Mapping QTLs for pre-harvest sprouting tolerance on chromosome 2D in a synthetic hexaploid wheat×common wheat cross

Based on segregation distortion of simple sequence repeat (SSR) molecular markers, we detected a significant quantitative trait loci (QTL) for pre-harvest sprouting (PHS) tolerance on the short arm of chromosome 2D (2DS) in the extremely susceptible population of F₂ progeny generated from the cross of PHS tolerant synthetic hexaploid wheat cultivar ‘RSP’ and PHS susceptible bread wheat cultivar ‘88–1643’. To identify the QTL of PHS tolerance, we constructed two SSR-based genetic maps of 2DS in 2004 and 2005. One putative QTL associated with PHS tolerance, designatedQphs.sau-2D, was identified within the marker intervalsXgwm261-Xgwm484 in 2004 and in the next year, nearly in the same position, between markerswmc112 andXgwm484. Confidence intervals based on the LOD-drop-off method ranged from 9 cM to 15.4 cM and almost completely overlapped with marker intervalXgwm261-Xgwm484. Flanking markers near this QTL could be assigned to the C-2DS1-0.33 chromosome bin, suggesting that the gene(s) controlling PHS tolerance is located in that chromosome region. The phenotypic variation explained by this QTL was about 25.73–27.50%. Genotyping of 48 F₆ PHS tolerant plants derived from the cross between PHS tolerant wheat cultivar ‘RSP’ and PHS susceptible bread wheat cultivar ‘MY11’ showed that the allele ofQphs.sau-2D found in the ‘RSP’ genome may prove useful for the improvement of PHS tolerance.     

Quantitative trait loci for resistance to spot blotch caused by Bipolarissorokiniana in wheat (T.aestivum L.) lines ‘Ning 8201’ and ‘Chirya 3’

Spot blotch caused by Bipolaris sorokiniana is a destructive disease of wheat in warm and humid wheat growing regions of the world. To identify quantitative trait loci (QTLs) for spot blotch resistance, two mapping populations were developed by making the crosses between common susceptible cultivar ‘Sonalika’ with the resistant breeding lines ‘Ning 8201’ and ‘Chirya 3’. Single seed descent derived F₆, F₇, F₈ lines of the first cross ‘Ning 8201’ × ‘Sonalika’ were evaluated for resistance to spot blotch in three blocks in each of the 3 years. After screening of 388 pairs of simple sequence repeat primers between the two parents, 119 polymorphic markers were used to genotype the mapping population. Four QTLs were identified on the chromosomes 2AS, 2BS, 5BL and 7DS and explained 62.9% of phenotypic variation in a simultaneous fit. The QTL on chromosome 2A was detected only in 1 year and explained 22.7% of phenotypic variation. In the second cross (‘Chirya 3’ × ‘Sonalika’), F₇ and F₈ population were evaluated in three blocks in each of the 2 years. In this population, five QTLs were identified on chromosomes 2BS, 2DS, 3BS, 7BS and 7DS. The QTLs identified in the ‘Chirya 3’ × ‘Sonalika’ population explained 43.4% of phenotypic variation in a simultaneous fit. The alleles for reduced disease severity in both the populations were derived from the respective resistant parent. The QTLs QSb.bhu-2B and QSb.bhu-7D from both populations were placed in the same deletion bins, 2BS1-0.53-0.75 and 7DS5-0.36-0.61, respectively. The closely linked markers Xgwm148 to the QTL on chromosome 2B and Xgwm111 to the QTL on chromosome 7D are potentially diagnostic markers for spot blotch resistance.     

