of 20 DNA markers specific for 6VL, six mapped in the distal 36% of the long arm, and nine mapped in the proximal 64% of 6VL. The breakpoint of the short arm of 6V?2 occurs between Xpsr106 and Xcdo270, and that of the long arm between Xpsr915 and Xmwg934. The powdery mildew resistance gene Pm21 is located on the short arm of chromosome 6V?2. Pm21 is present in del6V?2S-1, and can be further mapped in the proximal 58% of 6V?2S.