An intraspecific genetic map of water yam (<Emphasis Type="Italic">Dioscorea alata</Emphasis> L.) based on AFLP markers and QTL analysis for anthracnose resistance

Water yam (Dioscorea alata L.) is the most widely cultivated food yams. Despite its importance, its production is limited by anthracnose disease caused by Colletotrichum gloeosporioides (Penz.). The use of resistant yam varieties is the most reliable approach of management of this disease. The speed and precision of breeding can be improved by the development of genetic linkage maps which would provide the basis for locating and hence manipulating quantitative traits such as anthracnose resistance in breeding programmes. An F1 diploid population was developed by crossing ‘Boutou’ a female clone (with field resistance to anthracnose) with ‘Pyramide’ (susceptible). A linkage map was generated with 523 polymorphic markers from 26 AFLP primer combinations. The resulting map covered a total length of 1538 cM and included 20 linkage groups. It is the most saturated of all genetic linkage maps of yam to date. QTL analysis of anthracnose resistance was performed based on response to two isolates of C. gloeosporioides. Resistance to anthracnose appeared to be inherited quantitatively. Using a LOD significance threshold of 2.6 we identified a total of nine QTLs for anthracnose resistance. The phenotypic variance explained by each QTL ranged from 7.0 to 32.9% whereas the total amount of phenotypic variation for anthracnose resistance explained by all significant QTLs varied from 26.4 to 73.7% depending on the isolate and the variable considered. These QTLs displayed isolate-specific resistance as well as broad spectrum resistance. The availability of molecular markers linked to the QTLs of anthracnose resistance will facilitate marker-assisted selection in breeding programmes.     

Quantitative trait loci for resistance to Fusarium head blight in recombinant inbred wheat lines from the cross Huapei 57-2 / Patterson

Fusarium head blight (FHB), primarily caused by Fusarium graminearum in North America, often results in significant losses in yield and grain quality of wheat (Triticum aestivum L.). Evaluation of FHB resistance is laborious and can be affected by environmental conditions. The development of DNA markers associated with FHB quantitative trait loci (QTL) and their use in breeding programs could greatly enhance selection. The objective of this study was to identify the location and effect of QTLs for FHB resistance using simple sequence repeat (SSR) markers. A population of wheat recombinant inbred lines derived from the cross ‘Huapei57-2’/‘Patterson’ was characterized for type II resistance in one field experiment and two tests under controlled conditions in the greenhouse. Bulked segregant analysis followed by QTL mapping was used to identify the major segregating QTLs. Results indicate that ‘Huapei 57-2’ may have the same resistance allele as ‘Sumai3’ at a QTL located on the short arm of chromosome 3B. Other QTLs of lower effect size were identified on the long arm of 3Band on chromosomes 3A and 5B. Our findings along with results from other studies demonstrate that the effect of the QTL on3BS is large and consistent across a wide range of genetic backgrounds and environments. Pyramiding this QTL with other FHB QTLs using marker-assisted selection should be effective in improving FHB resistance in a wheat breeding program. This revised version was published online in August 2006 with corrections to the Cover Date.     

Marker selection for <Emphasis Type="Italic">Fusarium</Emphasis> head blight resistance based on quantitative trait loci (QTL) from two European sources compared to phenotypic selection in winter wheat

Fusarium head blight (FHB) infects all cereals including maize and is considered a major wheat disease, causing yield losses and mycotoxin contamination. This study aimed to compare the realized selection gain from marker and phenotypic selection in European winter wheat. A double cross (DC) combined three FHB resistance donor-QTL alleles (Qfhs.lfl-6AL and Qfhs.lfl-7BS from ‘Dream’, and one QTL on chromosome 2BL from ‘G16-92’) with two high yielding, susceptible winter wheats, ‘Brando’ and ‘LP235.1’. The base population of 600 DC derived F₁ lines was on one hand selected for the respective QTLs by SSR markers (marker-selected cycle, CM), resulting in 35 progeny possessing different combinations of beneficial donor-QTL alleles. On the other hand it was selected phenotypically, only by FHB rating, and the best 20 lines were recombined and selfed (phenotypically selected cycle, CP). The variants CP, CM, and an unselected variant (C0) were tested at four locations by inoculation of Fusarium culmorum. Resistance was measured as the mean of multiple FHB ratings (0–100%). FHB severity was reduced through both phenotypic and marker selection by 6.2 vs. 5.0%, respectively. On a per-year basis, marker selection by 2.5% was slightly superior to phenotypic selection with 2.1%, because the first variant saved 1 year. Marker-selected lines were on average 8.6 cm taller than phenotypically selected lines. A high genetic variation within the marker-selected variant for FHB resistance and the high effect of a resistance-QTL allele on straw length indicate that additional phenotypic selection will further enhance selection gain.     

