fw 2.2:a major QTL controlling fruit weight is common to both red- and green-fruited tomato species

We have shown that a major QTL for fruit weight (fw2.2) maps to the same position on chromosome 2 in the green-fruited wild tomato species, Lycopersicon pennellii and in the red-fruited wild tomato species, L. pimpinellifolium. An introgression line F₂ derived from L. esculentum (tomato) x L. pennellii and a backcross 1 (BC₁) population derived from L. esculentum x L. pimpinellifolium both place fw2.2 near TG91 and TG167 on chromosome 2 of the tomato highdensity linkage map. fw2.2 accounts for 30% and 47% of the total phenotypic variance in the L. pimpinellifolium and L. pennellii populations, respectively, indicating that this is a major QTL controlling fruit weight in both species. Partial dominance (d/a of 0.44) was observed for the L. pennellii allele of fw 2.2 as compared with the L. esculentum allele. A QTL with very similar phenotypic affects and gene action has also been identified and mapped to the same chromosomal region in other wild tomato accessions: L. cheesmanii and L. pimpinellifolium. Together, these data suggest that fw2.2 represents an orthologous QTL (i.e., derived by speciation as opposed to duplication) common to most, if not all, wild tomato species. High-resolution mapping may ultimately lead to the cloning of this key locus controlling fruit development in tomato.     

Effect of population size on the estimation of QTL: a test using resistance to barley stripe rust

The limited population sizes used in many quantitative trait locus (QTL) detection experiments can lead to underestimation of QTL number, overestimation of QTL effects, and failure to quantify QTL interactions. We used the barley/barley stripe rust pathosystem to evaluate the effect of population size on the estimation of QTL parameters. We generated a large (n=409) population of doubled haploid lines derived from the cross of two inbred lines, BCD47 and Baronesse. This population was evaluated for barley stripe rust severity in the Toluca Valley, Mexico, and in Washington State, USA, under field conditions. BCD47 was the principal donor of resistance QTL alleles, but the susceptible parent also contributed some resistance alleles. The major QTL, located on the long arm of chromosome 4H, close to the Mlo gene, accounted for up to 34% of the phenotypic variance. Subpopulations of different sizes were generated using three methods—resampling, selective genotyping, and selective phenotyping—to evaluate the effect of population size on the estimation of QTL parameters. In all cases, the number of QTL detected increased with population size. QTL with large effects were detected even in small populations, but QTL with small effects were detected only by increasing population size. Selective genotyping and/or selective phenotyping approaches could be effective strategies for reducing the costs associated with conducting QTL analysis in large populations. The method of choice will depend on the relative costs of genotyping versus phenotyping. Electronic Supplementary Material Supplementary material is available for this article at     

Susceptibility to <Emphasis Type="Italic">Fusarium </Emphasis>head blight is associated with the <Emphasis Type="Italic">Rht-D1b</Emphasis> semi-dwarfing allele in wheat

Fusarium head blight (FHB) is an important disease of wheat worldwide. The cultivar Spark is more resistant than most other UK winter wheat varieties but the genetic basis for this is not known. A mapping population from a cross between Spark and the FHB susceptible variety Rialto was used to identify quantitative trait loci (QTL) associated with resistance. QTL analysis across environments revealed nine QTL for FHB resistance and four QTL for plant height (PH). One FHB QTL was coincident with the Rht-1D locus and accounted for up to 51% of the phenotypic variance. The enhanced FHB susceptibility associated with Rht-D1b is not an effect of PH per se as other QTL for height segregating in this population have no influence on susceptibility. Experiments with near-isogenic lines supported the association between susceptibility and the Rht-D1b allele conferring the semi-dwarf habit. Our results demonstrate that lines carrying the Rht-1Db semi-dwarfing allele are compromised in resistance to initial infection (type I resistance) while being unaffected in resistance to spread within the spike (type II resistance).     