High-density genetic and physical bin mapping of wheat chromosome 1D reveals that the powdery mildew resistance gene <Emphasis Type="Italic">Pm24</Emphasis> is located in a highly recombinogenic region

Genetic maps of wheat chromosome 1D consisting of 57 microsatellite marker loci were constructed using Chinese Spring (CS) × Chiyacao F₂ and the International Triticeae Mapping Initiative (ITMI) recombinant inbred lines (RILs) mapping populations. Marker order was consistent, but genetic distances of neighboring markers were different in two populations. Physical bin map of 57 microsatellite marker loci was generated by means of 10 CS 1D deletion lines. The physical bin mapping indicated that microsatellite marker loci were not randomly distributed on chromosome 1D. Nineteen of the 24 (79.2%) microsatellite markers were mapped in the distal 30% genomic region of 1DS, whereas 25 of the 33 (75.8%) markers were assigned to the distal 59% region of 1DL. The powdery mildew resistance gene Pm24, originating from the Chinese wheat landrace Chiyacao, was previously mapped in the vicinity of the centromere on the short arm of chromosome 1D. A high density genetic map of chromosome 1D was constructed, consisting of 36 markers and Pm24, with a total map length of 292.7 cM. Twelve marker loci were found to be closely linked to Pm24. Pm24 was flanked by Xgwm789 (Xgwm603) and Xbarc229 with genetic distances of 2.4 and 3.6 cM, respectively, whereas a microsatellite marker Xgwm1291 co-segregated with Pm24. The microsatellite marker Xgwm1291 was assigned to the bin 1DS5-0.70-1.00 of the chromosome arm 1DS. It could be concluded that Pm24 is located in the ‘1S0.8 gene-rich region’, a highly recombinogenic region of wheat. The results presented here would provide a start point for the map-based cloning of Pm24.     

Mapping and diagnostic marker development for Soil-borne cereal mosaic virus resistance in bread wheat

Monogenically-inherited resistance to Soil-borne cereal mosaic virus (SBCMV) in hexaploid bread wheat cultivars ‘Tremie’ and ‘Claire’ was mapped on chromosome 5D. The two closest flanking markers identified in the Claire-derived mapping population, Xgwm469-5D and E37M49, are linked to the resistance locus at distances of 1 and 9 cm, respectively. Xgwm469-5D co-segregated with the SBCMV resistance in the Tremie-derived population and with the recently identified Sbm1 locus in the cv. Cadenza. This suggested that Tremie and Claire carry a resistance gene allelic to Sbm1, or one closely linked to it. The diagnostic value of Xgwm469-5D was assessed using a collection of SBCMV resistant and susceptible cultivars. Importantly, all susceptible genotypes carried a null allele of Xgwm469-5D, whereas resistant genotypes presumably related to either Claire and Tremie or Cadenza revealed a 152 or 154 bp allele of Xgwm469-5D, respectively. Therefore, Xgwm469-5D is well suited for marker assisted selection for SBCMV resistance.     

Mapping of resistance to spot blotch disease caused by Bipolaris sorokiniana in spring wheat

Spot blotch caused by Bipolaris sorokiniana is a destructive disease of wheat in warm and humid wheat growing regions of the world. The development of disease resistant cultivars is considered as the most effective control strategy for spot blotch. An intervarietal mapping population in the form of recombinant inbred lines (RILs) was developed from a cross ‘Yangmai 6’ (a Chinese source of resistance) × ‘Sonalika’ (a spot blotch susceptible cultivar). The 139 single seed descent (SSD) derived F₆, F₇, F₈ lines of ‘Yangmai 6’ × ‘Sonalika’ were evaluated for resistance to spot blotch in three blocks in each of the 3 years. Joint and/or single year analysis by composite interval mapping (CIM) and likelihood of odd ratio (LOD) >2.2, identified four quantitative trait loci (QTL) on the chromosomes 2AL, 2BS, 5BL and 6DL. These QTLs were designated as QSb.bhu-2A, QSb.bhu-2B, QSb.bhu-5B and QSb.bhu-6D, respectively. A total of 63.10% of phenotypic variation was explained by these QTLs based on the mean over years. Two QTLs on chromosomes 2B and 5B with major effects were consistent over 3 years. All QTL alleles for resistance were derived from the resistant parent ‘Yangmai 6’.     