Molecular mapping of quantitative trait loci (QTLs) controlling aluminium tolerance in bread wheat

Aluminium (Al) toxicity is a major constraint to crop productivity in acidic soils. A quantitative trait locus (QTL) analysis was performed to identify the genetic basis of Al tolerance in the wheat cultivar ‘Chinese Spring’. A nutrient solution culture approach was undertaken with the root tolerance index (RTI) and hematoxylin staining method as parameters to assess the Al tolerance. Using a set of D genome introgression lines, a major Al tolerance QTL was located on chromosome arm 4DL, explaining 31% of the phenotypic variance present in the population. A doubled haploid population was used to map a second major Al tolerance QTL to chromosome arm 3BL. This major QTL (Qalt _CS .ipk-3B) in ‘Chinese Spring’ accounted for 49% of the phenotypic variation. Linkage of this latter QTL to SSR markers opens the possibility to apply marker-assisted selection (MAS) and pyramiding of this new QTL to improve the Al tolerance of wheat cultivars in breeding programmes.     

Chromosomal regions associated with green plant regeneration in wheat (Triticum aestivum L.) anther culture

Genetic capacity for green plant regeneration in anther culture were mapped in a population comprising 50 doubled haploid lines from a cross between two wheat varieties ‘Ciano’ and ‘Walter’ with widely different capacity for green plant regeneration. Bulked segregant analysis with AFLP markers and composite interval mapping detected four QTLs for green plant percentage on chromosomes 2AL (QGpp.kvl-2A), 2BL (QGpp.kvl-2B.1 and QGpp.kvl-2B.2) and 5BL (QGpp.kvl-5B).The three QTLs detected on chromosome 2AL and 2BL all derived their alleles favouring green plant formation from the responsive parent ‘Ciano’.The remaining QTL on chromosome 5BL had the allele favouring green plants from the low responding parent ‘Walter’. In a multiple regression analysis the four QTLs could explain a total of 80% of the genotypic variation for green plant percentage. None of the chromosomal regions with QTLs for green plant percentage showed significant influence on either embryo formation or regeneration frequencies from the anther culture. The three major QTLs located on group two chromosomes were fixed in a second DH population derived from two parents ‘Ciano’ and ‘Benoist’,both with high capacity to produce green plants. A QTL explaining31.5% of the genetic variation for green plant formation were detected on chromosome 5BL in this cross as well. This revised version was published online in July 2006 with corrections to the Cover Date.     

QTL mapping for capsaicin and dihydrocapsaicin content in a population of Capsicum annuum ‘NB1’ × Capsicum chinense ‘Bhut Jolokia’

Capsaicinoids are pungent compounds used for industrial and medical purposes including food, medicine and cosmetics. The Indian local variety ‘Bhut Jolokia’ (Capsicum chinense Jacq.) is one of the world's hottest chilli peppers. It produces more than one million Scoville heat units (SHUs) in total capsaicinoids. In this study, our goal was to identify quantitative trait loci (QTLs) responsible for the high content of capsaicin and dihydrocapsaicin in ‘Bhut Jolokia’. Capsicum annuum ‘NB1’, a Korean pepper inbred line containing 14 000 SHUs, was used as a maternal line. An F₂ population derived by crossing between ‘NB1’ and ‘Bhut Jolokia’ was generated to map QTLs for capsaicinoids content. A total of 234 markers, including 201 HRM, 21 SSR, 2 CAPS and 10 gene‐based markers of the capsaicinoid synthesis pathway, were mapped. The final map covered a total distance of 1175.2 cM and contained 12 linkage groups corresponding to the basic chromosome number of chilli pepper. Capsaicin and dihydrocapsaicin content were analysed in 175 F₂ pepper fruits using the HPLC method. The maximum total capsaicinoids content was 1389 mg per 100g DW (dry weight), and the minimum content was 11 mg per 100g DW. Two QTLs (qcap3.1 and qcap6.1) for capsaicin content were identified on LG3 and LG6, and two QTLs (qhdc2.1 and qdhc2.2) for dihydrocapsaicin content were located on LG2. We did not detect QTLs for total capsaicinoids content. The QTL positions for capsaicin content were different from those for dihydrocapsaicin content. These results indicate that the complexity of selecting for more pungent chilli peppers must be considered in a chilli pepper breeding programme. The QTL‐linked markers identified here will be helpful to develop more pungent pepper varieties from ‘Bhut Jolokia’, a very hot pepper.     