Two stable QTL involved in adult plant resistance to powdery mildew in the winter wheat line RE714 are expressed at different times along the growing season

Powdery mildew (Blumeria graminis f. sp. tritici) is one of the major diseases of wheat (Triticum aestivum). Adult plant resistance (APR) to powdery mildew is considered more durable than resistance conferred by major race-specific resistance genes. The objective of the present study was a better understanding of the genetic basis of APR in RE714 by means of QTL analysis of several resistance scores along the growing season. A population of 160 recombinant inbred lines obtained from the cross between RE714 and Hardi (susceptible) was assessed for APR under natural infection conditions during 3 years and a genetic map with whole genome coverage was developed with microsatellite and AFLP markers in this population. Two major QTL on chromosomes 5D and 6A were detected each year, and 6 minor QTL were detected only in 1 or 2 years. The QTL on chromosome 5D was detected during all the growing season each year and its R ² value varied between 8.5 and 56.3%, whereas the QTL on chromosome 6A was detected at 1–4 scoring dates in the 3 years, and its R ² value varied between 6.1 and 20.5%. The two QTL explained between 24.4 and 52.1% of the phenotypic variance for AUDPC, depending on the year. The models including QTL and cofactors in the composite interval mapping explained between 29 and 72% of the variance. The molecular markers linked to the two major QTL could be used in marker-assisted selection for adult plant resistance to powdery mildew. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Genetic analysis of RFLPs, GATA microsatellites and RAPDs in a cross between L. esculentum and L. pimpinellifolium

A population of 257 BC₁ plants was developed from a cross between an elite processing line of tomato (Lycopersicon esculentum cvM82-1-7) and the closely related wild species L. pimpinellifolium (LA1589). The population was used to construct a genetic linkage map suitable for quantitative trait locus (QTL) analysis to be conducted in different backcross generations. The map comprises 115 RFLP, 3 RAPD and 2 morphological markers that span 1279 cM of the tomato genome with an average distance between markers of 10.7 cM. This map is comparable in length to that of the highdensity RFLP map derived from a L. esculentum x L. pennellii F₂ population. The order of the markers in the two maps is also in good agreement, however there are considerable differences in the distribution of recombination along the chromosomes. The segregation of six GATA-containing loci and 47 RAPD markers was also analyzed in subsets of the population. All of the microsatellite loci and 35 (75%) of the RAPDs mapped to clusters associated with centromeric regions.     

A major QTL introgressed from wild Lycopersicon hirsutum confers chilling tolerance to cultivated tomato (Lycopersicon esculentum)

Many plants of tropical or subtropical origin, such as tomato, suffer damage under chilling temperatures (under 10°C but above 0°C). An earlier study identified several quantitative trait loci (QTLs) for shoot turgor maintenance (stm) under root chilling in an interspecific backcross population derived from crossing chilling-susceptible cultivated tomato (Lycopersicon esculentum) and chilling-tolerant wild L. hirsutum. The QTL with the greatest phenotypic effect on stm was located in a 28 cM region on chromosome 9 (designated stm9), and enhanced chilling-tolerance was conferred by the presence of the Lycopersicon hirsutum allele at this QTL. Here, near-isogenic lines (NILs) were used to verify the effect of stm9, and recombinant sub-NILs were used to fine map its position. Replicated experiments were performed with NILs and sub-NILs in a refrigerated hydroponic tank in the greenhouse. Sub-NIL data was analyzed using least square means separations, marker-genotype mean t-tests, and composite interval mapping. A dominant QTL controlling shoot turgor maintenance under root chilling was confirmed on chromosome 9 using both NILs and sub-NILs. Furthermore, sub-NILs permitted localization of stm9 to a 2.7 cM interval within the original 28 cM QTL region. If the presence of the L. hirsutum allele at stm9 also confers chilling-tolerance in L. esculentum plants grown under field conditions, it has the potential to expand the geographic areas in which cultivated tomato can be grown for commercial production.     

