Mapping and use of seed protein loci for marker-assisted selection of growth habit and photoperiod response in <Emphasis Type="Italic">Nuña</Emphasis> bean (<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> L.)

The individual segregations of 14 seed protein loci named, SpA to SpM and Pha (phaseolin), were analyzed in a RIL population developed from the cross Xana × Cornell 49242. These seed protein loci were included in a genetic map previously developed in the same population. Protein loci, SpA, SpB, SpE, SpI, SpJ, and Pha, are organized in two different clusters, both located in linkage group (LG) 7; SpF, SpG, SpK, SpL, and SpM, form a single cluster in LG 4; SpC, is located in LG 3; and SpD, in LG 1. A close linkage was identified between the SpD seed protein locus, and the fin gene, controlling determinate growth habit. The usefulness of the SpD locus as a marker for the indirect selection of determinate growth habit and photoperiod insensitivity was checked in a F₂ population derived from the cross G12587 (an indeterminate and photoperiod sensitive nuña bean) × Sanilac (determinate and photoperiod insensitive) and in a set of Mesoamerican and Andean genotypes. Results indicate that SpD protein locus was useful to detect individuals having determinate growth habit and photoperiod insensitivity in the cross G12587 × Salinac although some recombinants were found. However, the linkage between the SpD locus and the genes controlling growth habit and photoperiod sensitivity should be checked before using the SpD locus for the indirect selection of these traits in different backgrounds.     

Fine mapping the <Emphasis Type="Italic">BjPl1</Emphasis> gene for purple leaf color in B2 of <Emphasis Type="Italic">Brassica juncea</Emphasis> L. through comparative mapping and whole-genome re-sequencing

Purple plants with higher anthocyanin content have attracted increasing attention in recent years due to their advantageous biological functions and nutritional value. A spontaneous mutant with purple leaves, designated 1280-1, was discovered in Brassica juncea line 1280. A previous genetic analysis indicated that the purple leaf trait in 1280-1 was controlled by a dominant gene (BjPl1). In the present study, an analysis of total anthocyanin content further indicated that the purple leaf trait was controlled by a complete dominance gene. According to a survey of 426 primers available from public resources, BjPl1 was assigned to linkage group B2 of B. juncea. In the early stage of this research, based on comparative mapping in Brassica, two simple sequence repeat (SSR) markers developed from A2 of B. rapa delimited the BjPl1 gene to a 0.7-cM genetic interval in the corresponding linkage map. According to information on the B. juncea genome released recently, the location of BjPl1 was further narrowed to a 225-kb interval (17.74–17.97 Mb). Within the target region, whole-genome re-sequencing identified two candidate regions (17.74–17.78 Mb and 17.93–17.96 Mb). Through Blast analysis of the two candidate intervals, four homologous anthocyanin biosynthetic genes were identified and localized to a 17.93–17.96 Mb interval of B2 (approximately 27 kb), which might include BjPl1. This work lays the foundation for the isolation of BjPl1 and will further improve our understanding of the molecular mechanisms of the anthocyanin metabolic pathway in Brassica.     

Induced expression of selected plant defence related genes in pot azalea, <Emphasis Type="Italic">Rhododendron simsii</Emphasis> hybrid

A set of putative marker genes to study plant defense responses against Polyphagotarsonemus latus, a key pest in the production of Rhododendron simsii hybrids, was selected and validated. Genes belonged to the biosynthetic pathway of phytohormones jasmonic acid (JA) (RsLOX, RsAOS, RsAOC, RsOPR3 and RsJMT) and salicylic acid (SA) (RsPAL and RsICS). Furthermore, RsPPO, a putative marker gene for oxidative stress response was successfully cloned from R. simsii. A CTAB-based extraction protocol was optimized to assure excellent RNA quality for subsequent RT-qPCR analysis. The RT-qPCR protocol was extensively tested and RsRG7 and RsRG14 were selected as reference genes from a geNorm pilot study. Validation of the marker genes was done after application with elicitors [methyl jasmonate (MeJA), coronatine, β-aminobutyric acid and acibenzolar-Smethyl] or wounding. Both 100 μM MeJA and 0.1 μM coronatine had a significant effect on the expression of all marker genes. Foliar application of MeJA on the shoots resulted in a significantly earlier response when compared to root application and subsequent sampling of the shoots. Expression patterns after MeJA treatment were generally the same in six R. simsii genotypes: ‘Nordlicht’, ‘Elien’, ‘Aiko Pink’, ‘Michelle Marie’, ‘Mevrouw Gerard Kint’ and ‘Sachsenstern’. Wounding resulted in the same expression patterns as MeJA treatment except for RsJMT. None of the genotypes showed a significant induction of the latter gene 6 h upon wounding. Findings of these experiments indicated that the tolerant genotype ‘Elien’ has low basal expression levels of RsPPO. This might be the first step towards the breeding of mite-tolerant genotypes.     