Using molecular markers to pyramid genes for resistance to ascochyta blight and anthracnose in lentil (Lens culinaris Medik)

Ascochyta blight caused by the fungus Ascochyta lentis Vassilievsky and anthracnose caused by Colletotrichum truncatum [(Schwein.) Andrus & W.D. Moore] are the most destructive diseases of lentil in Canada. The diseases reduce both seed yield and seed quality. Previous studies demonstrated that two genes, ral1 and AbR1, confer resistance toA. lentis and a major gene controls the resistance to 95B36 isolate of C. truncatum. Molecular markers linked to each gene have been identified. The current study was conducted to pyramid the two genes for resistance to ascochyta blight and the gene for resistance to anthracnose into lentil breeding lines. A population (F_6:7) consisting of 156 recombinant inbred lines (RILs) was developed from across between ‘CDC Robin’ and a breeding line ‘964a-46’. The RILs were screened for reaction to two isolates (A1 and 3D2) ofA. lentis and one isolate (95B36) ofC. truncatum. χ² analysis of disease reactions demonstrated that the observed segregation ratios of resistant versus susceptible fit the two gene model for resistance to ascochyta blight and a single gene model for resistance to anthracnose. Using markers linked to ral1 (UBC 227₁₂₉₀), to AbR1(RB18₆₈₀) and to the major gene for resistance to anthracnose (OPO6₁₂₅₀),respectively, we confirmed that 11 RILs retained all the three resistance genes. More than 82% of the lines that had either or both RB18₆₈₀ and UBC227₁₂₉₀markers were resistant to 3D2 isolate and had a mean disease score lower than 2.5. By contrast, 80% of the lines that had none of the RAPD markers were susceptible and had a mean disease score of 5.8. For the case of A1 isolate of A. lentis, more than 74% of the lines that carriedUBC227₁₂₉₀ were resistant, whereas more than 79% of the lines that do not have the marker were susceptible. The analysis of the RILs usingOPO6₁₂₅₀ marker demonstrated that 11out of 72 resistant lines carried the marker, whereas 66 out of 84 susceptible lines had the marker present. Therefore, selecting materials with both markers for resistance to ascochyta blight and a marker for resistance to anthracnose can clearly make progress toward resistance in the population. This revised version was published online in July 2006 with corrections to the Cover Date.     

Identification of molecular markers associated with sugar-related traits in a <Emphasis Type="Italic">Saccharum</Emphasis> interspecific cross

The cultivated sugarcane (Saccharum spp. hybrids, 2n = 100–130) is one crop for which interspecific hybridization involving wild germplasm has provided a major breakthrough in its improvement. Few clones were used in the initial hybridization event leading to a narrow genetic base for continued cultivar development. Molecular breeding would facilitate the identification and introgression of novel alleles/genes from the wild germplasm into cultivated sugarcane. We report the identification of molecular markers associated with sugar-related traits using an F₁ population derived from a cross between S. officinarum ‘Louisiana Striped’ × S. spontaneum ‘SES 147B’, the two major progenitor species of cultivated sugarcane. Genetic linkage maps of the S. officinarum and S. spontaneum parents were produced using the AFLP, SRAP and TRAP molecular marker techniques. The mapping population was evaluated for sugar-related traits namely, Brix (B) and pol (P) at the early (E) and late (L) plant growing season in the plant cane (04) and first ratoon (05) crops (04EB, 04LB, 04LP, 05EB and 05EP). For S. officinarum, combined across all the traits, a total of 30 putative QTLs was observed with LOD scores ranging from 2.51 to 7.48. The phenotypic variation (adj. R²) explained by all QTLs per trait ranged from 22.1% (04LP) to 48.4% (04EB). For S. spontaneum, a total of 11 putative QTLs was observed with LOD scores ranging from 2.62 to 4.70 and adj. R² ranging from 9.3% (04LP) to 43.0% (04LB). Nine digenic interactions (iQTL) were observed in S. officinarum whereas only three were observed in S. spontaneum. About half of the QTLs contributed by both progenitor species were associated with effects on the trait that was contrary to expectations based on the phenotype of the parent contributing the allele. Quantitative trait loci and their associated effects were consistent across crop-years and growing seasons with very few QTLs being unique to the early season. When the data were reanalyzed using the non-parametric discriminant analysis (DA) approach, significant marker-trait associations were detected for markers that were either identical to or in the vicinity of markers previously identified using the traditional QTL approach. Discriminant analysis also pointed to previously unidentified markers some of which remained unlinked on the map. These preliminary results suggest that DA could be used as a complementary approach to traditional QTL analysis in a crop like sugarcane for which saturated linkage maps are unavailable or difficult to obtain.     

QTL mapping of powdery mildew susceptibility in hop (Humulus lupulus L.)