Genetic analysis of organoleptic quality in fresh market tomato. 1. Mapping QTLs for physical and chemical traits

Improving organoleptic quality is an important but complex goal for fresh market tomato breeders. A total of 26 traits involved in organoleptic quality variation were evaluated, in order to understand the genetic control of this characteristic. A recombinant inbred line (RIL) population was developed from an intraspecific cross between a cherry tomato line with a good overall aroma intensity and an inbred line with a common taste but with bigger fruits. Physical traits included fruit weight, diameter, color (L,a,b), firmness and elasticity. Chemical traits were dry matter weight, titratable acidity, pH, and the contents of soluble solids, sugars, lycopene, carotene and 12 aroma volatiles. RILs showed a large range of variation for most of the traits and many of them were transgressive. Some correlations between aroma volatiles were in accordance with the metabolic pathway they originated from. A total of 81 significant QTLs were detected for the 26 traits by simple and composite interval mapping. They were mainly distributed in a few regions on chromosomes 2, 3, 4, 8, 9, 11 and 12. Major QTLs (R²>30%) were detected for fruit weight, diameter, and color, and for six aroma volatiles. Co-localization of QTLs controlling correlated traits was mainly found on chromosome 2. QTLs for fruit weight and sugar content or dry matter weight were often co-localized. However, a QTL for soluble-solids content and dry matter weight have been detected on chromosome 9 in a region without fruit weight QTLs. QTLs for seven aroma volatiles, lycopene content and fruit color were also co-localized. The QTL localizations were compared with those detected in crosses between Lycopersicon esculentum and wild tomato species. Received: 19 January 2000 / Accepted: 26 May 2000     

Identification of QTL associated with resistance to bacterial spot race T4 in tomato

Bacterial spot of tomato (Solanum lycopersicum L.), caused by several Xanthomonas sp., is a serious but difficult disease to control by chemical means. Development of resistance has been hindered by emergence of races virulent to tomato, by the quantitative inheritance of resistance, and by a low correlation between seedling assays and resistance in the field. Resistance to multiple races, including race T4, has been described in the S. lycopersicum var. cerasiformae accession PI 114490. We used molecular markers to identify associations with quantitative trait loci (QTL) in an elite inbred backcross (IBC) population derived from OH 9242, PI 114490 and Fla. 7600, a breeding line with tomato accession Hawaii 7998 (H7998) in its pedigree. Race T4 resistance has also been described in the advanced breeding lines Fla. 8233, Fla. 8517, and Fla. 8326, and a selective genotyping approach was used to identify introgressions associated with resistance in segregating progeny derived from crosses with these lines. In the IBC population, loci on chromosomes 11 and 3, respectively, explained as much as 29.4 and 4.8% of resistance variation. Both these loci were also confirmed by selective genotyping: PI 114490 and H7998 alleles on chromosome 11 each provided resistance. The PI 114490 allele on chromosome 3 was confirmed in the Fla. 8517 population, and an allele of undetermined descent was confirmed at this locus in the Fla. 8326 population. A chromosome 12 allele was associated with susceptibility in the Fla. 8517 population. Additional loci contributing minor effects were also implicated in the IBC population or by selective genotyping. Selection for the major QTL in a marker-directed phenotyping approach should significantly improve the efficiency of breeding for resistance to bacterial spot race T4, although as yet undetected QTL would be necessary to carry out strict marker assisted selection.     

QTL analysis of horticultural traits differentiating the cultivated tomato from the closely related species Lycopersicon pimpinellifolium

Molecular markers were used to map and characterize quantitative trait loci (QTLs) for several characters of agronomic and biological importance in an interspecific backcross of tomato. The parents of the cross were an elite processing inbred Lycopersicon esculentum cv M82-1-7 and the closely related red-fruited wild species L. pimpinellifolium (LA1589). A total of 257 BC₁ plants were grown under field conditions in Ithaca, New York and scored for 19 quantitative traits. A genetic linkage map was constructed for the same population using 115 RFLP, 3 RAPD and 2 morphological markers that spanned 1,279 cM of the tomato genome with an average interval length of 10.7 cM. A minimum of 54 putatively significant QTLs (P<0.001; LOD> 2.4) were detected for all characters with a range of 1–7 QTLs detected per character. Of the total 54 QTLs 11% had alleles with effects opposite to those predicted by the parental phenotypes. The percentage of phenotypic variation associated with single QTLs ranged from 4% to 47%. Multilocus analysis showed that the cumulative action of all QTLs detected for each trait accounted for 12–59% of the phenotypic variation. The difference in fruit weight was controlled largely by a single major QTL (fw2.2). Digenic epistasis was not evident. Several regions of the genome (including the region near sp on chromosome 6) showed effects on more than one trait. Implications for variety improvement and inferences about the domestication of the cultivated tomato are discussed.     