Identification of genetic factors controlling kernel hardness and related traits in a recombinant inbred population derived from a soft × ‘extra-soft’ wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) cross

Kernel hardness or texture, used to classify wheat (Triticum aestivum L.) into soft and hard classes, is a major determinant of milling and baking quality. Wheat genotypes in the soft class that are termed ‘extra-soft’ (with kernel hardness in the lower end of the spectrum) have been associated with superior end-use quality. In order to better understand the relationship between kernel hardness, milling yield, and various agronomic traits, we performed quantitative trait mapping using a recombinant inbred line population derived from a cross between a common soft wheat line and a genotype classified as an ‘extra-soft’ line. A total of 47 significant quantitative trait loci (QTL) (LOD ≥ 3.0) were identified for nine traits with the number of QTL affecting each trait ranging from three to nine. The percentage of phenotypic variance explained by these QTL ranged from 3.7 to 50.3%. Six QTL associated with kernel hardness and break flour yield were detected on chromosomes 1BS, 4BS, 5BS, 2DS, 4DS, and 5DL. The two most important QTL were mapped onto orthologous regions on chromosomes 4DS (Xbarc1118–Rht-D1) and 4BS (Xwmc617–Rht-B1). These results indicated that the ‘extra-soft’ characteristic was not controlled by the Hardness (Ha) locus on chromosome 5DS. QTL for eight agronomic traits occupied two genomic regions near semi-dwarf genes Rht-D1 on chromosome 4DS and Rht-B1 on chromosome 4BS. The clustering of these QTL is either due to the pleiotropic effects of single genes or tight linkage of genes controlling these various traits.     

Identification of a new QTL for <Emphasis Type="Italic">Fusarium</Emphasis> head blight resistance in the wheat genotype “Wang shui-bai”

Fusarium head blight (FHB) is one of the most devastating wheat diseases, causing both yield loss and quality reduction. To detect quantitative trait loci (QTL) responsible for FHB resistance, plants of the F _2:3 population derived from a ‘Wangshui-bai’ × ‘Sy95-7’ cross were artificially inoculated. Of 396 simple sequence repeats (SSRs), 125 amplified fragment length polymorphisms were used for FHB resistance QTL analysis. Five QTLs for FHB resistance were detected on chromosomes 3B, 6B, 7A, 1B and 2D. The effect of the QTL located on chromosome 3B on phenotypic variation was 31.69%, while that of the QTL found on 2D was the smallest and only accounted for 4.98% of the variation. The resistance alleles originated from ‘Wangshibai’ and association of the QTLs using these SSR markers may facilitate marker-assisted selection to improve FHB resistance in the wheat breeding programs of southwest China.     

Identification and validation of a major QTL conferring crown rot resistance in hexaploid wheat

Crown rot (CR), caused by various Fusarium species, is a chronic wheat disease in Australia. As part of our objective of improving the efficiency of breeding CR resistant wheat varieties, we have been searching for novel sources of resistance. This paper reports on the genetic control of one of these newly identified resistant genotypes, ‘CSCR6’. A population derived from a cross between CSCR6 and an Australian variety ‘Lang’ was analyzed using two Fusarium isolates belonging to two different species, one Fusarium pseudograminearum and the other Fusarium graminearum. The two isolates detected QTL with the same chromosomal locations and comparable magnitudes, indicating that CR resistance is not species-specific. The resistant allele of one of the QTL was derived from ‘CSCR6’. This QTL, designated as Qcrs.cpi-3B, was located on the long arm of chromosome 3B and explains up to 48.8% of the phenotypic variance based on interval mapping analysis. Another QTL, with resistant allele from the variety ‘Lang’, was located on chromosome 4B. This QTL explained up to 22.8% of the phenotypic variance. A strong interaction between Qcsr.cpi-3B and Qcsr.cpi-4B was detected, reducing the maximum effect of Qcrs.cpi-3B to 43.1%. The effects of Qcrs.cpi-3B were further validated in four additional populations and the presence of this single QTL reduced CR severity by up to 42.1%. The fact that significant effects of Qcrs.cpi-3B were detected across all trials with different genetic backgrounds and with the use of isolates belonging to two different Fusarium species make it an ideal target for breeding programs as well as for further characterization of the gene(s) involved in its resistance.     