Linkage between genes for hairy first leaf and chlorophyll deficiency in Brassica oleracea

The fourth linkage group of B. oleracea L. has two genes: Hr-1, (hairy first leaf), a dominant seedling marker from “Dwarf Green” curly kale, and pg-2, (pale green seedling), a recessive chlorophyll mutant from green sprouting broccoli. Recombination between Hr-1 and pg-2 ranged from 7.4 to 20.1% in the six progenies studied, with a mean of 13.15±0.68%. Hr-1 segregated independently of the three other linkage groups (two genes of each were tested) and of two unlocated genes for male sterility.     

Identification of the <Emphasis Type="Italic">S</Emphasis> locus core sequences determining self-incompatibility and <Emphasis Type="Italic">S</Emphasis> multigene family from draft genome sequences of radish (<Emphasis Type="Italic">Raphanus sativus</Emphasis> L.)

The S core and its flanking sequences were identified from two independent draft genome sequences of radish (Raphanus sativus L.). After gap-filling with PCR, the S core regions and full-length S receptor kinase (SRK) genes from two radish genomes were obtained. Phylogenetic analysis of the SRK genes clearly showed that one S core region belonged to the class I S haplotypes, but the other was included in the class II S haplotypes. Three sequences showing homology with known transposable elements were identified in the core regions, and one intact copia-type long terminal repeat (LTR)-retrotransposon containing a 4125-bp open reading frame (ORF) was identified in the class I S haplotype. A total of 61 genes showing homology with the SRK genes were identified from two draft genome sequences. Among them, the RsKD1 showed the highest homology with the SRK genes. There was 90% nucleotide sequence identity between the RsKD1 and RsSRK1 genes in the kinase domains. The phylogenetic tree of SRK genes and 13 most closely related homologs showed that all homologs were more closely related to the class II SRK genes than to the class I SRKs. Physical mapping of radish SRK-homologous genes and their B. rapa orthologs showed that two radish homologs and their B. rapa orthologs were tightly linked to the SRK genes in radish and B. rapa genomes. Sequence information about multiple SRK-homologs identified in this study would be helpful for designing reliable primer pairs for faithful PCR amplification of the SRK alleles, leading to improvement of the S haplotyping system in radish breeding programs.     

Mapping QTLs for plant height and flowering time in a Chinese summer planting soybean RIL population

Soybean is a primary source of plant oil and protein and has a high nutritional value. Plant height (PH) and flowering time (FT) are two important agronomic traits in breeding programs for soybean. In this study, we mapped QTLs associated with PH and FT in three environments using a population with determinate growth including 236 recombinant inbred lines (NJZY-RIL) derived from a cross between two summer planting varieties, ZXD and NN1138-2. A high-density genetic map with 3255 SLAF-markers was constructed that spanned 2144.85 cM of the soybean genome with an average marker distance of 0.66 cM. Altogether, six QTLs controlling PH and eleven QTLs controlling FT were mapped using mixed-model-based composite interval mapping and composite interval mapping methods. qPH-1-1 and qFT-15-2 were two novel main effect QTLs identified in this study; qFT-6-2, qFT-15-2, qFT-16-1, qPH-1-1, qPH-15-1 and qPH-16-1 were consistently detected across environments and by the two mapping methods. Two pairs of QTLs, qFT-15-2 and qPH-15-1 as well as qFT-16-1 and qPH-16-1, which were located in the same marker interval on chromosomes 15 and 16, respectively, were found to have close linkage or pleiotropy. These results may increase our understanding of the genetic control of PH and FT in soybean and provide support for implementing marker-assisted selection in developing soybean cultivars with high yield and early maturity in summer planting regions.     