The effects of mullein plants (Verbascum thapsus) on the population dynamics of Dicyphus hesperus (Heteroptera: Miridae) in tomato greenhouses

The response of Dicyphus hesperus Knight (Heteroptera: Miridae) to whitefly populations in tomato greenhouses was measured in the presence and absence of mullein (Verbascum thapsus L.) as an alternative host plant. The dynamics of the D. hesperus population on tomato (Lycopersicon esculentum Mill.) and on mullein plants were followed through an entire growing season. In houses with mullein plants, more predators occurred on mullein when whitefly density was low on tomato. A mark-release-recapture experiment where rabbit IgG was used as an external marker showed that D. hesperus adults moved from mullein plants to tomato plants. D. hesperus was always more abundant in houses with mullein than in the houses with tomato plants alone. Movements between tomato and mullein plants are discussed as a strategy to optimize predator foraging. The use of mullein as an alternative host plant may contribute to the establishment of D. hesperus and help to preserve the predator population when prey on tomato crops is scarce.     

QTL analysis of plant development and fruit traits in pepper and performance of selective phenotyping

A QTL analysis was performed to determine the genetic basis of 13 horticultural traits conditioning yield in pepper (Capsicum annuum). The mapping population was a large population of 297 recombinant inbred lines (RIL) originating from a cross between the large-fruited bell pepper cultivar ‘Yolo Wonder’ and the small-fruited chilli pepper ‘Criollo de Morelos 334’. A total of 76 QTLs were detected for 13 fruit and plant traits, grouped in 28 chromosome regions. These QTLs explained together between 7% (internode growth time) and 91% (fruit diameter) of the phenotypic variation. The QTL analysis was also performed on two subsets of 141 and 93 RILs sampled using the MapPop software. The smaller populations allowed for the detection of a reduced set of QTLs and reduced the overall percentage of trait variation explained by QTLs. The frequency of false positives as well as the individual effect of QTLs increased in reduced population sets as a result of reduced sampling. The results from the QTL analysis permitted an overall glance over the genetic architecture of traits considered by breeders for selection. Colinearities between clusters of QTLs controlling fruit traits and/or plant development in distinct pepper species and in related solanaceous crop species (tomato and eggplant) suggests that shared mechanisms control the shape and growth of different organs throughout these species.     

QTLs mapping for fruit size and shape in chromosomes 2 and 4 in pepper and a comparison of the pepper QTL map with that of tomato

Quantitative trait locus (QTL) mapping for fruit weight and shape in pepper (Capsicum spp.) was performed using C. chinense and C. frutescens introgression lines of chromosomes 2 and 4. In chromosome 2, a single major fruit-weight QTL, fw2.1, was detected in both populations that explained 62% of the trait variation. This QTL, as well as a fruit-shape QTL, fs2.1, which had a more minor effect, were localized to the tomato fruit-shape gene ovate. The cloned tomato fruit-weight QTL, fw2.2, did not play a major role in controlling fruit size variations in pepper. In chromosome 4, two fruit-weight QTLs, fw4.1 and fw4.2, were detected in the same genomic regions in both mapping populations. In addition, a single fruit-shape QTL was detected in each of the mapping populations that co-localized with one of the fruit-weight QTLs, suggesting pleiotropy or close linkage of the genes controlling size and shape. fw2.1 and fw4.2 represent major fruit-weight QTLs that are conserved in the three Capsicum species analyzed to date for fruit-size variations. Co-localization of the pepper QTLs with QTLs identified for similar traits in tomato suggests that the pepper and tomato QTLs are orthologous. Compared to fruit-shape QTLs, fruit-weight QTLs were more often conserved between pepper and tomato. This implies that different modes of selection were employed for these traits during domestication of the two Solanaceae species.S. Zygier and A. Ben Chaim contributed equally to this work.     