Development and QTL assessment of Triticum aestivum–Aegilops tauschii introgression lines

A set of 84 bread wheat lines, each containing a single homozygous introgression of the Aegilops tauschii genome was produced in the ‘Chinese Spring’ background via backcrossing of the D-genome chromosome substitution lines ‘Chinese Spring’/Sears’s ‘Synthetic 6x’ with the recurrent parent and subsequent selfing. The development of the lines was accompanied by microsatellite marker assisted selection. With the exception of three telomeric regions at chromosomes 1DL, 4DL and 7DS, and a region of less than 24 cM on the chromosome arm 3DL, the genome of Ae. tauschii is fully represented in these lines. The newly developed lines were used for the discovery of morphological and agronomical quantitative trait loci (QTLs) from the wild species. Fifty-two introgression lines were grown in the field and evaluated for six traits including flowering time, plant height, ear length, spikelet number, fertility and grain weight per ear. Seventeen significant QTLs were detected, Ae. tauschii contributed favourable alleles at nine loci influencing five traits. The whole set of 84 homozygous lines provides a tool for further testing the effects and stability of the detected QTLs and for the evaluation of new traits.     

Genetic dissection of grain yield in bread wheat. I. QTL analysis

Grain yield forms one of the key economic drivers behind a successful wheat (Triticum aestivum L.) cropping enterprise and is consequently a major target for wheat breeding programmes. However, due to its complex nature, little is known regarding the genetic control of grain yield. A doubled-haploid population, comprising 182 individuals, produced from a cross between two cultivars ‘Trident’ and ‘Molineux’, was used to construct a linkage map based largely on microsatellite molecular makers. ‘Trident’ represents a lineage of wheat varieties from southern Australia that has achieved consistently high relative grain yield across a range of environments. In comparison, ‘Molineux’ would be rated as a variety with low to moderate grain yield. The doubled-haploid population was grown from 2002 to 2005 in replicated field experiments at a range of environments across the southern Australian wheat belt. In total, grain yield data were recorded for the population at 18 site-year combinations. Grain yield components were also measured at three of these environments. Many loci previously found to be involved in the control of plant height, rust resistance and ear-emergence were found to influence grain yield and grain yield components in this population. An additional nine QTL, apparently unrelated to these traits, were also associated with grain yield. A QTL associated with grain yield on chromosome 1B, with no significant relationship with plant height, ear-emergence or rust resistance, was detected (LOD ≥2) at eight of the 18 environments. The mean yield, across 18 environments, of individuals carrying the ‘Molineux’ allele at the 1B locus was 4.8% higher than the mean grain yield of those lines carrying the ‘Trident’ allele at this locus. Another QTL identified on chromosome 4D was also associated with overall gain yield at six of the 18 environments. Of the nine grain yield QTL not shown to be associated with plant height, phenology or rust resistance, two were located near QTL associated with grain yield components. A third QTL, associated with grain yield components at each of the environments used for testing, was located on chromosome 7D. However, this QTL was not associated with grain yield at any of the environments. The implications of these findings on marker-assisted selection for grain yield are discussed.     

Fine-mapping of qRL6.1, a major QTL for root length of rice seedlings grown under a wide range of NH4 + concentrations in hydroponic conditions

Root system development is an important target for improving yield in cereal crops. Active root systems that can take up nutrients more efficiently are essential for enhancing grain yield. In this study, we attempted to identify quantitative trait loci (QTL) involved in root system development by measuring root length of rice seedlings grown in hydroponic culture. Reliable growth conditions for estimating the root length were first established to renew nutrient solutions daily and supply NH₄ ⁺ as a single nitrogen source. Thirty-eight chromosome segment substitution lines derived from a cross between ‘Koshihikari’, a japonica variety, and ‘Kasalath’, an indica variety, were used to detect QTL for seminal root length of seedlings grown in 5 or 500 μM NH₄ ⁺. Eight chromosomal regions were found to be involved in root elongation. Among them, the most effective QTL was detected on a ‘Kasalath’ segment of SL-218, which was localized to the long-arm of chromosome 6. The ‘Kasalath’ allele at this QTL, qRL6.1, greatly promoted root elongation under all NH₄ ⁺ concentrations tested. The genetic effect of this QTL was confirmed by analysis of the near-isogenic line (NIL) qRL6.1. The seminal root length of the NIL was 13.5–21.1% longer than that of ‘Koshihikari’ under different NH₄ ⁺ concentrations. Toward our goal of applying qRL6.1 in a molecular breeding program to enhance rice yield, a candidate genomic region of qRL6.1 was delimited within a 337 kb region in the ‘Nipponbare’ genome by means of progeny testing of F₂ plants/F₃ lines derived from a cross between SL-218 and ‘Koshihikari’.     