<Emphasis Type="Italic">Er3</Emphasis> gene,conferring resistance to powdery mildew in pea,is located in pea LGIV

Three genes for resistance to Erysiphe pisi, named er1, er2 and Er3 have been described in pea so far. er1 gene is located in pea linkage group VI, while er2 gene has been mapped in LGIII. SCAR and RAPD markers tightly linked to Er3 gene have been identified, but the position of these markers in the pea genetic map was unknown. The objective of this study was to localize Er3 gene in the pea genetic map. Towards this aim, the susceptible pea cv. Messire (er3er3) and a resistant near isogenic line of Messire (cv. Eritreo, Er3Er3) were surveyed with SSRs with known position in the pea map. Three SSRs were polymorphic between “Messire” and “Eritreo” and further surveyed in two contrasting bulks formed by homozygous Er3Er3/er3er3 individuals obtained from a F₂ population derived from the cross C2 (Er3Er3)?×?Messire (er3er3). A single marker, AA349, was polymorphic between the bulks. Subsequently, other ten markers located in the surrounding of AA349 were selected and analysed in Er3Er3 and er3er3 plants. As a results, another SSR, AD61, was found to be polymorphic between Er3Er3 and er3er3 plants. Further linkage analysis confirmed that SSRs AA349 and AD61 were linked to Er3 and to the RAPD and SCAR markers previously reported to be linked to this gene. Er3 gene was located in pea LGIV at 0.39 cM downstream of marker AD61. The location of Er3 gene in the pea map is a first step toward the identification of this gene.     

Evaluating the contribution of Yr genes to stripe rust resistance breeding through marker-assisted detection in wheat

Numerous stripe rust resistance genes have been identified from wheat, and new virulent races of Puccinia striiformis f. sp. tritici have also emerged in recent years. Deployment of diverse combinations of resistance genes is an efficient way to combat virulent evolution of strip rust pathogen. In this study, publically available molecular markers were used to identify the distribution of 36 Yr genes in 672 wheat accessions. The effectiveness of Yr genes individually and in combinations was also evaluated in field conditions. The result showed effective resistance of some recently applied genes, such as Yr15 and Yr65. It also showed the lost efficacy of some once widely used genes, such as Yr9 and Yr10. Moreover, significant additive effects were observed in some gene combinations, such as Yr9 + Yr18 and Yr30 + Yr46. Proper deploying of Yr genes and utilizing the positive interactions will be helpful for durable resistance breeding in wheat.     

Development of NBS-related microsatellite (NRM) markers in hexaploid wheat

NBS (nucleotide binding site) genes, one type of the most important disease-resistance genes in the plant kingdom, are usually found clustered in genome. In this study, a total of 2288 full-length NBS protein-coding sequences were isolated from the wheat (Triticum aestivum L.) genome, and 903 TaNBSs of which were found expressed in wheat. Meanwhile, 2203 microsatellite loci were detected within 1061 scaffolds containing TaNBS. The distribution of these microsatellite loci across wheat homologous groups (HG) is 20% HG2, 16% HG7, 15% HG1, 15% HG6, 12% HG4, 12% HG5 and 10% HG3. We developed 1830 NBS-related microsatellite (NRM) markers for the microsatellite loci on TaNBS-scaffold sequences.Among them, 342 NRM markers were developed for HG2 with the largest number of microsatellite loci, and 69 out of these markers were anchored to the wheat genetic map using mapping population. Then, a total of 26 2AS-NRM markers, nine 2BL-NRM markers and nine 2DL-NRM markers were integrated into the genetic maps carrying Yr69, Pm51 and Pm43, respectively. Finally, candidate sequences, within the gene clusters where Yr5 and Sr21 located, were analyzed according to the genomic position information of TaNBS and NRM markers. These NRM markers have clear chromosome locations and are correlated with potential disease resistance sequences, which can be manipulated to mapping or adding linkage markers of disease-resistance genes or QTLs, especially for those in the NBS gene clusters.     