A major QTL controlling seed dormancy and pre-harvest sprouting resistance on chromosome 4A in a Chinese wheat landrace

Wheat pre-harvest sprouting (PHS) can cause significant reduction in yield and end-use quality of wheat grains in many wheat-growing areas worldwide. To identify a quantitative trait locus (QTL) for PHS resistance in wheat, seed dormancy and sprouting of matured spikes were investigated in a population of 162 recombinant inbred lines (RILs) derived from a cross between the white PHS-resistant Chinese landrace Totoumai A and the white PHS-susceptible cultivar Siyang 936. Following screening of 1,125 SSR primers, 236 were found to be polymorphic between parents, and were used to screen the mapping population. Both seed dormancy and PHS of matured spikes were evaluated by the percentage of germinated kernels under controlled moist conditions. Twelve SSR markers associated with both PHS and seed dormancy were located on the long arm of chromosome 4A. One QTL for both seed dormancy and PHS resistance was detected on chromosome 4AL. Two SSR markers, Xbarc 170 and Xgwm 397, are 9.14 cM apart, and flanked the QTL that explained 28.3% of the phenotypic variation for seed dormancy and 30.6% for PHS resistance. This QTL most likely contributed to both long seed dormancy period and enhanced PHS resistance. Therefore, this QTL is most likely responsible for both seed dormancy and PHS resistance. The SSR markers linked to the QTL can be used for marker-assisted selection of PHS-resistant white wheat cultivars. Shi-Bin Cai and Cui-Xia Chen contributed equally to this work.     

Enhancement of growth and yield of tomato by <Emphasis Type="Italic">Rhodopseudomonas</Emphasis> sp. under greenhouse conditions

A greenhouse test was carried out to examine the effects on tomato growth of application of purple non-sulfur bacterium Rhodopseudomonas sp. which had enhanced germination and growth of tomato seed under axenic conditions. The shoot length of tomato plant inoculated by Rhodopseudomonas sp. KL9 increased by 34.6% compared to that of control in 8 weeks of cultivation. During the same period, this strain increased 120.6 and 78.6% of dry weight of shoot and root of tomato plants, respectively. The formation ratio of tomato fruit from flower was also raised by inoculation of KL9. In addition, Rhodopseudomonas sp. KL9 treatment enhanced the fresh weight and lycopene content in the harvested tomato fruits by 98.3 and 48.3%, respectively compared to those of the uninoculated control. When the effect on the indigenous bacterial community and fate of the inoculated Rhodopseudomonas sp. KL9 were monitored by denaturing gradient gel electrophoresis analysis, its application did not affect the native bacterial community in tomato rhizosphere soil, but should be repeated to maintain its population size. This bacterial capability may be applied as an environment-friendly biofertilizer to cultivation of high quality tomato and other crops including lycopene-containing vegetables and fruits.     

Mapping minor QTL for increased stearic acid content in sunflower seed oil

Increased stearic acid (C18:0) content in the seed oil of sunflower would improve the oil quality for some edible uses. The sunflower line CAS-20 (C18:0 genotype Es1Es1es2es2), developed from the high C18:0 mutant line CAS-3 (C18:0 genotype es1es1es2es2; 25% C18:0), shows increased C18:0 levels in its seed oil (8.6%). The objective of this research was to map quantitative trait loci (QTL) conferring increased C18:0 content in CAS-20 in an F₂ mapping population developed from crosses between HA-89 (wild type Es1Es1Es2Es2; low C18:0) and CAS-20, which segregates independently of the macromutation Es1 controlling high C18:0 content in CAS-3. Seed oil fatty acid composition was measured in the F₂ population by gas-liquid chromatography. A genetic linkage map of 17 linkage groups (LGs) comprising 80 RFLP and 19 SSR marker loci from this population was used to identify QTL controlling fatty acid composition. Three QTL affecting C18:0 content were identified on LG3, LG11, and LG13, with all alleles for increased C18:0 content inherited from CAS-20. In total, these QTL explained 43.6% of the C18:0 phenotypic variation. Additionally, four candidate genes (two stearate desaturase genes, SAD6 and SAD17, and a FatA and a FatB thioesterase gene) were placed on the QTL map. On the basis of positional information, QTL on LG11 was suggested to be a SAD6 locus. The results presented show that increased C18:0 content in sunflower seed oil is not a simple trait, and the markers flanking these QTL constitute a powerful tool for plant breeding programs.     