Detection and verification of malting quality QTLs using wild barley introgression lines

A malting quality quantitative trait locus (QTL) study was conducted using a set of 39 wild barley introgression lines (hereafter abbreviated with S42ILs). Each S42IL harbors a single marker-defined chromosomal segment from the wild barley accession ‘ISR 42-8’ (Hordeum vulgare ssp. spontaneum) within the genetic background of the elite spring barley cultivar ‘Scarlett’ (Hordeum vulgare ssp. vulgare). The aim of the study was (1) to verify genetic effects previously identified in the advanced backcross population S42, (2) to detect new QTLs, and (3) to identify S42ILs exhibiting multiple QTL effects. For this, grain samples from field tests in three different environments were subjected to micro malting. Subsequently, a line × phenotype association study was performed with the S42ILs in order to localize putative QTL effects. A QTL was accepted if the trait value of a particular S42IL was significantly (P < 0.05) different from the recurrent parent as a control, either across all tested environments or in a particular environment. For eight malting quality traits, altogether 40 QTLs were localized, among which 35 QTLs (87.5%) were stable across all environments. Six QTLs (15.0%) revealed a trait improving wild barley effect. Out of 36 QTLs detected in a previous advanced backcross QTL study with the parent BC₂DH population S42, 18 QTLs (50.0%) could be verified with the S42IL set. For the quality parameters α-amylase activity and Hartong 45°C, all QTLs assessed in population S42 were verified by S42ILs. In addition, eight new QTL effects and 17 QTLs affecting two newly investigated traits were localized. Two QTL clusters harboring simultaneous effects on eight and six traits, respectively, were mapped to chromosomes 1H and 4H. In future, fine-mapping of these QTL regions will be conducted in order to shed further light on the genetic basis of the most interesting QTLs.     

Genetic dissection of grain yield in bread wheat. II. QTL-by-environment interaction

The grain yield of wheat is influenced by genotype, environment and genotype-by-environment interaction. A mapping population consisting of 182 doubled haploid progeny derived from a cross between the southern Australian varieties ‘Trident’ and ‘Molineux’, was used to characterise the interaction of previously mapped grain yield quantitative trait locus (QTL) with specific environmental covariables. Environments (17) used for grain yield assessment were characterised for latitude, rainfall, various temperature-based variables and stripe rust infection severity. The number of days in the growing season in which the maximum temperature exceeded 30°C was identified as the variable with the largest effect on site mean grain yield. However, the greatest QTL-by-environmental covariable interactions were observed with the severity of stripe rust infection. The rust resistance allele at the Lr37/Sr38/Yr17 locus had the greatest positive effect on grain yield when an environment experienced a combination of high-stripe rust infection and cool days. The grain yield QTL, QGyld.agt-4D, showed a very similar QTL-by-environment covariable interaction pattern to the Lr37/Sr38/Yr17 locus, suggesting a possible role in rust resistance or tolerance. Another putative grain yield per se QTL, QGyld.agt-1B, displayed interactions with the quantity of winter and spring rainfall, the number of days in which the maximum temperature exceeded 30°C, and the number of days with a minimum temperature below 10°C. However, no cross-over interaction effect was observed for this locus, and the ‘Molineux’ allele remained associated with higher grain yield in response to all environmental covariables. The results presented here confirm that QGyld.agt-1B may be a prime candidate for marker-assisted selection for improved grain yield and wide adaptation in wheat. The benefit of analysing the interaction of QTL and environmental covariables, such as employed here, is discussed.     