Effects of introgressions from <Emphasis Type="Italic">Festuca pratensis</Emphasis> on winter hardiness of <Emphasis Type="Italic">Lolium perenne</Emphasis>

Previous studies reported that some genotypes with introgressed Festuca chromosome segment(s) in Lolium genome showed enhanced winter hardiness compared to Lolium. The aim of this study was to search comprehensively for the Festuca pratensis chromosome regions affecting winter hardiness-related traits when introgressed into the Lolium perenne genome. Association between F. pratensis introgression and winter hardiness-related traits (fall and winter hardiness indexes, early-spring dry matter yield, and freezing tolerance) were screened in the diploid introgression populations (n = 203) that had some F. pratensis chromosome segments introgressed. Eighty-four intron markers corresponding to unique rice genes randomly distributed across the genome were used for genotyping. Winter hardiness of almost all plants in the introgression populations was lower than that of the F. pratensis and triploid hybrid parents, but the average was higher than that of L. perenne. A significant positive effect of F. pratensis introgression on early-spring dry matter yield was detected on chromosome 7. This quantitative trait locus (QTL) was confirmed by linkage analysis using a backcross population with F. pratensis introgression in the target region of chromosome 7. However, the contribution of the newly identified QTL was rather small (6.7–9.6%), suggesting that superior winter hardiness of F. pratensis compared to L. perenne is conferred by multiple small-effect QTLs. We also detected a previously unreported negative effect of Festuca introgression on winter hardiness. Newly obtained QTL information in this study would contribute to the design of Festuca/Lolium hybrid breeding.     

High-density genetic map construction and QTL mapping for fiber strength on Chr24 across multiple environments in a CCRI70 recombinant inbred lines population

Upland cotton is an important economic crop that produces high-quality fiber for the textile industry. With the development of next-generation sequencing technology and improvements in human living standards, it has become possible to improve the fiber quality and yield of cotton with high-throughput molecular markers. Upland cotton 901-001 is an excellent, high-quality, non-transgenic cultivar, while the sGK156 strain shows high resistance to verticillium wilt. The phenotype of F₁ plants, certified in 2008 as national variety CCRI70, shows positive transgressive characteristics such as high quality, high yield, and resistance to verticillium wilt. We developed a population of 250 recombination inbred lines from a cross between 901-001 and sGK156. The fiber strength trait of plants from nine environments was collected, and a genetic linkage map of Chr24 comprising 168 SNP marker loci covering a genetic distance of 107.46 cM and with an average distance of 0.64 cM was generated. QTLs were identified across the nine environments using the composite interval mapping method. A total of eight QTLs for FS were identified on Chr24, three of which were stably expressed in at least five environments. Some candidate genes located in qFS-c24-2 and qFS-c24-4 were functionally annotated as potentially playing important roles in fiber development, with homologous genes reported in Arabidopsis thaliana. These results suggest that QTLs identified in the present study could contribute to improving FS and may be applicable for marker-assisted selection.     

Molecular markers for improving control of soil-borne pathogen <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> in sugar beet

Fusarium spp. cause severe damage in many agricultural crops, including sugar beet, with Fusarium oxysporum historically being considered as the most damaging of all species. Sugar beet needs to be protected from this class of soil-borne pathogens in order to ensure an optimal sugar yield in the field. Genetic control of the disease is crucial in managing these pathogens. Identification of single nucleotide polymorphism (SNP) markers linked to resistance can be a powerful tool for the introgression of valuable genes needed to develop Fusarium-resistant varieties. A candidate gene approach was carried out to identify SNP markers linked to putative Fusarium resistance sources in sugar beet. Five resistant analogue genes (RGAs) were screened by means of high resolution melting (HRM) analysis in a set of sugar beet lines, considered as resistant and susceptible to Fusarium oxysporum. HRM polymorphisms were observed in 80% of amplicons. Two HRM polymorphisms were significantly associated with Fusarium resistance (P < 0.05). The amplicons that showed association were sequenced and two SNPs were identified. The association was further validated on 96 susceptible and 96 resistant plants using competitive allele-specific PCR (KASPar) technology. The selected SNPs could be used for marker-assisted breeding of Fusarium resistance in sugar beet.     

Identification and fine mapping of molecular markers closely linked to fruit spines size <Emphasis Type="Italic">ss</Emphasis> gene in cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis> L.)