Quantitative trait loci for aluminum resistance in wheat

Quantitative trait loci (QTL) for wheat resistance to aluminum (Al) toxicity were analyzed using simple sequence repeats (SSRs) in a population of 192 F₆ recombinant inbred lines (RILs) derived from a cross between an Al-resistant cultivar, Atlas 66 and an Al-sensitive cultivar, Chisholm. Wheat reaction to Al was measured by relative root growth and root response to hematoxylin stain in nutrient-solution culture. After screening 1,028 SSR markers for polymorphisms between the parents and bulks, we identified two QTLs for Al resistance in Atlas 66. One major QTL was mapped on chromosome 4D that co-segregated with the Al-activated malate transporter gene (ALMT1). Another minor QTL was located on chromosome 3BL. Together, these two QTLs accounted for about 57% of the phenotypic variation in hematoxylin staining score and 50% of the variation in net root growth (NRG). Expression of the minor QTL on 3BL was suppressed by the major QTL on 4DL. The two QTLs for Al resistance in Atlas 66 were also verified in an additional RIL population derived from Atlas 66/Century. Several SSR markers closely linked to the QTLs were identified and have potential to be used for marker-assisted selection (MAS) to improve Al-resistance of wheat cultivars in breeding programs.     

Stability over genetic backgrounds, generations and years of quantitative trait locus (QTLs) for organoleptic quality in tomato

The efficiency of marker-assisted backcross for the introgression of a quantitative trait locus (QTL) from a donor line into a recipient line depends on the stability of QTL expression. QTLs for six quality traits in tomato (fruit weight, firmness, locule number, soluble solid content, sugar content and titratable acidity) were studied in order to investigate their individual effect and their stability over years, generations and genetic backgrounds. Five chromosome regions carrying fruit quality QTLs were transferred following a marker-assisted backcross scheme from a cherry tomato line into three modern lines with larger fruits. Three sets of genotypes corresponding to three generations were compared: (1) an RIL population, which contained 50% of each parental genome, (2) three BC3S1 populations which segregated simultaneously for the five regions of interest but were almost fully homozygous for the recipient genome on the eight chromosomes carrying no QTL and (3) three sets of QTL-NILs (BC3S3 lines) which differed from the recipient line only in one of the five regions. QTL detection was performed in each generation, in each genetic background and during 2 successive years for QTL-NILs. About half of the QTLs detected in QTL-NILs were detected in both years. Eight of the ten QTLs detected in RILs were recovered in the QTL-NILs with the genetic background used for the initial QTL mapping experiment, with the exception of two QTLs for fruit firmness. Several new QTLs were detected. In the two other genetic backgrounds, the number of QTLs in common with the RILs was lower, but several new QTLs were also detected in advanced generations.     