Mapping quantitative trait loci associated with regeneration ability of seed callus in rice, Oryza sativa L.

 Quantitative trait loci (QTL) controlling the regeneration ability of rice seed callus were detected using 245 RFLP markers and 98 BC₁F₅ lines derived from two varieties, ‘Nipponbare’ and ‘Kasalath’. Regeneration ability was evaluated by two indices: average number of regenerated shoots per callus (NRS) and regeneration rate (RR). The BC₁F₅ lines showed continuous segregation for both indices. Five putative QTL for NRS (tentatively named qRg1, qRg2, qRg4a, qRg4b and qRg4c) located on chromosomes 1, 2 and 4 were detected. Digenic interaction among these detected QTL was not significant (P<0.01). Among the five QTL detected, four ‘Kasalath’ alleles and one ‘Nipponbare’ allele increased NRS. According to an estimate based on the nearest marker loci, the five QTL accounted for 38.5% of the total phenotypic variation of the BC₁F₅ lines. For RR, four putative QTL were detected on chromosomes 2 and 4, and all of these were in the same chromosomal regions as the NRS QTL. The four RR QTL accounted for 32.6% of the total phenotypic variation. Received: 7 November 1996 / Accepted: 25 April 1997     

Comparative mapping of the wheat chromosome 5A Vrn-A1 region with rice and its relationship to QTL for flowering time

 The vernalization gene Vrn-A1 on chromosome 5A is the predominant gene determining the spring/winter habit difference in bread wheat. Vrn-A1 was physically mapped using a set of deletion lines which located it to the region of chromosome 5A flanked by deletion breakpoints 0.68 and 0.78. This interval was shown to be homoeologous to a region of rice chromosome 3 that contains the flowering-time QTL Hd-6, previously mapped in a Nipponbare×Kasalath cross, and FLTQ1, a novel QTL identified by analysis of 78 F₃ families derived from a cross of ‘IR20’ב63–83’. Possible relationships between Vrn-A1 and rice QTL are discussed. Analysis of the chromosome 5A deletion lines showed evidence for a second, more proximal flowering-time effect located between deletion breakpoints 0.56 and 0.64. The proximal part of chromosome 5A is homoeologous to rice chromosome 9, on which two QTL were detected in the ‘IR20ב63–83’ cross. The possible relationship between these effects is also discussed. Received: 23 December 1997 / Accepted: 12 January 1998     

A Novel STS Marker for Polyphenol Oxidase Activity in Bread Wheat

The enzyme activity of polyphenol oxidase (PPO) in grain has been related to undersirable brown discoloration of bread wheat (Triticum aestivum L.) based end-products, particularly for Asian noodles. Breeding wheat cultivars with low PPO activity is the best approach to reduce the undesirable darkening. Molecular markers could greatly improve selection efficiency in breeding programs. Based on the sequences of PPO genes (GenBank Accession Numbers AY596268, AY596269 and AY596270) conditioning PPO activity during kernel development, 28 pairs of primers were designed using the software ‘DNAMAN’. One of the markers from AY596268, designated as PPO18, can amplify a 685-bp and an 876-bp fragment in the cultivars with high and low PPO activity, respectively. The difference of 191-bp size was detected in the intron region of the PPO gene. The STS marker PPO18 was mapped to chromosome 2AL using a DH population derived from a cross Zhongyou 9507× CA9632, a set of nulli-tetrasomic lines and ditelosomic line 2AS of Chinese Spring. QTL analysis indicated that the PPO gene co-segregated with the STS marker PPO18 and is closely linked to Xgwm312 and Xgwm294 on chromosome 2AL, explaining 28–43% of phenotypic variance for PPO activity across three environments. A total of 233 Chinese wheat cultivars and advanced lines were used to validate the correlation between the polymorphic fragments of PPO18 and grain PPO activity. The results showed that PPO18 is a co-dominant, efficient and reliable molecular marker for PPO activity and can be used in wheat breeding programs targeted for noodle quality improvement.     