Fruit spine size is one of the importantly external quality traits effected the economic value of cucumber fruit. Morphological–cytological observation of the fruit spine size phenotype indicated that large spine formation arises from an increasing of spiny pedestal cell number caused by cell division, and best periods to accurately score fruit spine size trait was 4th day before flowering to 7th day after flowering according the continuous observation. Genetic analysis showed that a single dominant gene determined the fruit spine size trait in cucumber. BC₁ population (189 individuals) of two inbred lines (large spine PI197088 and small spine SA0422) was used for primary mapping of the SS/ss locus with 7 markers covering an interval of 37.1 cM. An F₂ segregating population of 1032 individuals constructed from the same two parents (PI197088 and SA0422) was used to fine mapping of the SS/ss locus. Six new markers linked to the gene were successfully screened for construction of a fine linkage map, in which the SS/ss locus was located in the region flanked by marker SE1 (3 recombinants) and SSR43 (2 recombinants) with a 189 kb physical distance. Markers from this study will be valuable for candidate gene cloning and marker-assisted selection for cucumber breeding.     

Identification of drought responsive QTLs during vegetative growth stage of rice using a saturated GBS-based SNP linkage map

Drought is a major abiotic constraint for rice production worldwide. The quantitative trait loci (QTLs) for drought tolerance traits identified in earlier studies have large confidence intervals due to low density linkage maps. Further, these studies largely focused on the above ground traits. Therefore, this study aims to identify QTLs for root and shoot traits at the vegetative growth stage using a genotyping by sequencing (GBS) based saturated SNP linkage map. A recombinant inbred line (RIL) population from a cross between Cocodrie and N-22 was evaluated for eight morphological traits under drought stress. Drought was imposed to plants grown in 75 cm long plastic pots at the vegetative growth stage. Using a saturated SNP linkage map, 14 additive QTLs were identified for root length, shoot length, fresh root mass, fresh shoot mass, number of tillers, dry root mass, dry shoot mass, and root-shoot ratio. Majority of the drought responsive QTLs were located on chromosome 1. The expression of QTLs varied under stress and irrigated condition. Shoot length QTLs qSL1.38 and qSL1.11 were congruent to dry shoot mass QTL qDSM1.38 and dry root mass QTL qDRM1.11, respectively. Analysis of genes present within QTL confidence intervals revealed many potential candidate genes such as laccase, Calvin cycle protein, serine threonine protein kinase, heat shock protein, and WRKY protein. Another important gene, Brevis radix, present in the root length QTL region, was known to modulate root growth through cell proliferation and elongation. The candidate genes and the QTL information will be helpful for marker-assisted pyramiding to improve drought tolerance in rice.     

Genotyping-by-sequencing of waxy and glossy near-isogenic broccoli lines

Wild-type Brassica oleracea L. have matte blue-green leaves caused by an interaction between leaf pigmentation and a waxy bloom coating the surface. Glossy mutants have reduced and/or altered epicuticular wax giving the leaves a shiny green appearance and have been identified in most B. oleracea crop varieties, including cauliflower, kale, broccoli, Brussels sprouts, cabbage, and collard. The genetic basis of glossy mutants has not been studied in B. oleracea. Glossiness can confer resistance to multiple herbivores mediated by modification of herbivore feeding behavior and foraging efficiency of their predators. The USDA-ARS-U.S. Vegetable Laboratory released two pairs of near-isogenic broccoli lines (NILs) that visibly differ only for glossiness, providing ideal germplasm for the genetic study of epicuticular wax in B. oleracea. Genotyping-by-sequencing resulted in hundreds of polymorphisms between each pair of NILs. Polymorphisms were identified in or near three different wax synthesis genes suggesting the two glossy mutants were caused by alteration of different steps of the pathway.     

Map-based cloning of the fertility restoration locus <Emphasis Type="Italic">Rfm1</Emphasis> in cultivated barley (<Emphasis Type="Italic">Hordeum vulgare</Emphasis>)

Hybridization technology has proven valuable in enhancing yields in many crops, but was only recently adopted in the small grain cereals. Hybrid varieties in barley (Hordeum vulgare) rely on the cytoplasmic male sterility (CMS) system msm1 derived from Hordeum vulgare ssp. spontaneum. The major restorer gene described for the msm1 system is known as Rfm1 and maps to the top of chromosome 6H. To gain further insight into mechanisms underlying male fertility restoration in barley, we used a map-based cloning approach to identify the nuclear gene involved in the restoration mechanism of this hybridization system. Taking advantage of the available genomic resources in barley in combination with a custom-made non-gridded BAC library developed from a restorer line, we cloned and sequenced the Rfm1 restorer locus. The characterization and annotation of the nucleotide sequence for the Rfm1 restorer allele allowed for the identification of the candidate gene for Rfm1. The Rfm1 locus carries a tandem repeat of a gene encoding a pentatricopeptide repeat (PPR) protein. Surprisingly, Rfm1 belongs to the PLS-DYW subfamily of PPR genes known for their involvement in RNA editing in plants organelles, but that to date have not been identified as restorer genes.     