Mapping of quantitative trait loci for kernel row number in maize across seven environments

Genetic factors controlling quantitative inheritance of grain yield and its components have been intensively investigated during recent decades using diverse populations in maize (Zea mays L.). Notwithstanding this, quantitative trait loci (QTL) for kernel row number (KRN) with large and consistent effect have not been identified. In this study, a linkage map of 150 simple sequence repeat (SSR) loci was constructed by using a population of 500 F2 individuals derived from a cross between elite inbreds Ye478 and Dan340. The linkage map spanned a total of 1478 cM with an average interval of 10.0 cM. A total of 397 F2:3 lines were evaluated across seven diverse environments for mapping QTL for KRN. Some QTL for grain yield and its components had previously been confirmed with this population across environments. A total of 13 QTL for KRN were identified, with each QTL explaining from 3.0 to 17.9% of phenotypic variance. The gene action for KRN was mainly additive to partial dominance. A large-effect QTL (qkrn7) with partial dominance effect accounting for 17.9% of the phenotypic variation for KRN was identified on chromosome 7 near marker umc1865 with consistent gene effect across seven diverse environments. This study has laid a foundation for map-based cloning of this major QTL and for developing molecular markers for marker-assisted selection of high KRN.     

Marker-assisted introgression of blackmold resistance QTL alleles from wild Lycopersicon cheesmanii to cultivated tomato (L. esculentum) and evaluation of QTL phenotypic effects

Blackmold, caused by the fungus Alternaria alternata, is a major ripe fruit disease of processing tomatoes. Previously, we found blackmold resistance in a wild tomato (Lycopersicon cheesmanii) and quantitative trait loci (QTL) for resistance were mapped in an interspecific population. Five QTLs were selected for introgression from L. cheesmanii into cultivated tomato using marker-assisted selection (MAS). Restriction fragment length polymorphism and PCR-based markers flanking, and within, the chromosomal regions containing QTLs were used for MAS during backcross and selfing generations. BC₁ plants heterozygous at the QTLs, and subsequent BC₁S₁ and BC₁S₂ lines possessing different homozygous combinations of alleles at the target QTLs, were identified using DNA markers. Field experiments were conducted in 1998 (with 80 marker-selected BC₁S₂ lines) and 1999 (with 151 marker-selected BC₁S₂ and BC₁S₃ lines) at three California locations. Blackmold resistance was assessed during both years, and horticultural traits were evaluated in 1999. The BC₁S₂ and BC₁S₃ lines containing L. cheesmanii alleles at the QTLs were associated with a large genetic variance for resistance to blackmold and moderate heritability, suggesting that significant genetic gain may be achieved by selection in this genetic material. L. cheesmanii alleles at three of the five introgressed QTLs showed a significant, positive effect on blackmold resistance. A QTL on chromosome 2 had the largest positive effect on blackmold resistance, alone and in combination with other QTLs, and was also associated with earliness, a positive horticultural trait. The other four QTLs were associated primarily with negative horticultural traits. Fine mapping QTLs using near isogenic lines could help determine if such trait associations are due to linkage drag or pleiotropy.     

Identification of a major quantitative trait locus (QTL) for yellow spot (<Emphasis Type="Italic">Mycovellosiella koepkei</Emphasis>) disease resistance in sugarcane

In this study we used amplified fragment length polymorphism (AFLP) and microsatellite (short sequence repeat or SSR) markers to identify a major quantitative trail locus (QTL) for yellow spot (Mycovellosiella koepkei) disease resistance in sugarcane. A bi-parental cross between a resistant variety, M 134/75, and a susceptible parent, R 570, generated a segregating population of 227 individuals. These clones were evaluated for yellow spot infection in replicated field trials in two locations across two consecutive years. A χ²-test (χ² at 98% confidence level) of the observed segregation pattern for yellow spot infection indicated a putative monogenic dominant inheritance for the trait with a 3 (resistant):1(susceptible) ratio. The AFLP and SSR markers identified 666 polymorphisms as being present in the resistant parent and absent in the susceptible one. A genetic map of M 134/75 was constructed using 557 single-dose polymorphisms, resulting in 95 linkage groups containing at least two markers based on linkages in coupling. QTL analysis using QTLCartographer v1.17d and MAPMAKER/QTL v1.1 identified a single major QTL located on LG87, flanked by an AFLP marker, actctc10, and an SSR marker, CIR12284. This major QTL, which was found to be linked at 14 cM to an AFLP marker, was detected with LOD 8.7, had an additive effect of −10.05% and explained 23.8% of the phenotypic variation of yellow spot resistance.