Dissection of quantitative and durable leaf rust resistance in Swiss winter wheat reveals a major resistance QTL in the<Emphasis Type="Italic"> Lr34</Emphasis> chromosomal region

The Swiss winter bread wheat cv. Forno has a highly effective, durable and quantitative leaf rust (Puccinia triticina Eriks.) resistance which is associated with leaf tip necrosis (LTN). We studied 240 single seed descent lines of an Arina×Forno F_5:7 population to identify and map quantitative trait loci (QTLs) for leaf rust resistance and LTN. Percentage of infected leaf area (%) and the response to infection (RI) were evaluated in seven field trials and were transformed to the area under the disease progress curves (AUDPC). Using composite interval mapping and LOD >4.4, we identified eight chromosomal regions specifically associated with resistance. The largest and most consistent leaf rust resistance locus was identified on the short arm of chromosome 7D (32.6% of variance explained for AUDPC_% and 42.6% for AUDPC_RI) together with the major QTL for LTN (R ²=55.6%) in the same chromosomal region as Lr34 (Xgwm295). A second major leaf rust resistance QTL (R ²=28% and 31.5%, respectively) was located on chromosome arm 1BS close to Xgwm604 and was not associated with LTN. Additional minor QTLs for LTN (2DL, 3DL, 4BS and 5AL) and leaf rust resistance were identified. These latter QTLs might correspond to the leaf rust resistance genes Lr2 or Lr22 (2DS) and Lr14a (7BL).Electronic Supplementary Material Supplementary material ist available in the online version ot this article at Communicated by H.C. Becker     

Fine mapping of <Emphasis Type="Italic">Sta1</Emphasis>, a quantitative trait locus determining stele transversal area,on rice chromosome 9

The stele (root vascular cylinder) in plants plays an important role in the transport of water and nutrients from the root to the shoot. A quantitative trait locus (QTL) on rice chromosome 9 that controls stele transversal area (STA) was previously detected in an F₃ mapping population derived from a cross between the lowland cultivar ‘IR64’, with a small STA, and the upland cultivar ‘Kinandang Patong’, with a large STA. To identify the gene(s) underlying this QTL, we undertook fine mapping of the locus. We screened eight plants from BC₂F₃ lines in which recombination occurred near the QTL. Progeny testing of BC₂F₄ plants was used to determine the genotype classes for the QTL in each BC₂F₃ line. Accordingly, the STA QTL Sta1 (Stele Transversal Area 1) was mapped between the InDel markers ID07_12 and ID07_14. A candidate genomic region for Sta1 was defined more precisely between markers RM566 and RM24334, which delimit a 359-kb interval in the reference cultivar ‘Nipponbare’. A line homozygous for the ‘Kinandang Patong’ allele of Sta1 had an STA approximately 28.4% larger than that of ‘IR64’. However, Sta1 did not influence maximum or total root length, suggesting that this QTL specifically controls STA.     

Genetic mapping of a major gene affecting onion bulb fructan content

The non-structural dry matter content of onion bulbs consists principally of fructose, glucose, sucrose and fructans. The objective of this study was to understand the genetic basis for the wide variation observed in the relative amounts of these carbohydrates. Bulb carbohydrate composition was evaluated in progeny from crosses between high dry matter storage onion varieties and sweet, low dry matter varieties. When samples were analysed on a dry weight basis, reducing sugar and fructan content exhibited high negative correlations and bimodal segregation suggestive of the action of a major gene. A polymorphic SSR marker, ACM235, was identified which exhibited strong disequilibrium with bulb fructan content in F_2:3 families from the ‘W202A’ × ‘Texas Grano 438’ mapping population evaluated in two environments. This marker was mapped to chromosome 8 in the interspecific population ‘Allium cepa × A. roylei’. Mapping in the ‘Colossal Grano PVP’ × ‘Early Longkeeper P12’ F₂ population showed that a dominant major gene conditioning high-fructan content lay in the same genomic region. QTL analysis of total bulb fructan content in the intraspecific mapping population ‘BYG15-23’ × ‘AC43’ using a complete molecular marker map revealed only one significant QTL in the same chromosomal region. This locus, provisionally named Frc, may account for the major phenotypic differences in bulb carbohydrate content between storage and sweet onion varieties.