A new gene <Emphasis Type="Italic">Bph33</Emphasis>(<Emphasis Type="Italic">t</Emphasis>) conferring resistance to brown planthopper (BPH), <Emphasis Type="Italic">Nilaparvata lugens</Emphasis> (Stål) in rice line RP2068-18-3-5

The rice brown planthopper (BPH) Nilaparvata lugens (Stål) is one of the major pests of rice across Asia. Host-plant resistance is the most ecologically acceptable means to manage this pest. A rice breeding line RP2068-18-3-5 (RP2068) derived from the land race Velluthacheera is reported to be resistant to BPH populations across India. We identified a new R gene [Bph33(t)] in this line using advanced generation RILs derived from TN1 × RP2068 cross through phenotyping at two locations and linkage analysis with 99 polymorphic SSR markers. QTL analysis through IciMapping identified at least two major QTL on chromosome 1 influencing seedling damage score in seed box screening, honey dew excretion by adults and nymphal survival. Since no BPH R gene has been reported on chromosome 1, we designate this locus as a new gene Bph33(t) which accounted for over 20% of phenotypic variance. Scanning the region for candidate gene suggested two likely candidates a leucine rich repeat (LRR) gene and a heat shock protein (HSP) coding gene. Expression profiling of the two genes in the two contrasting parents and RILs showed induction of the HSP gene (LOC_Os01g42190.1) at 6 h after infestation while LRR gene did not show such induction. It is likely that the HSP represented Bph33(t).     

Hybrids of <Emphasis Type="Italic">Passiflora</Emphasis>: <Emphasis Type="Italic">P. gardneri</Emphasis> versus <Emphasis Type="Italic">P. gibertii,</Emphasis> confirmation of paternity,morphological and cytogenetic characterization

Two new varieties of interspecific hybrids of Passiflora have been developed from the cross between P. gardneri versus P. gibertii, both registered under the Passiflora Society International. Twelve putative hybrids were analyzed. Hybridization was confirmed using RAPD and SSR markers. Primer UBC11 (5′-CCGGCCTTAC-3′) generated informative bands. Primer SSR Pe75 has amplified species-specific fragments and a heterozygote status was observed with two parent bands 300 and 350 bp. The molecular markers generated have been analyzed for the presence or absence of specific informative bands. Based on the morphological characterization, we have identified two hybrid varieties: P. ‘Gabriela’ and P. ‘Bella’. P. ‘Gabriela’ produced flowers in bluish tones, bluish petals on the adaxial and abaxial faces, light blue sepals on the adaxial and light green on the abaxial faces, corona with the base of filaments in intense lilac color and white apex. P. ‘Bella’ produced flowers in lilac tones, intense lilac petals on the adaxial and abaxial faces, dark lilac sepals with whitish edges on the adaxial and light green on the abaxial faces, corona with the base of filaments in intense lilac color and white apex. The cytogenetic analysis verified that the hybrids have the same chromosomal number as the parents (2n = 18); the formation of bivalents between the homeologous chromosomes (n = 9) was observad, leading to regular meiosis, which allows the sexual reproduction and use of these hybrids in breeding programs.     

Genetic mapping of the <Emphasis Type="Italic">qGCR6</Emphasis> locus affecting glossiness of cooked rice

A japonica variety, Koshihikari, is known to have favorable eating quality. Two rice backcross inbred lines (BILs) developed from Koshihikari exhibited significantly different glossiness of cooked rice (GCR), an eating quality trait measured using the Toyo-taste meter. Genetic analysis indicated that the genetic composition of these two BILs differed only on the short arm of chromosome 6, which led to the identification of the qGCR6 locus. Through high-resolution genetic mapping, the qGCR6 locus was further delimited to a 43.9 kb chromosomal region containing ten putative genes. The DNA marker SNP2175, which tightly links to qGCR6, was developed and can be used in marker-assisted breeding programs.