Hop powdery mildew [Podosphaera macularis (Wallr.) U. Braun & S. Takam.] is best controlled via the production of resistant varieties. Recent evidence supports selection against plant susceptibility genes to fungal pathogens as a more durable resistance mechanism than selection for resistance genes. The objective of this study was to identify molecular-based QTLs, their genetic effects and epistasis among QTLs associated with susceptibility to powdery mildew. Parents and offspring from the cross, ‘Perle’ × ‘USDA 19058M’, were clonally replicated and inoculated in a greenhouse using a CRD experimental design in Corvallis, OR. DNA was extracted, purified and analyzed via three different marker systems. Analysis of the resulting markers was based upon the “two-way pseudo-testcross” procedure. QTL mapping using multiple interval mapping and Bayesian interval mapping analyses were performed using WinQTL Cartographer 2.5_003. Comparison amongst mapping analyses identified three persistent QTLs on three linkage groups without significant epistatic effect upon expression. The persistent QTL on linkage group C7 had both additive and dominant effects controlling phenotype expression. The presence or absence of the two AFLP markers bordering the QTL on C7 defined susceptibility in offspring. This is the first report in hop identifying molecular markers linked to QTLs associated with disease susceptibility.     

Identification of QTLs for stay green trait in wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) in the ‘Chirya 3’ × ‘Sonalika’ population

Stay green or delayed senescence is considered to play a crucial role in grain development in wheat when assimilates are limited. We identified three QTLs for stay green on the chromosomes 1AS, 3BS and 7DS using a recombinant inbred (RI) population developed by making crosses between the stay green parent ‘Chirya 3’ and non-stay green ‘Sonalika’. The RI lines were evaluated in natural field conditions for 2 years in replicated trial. The QTL on chromosome 1A was identified in both the years, while the QTLs on 3BS and 7DS were identified only in 1st and 2nd year, respectively. The QTLs explained up to 38.7% of phenotypic variation in a final simultaneous fit. The alleles for higher stay green values derived from the stay green parent ‘Chirya 3’. The QTLs were named as QSg.bhu-1A, QSg.bhu-3B and QSg.bhu-7D. The QTL QSg.bhu-3B and QSg.bhu-7D were placed in the 3BS9-0.57-0.78 and 7DS5-0.36-0.61 deletion bins, respectively.     

Identification of quantitative trait loci associated with resistance to foliar diseases in sorghum [<Emphasis Type="Italic">Sorghum bicolor</Emphasis> (L.) Moench]

Forage sorghum cultivars grown in India are susceptible to various foliar diseases, of which anthracnose, rust, zonate leaf spot, drechslera leaf blight and target leaf spot cause severe damage. We report here the quantitative trait loci (QTLs) conferring resistance to these foliar diseases. QTL analysis was undertaken using 168 F₇ recombinant inbred lines (RILs) of a cross between a female parental line 296B (resistant) and a germplasm accession IS18551 (susceptible). RILs and parents were evaluated in replicated field trials in two environments. A total of twelve QTLs for five foliar diseases on three sorghum linkage groups (SBI-03, SBI-04 and SBI-06) were detected, accounting for 6.9–44.9% phenotypic variance. The morphological marker Plant color (Plcor) was associated with most of the QTL across years and locations. The QTL information generated in this study will aid in the transfer of foliar disease resistance into elite susceptible sorghum breeding lines through marker-assisted selection.     

A quantitative trait locus influencing tolerance to Phytophthora root rot in the soybean cultivar ‘Conrad’

‘Conrad’, a soybean cultivar tolerant to Phytophthora root rot (PRR), and ‘OX760-6-1’, a breeding line with low tolerance to PRR, were crossed. F₂ derived recombinant inbred lines were advanced to F₆ to generate a population through single-seed descent. This population was used to identify quantitative trait loci (QTLs) influencing PRR tolerance in ‘Conrad’. A total of 99 simple sequence repeat (SSR), or microsatellite, markers that were polymorphic and clearly segregated in the F₆ mapping population were used for QTL detection. Based on the data of PRR in the field at two planting locations, Woodslee and Weaver, for the years 2000 and 2001, one putative QTL, designated as Qsatt414-596, was detected using MapMaker/QTL. Qsatt414-596 was flanked by two SSR markers from the linkage group MLG J, Satt414 and Satt596. Satt414 and Satt596 were also detected to be significantly (P < 0.005) associated with PRR using the SAS GLM procedure and were estimated to explain 13.7% and 21.5% of the total phenotypic variance, respectively.     

Inheritance of resistance to <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">melonis</Emphasis> races 0 and 2 in melon accession Tortuga

The inheritance of the resistance to Fusarium oxysporum f. sp. melonis (F.o.m.) races 0 and 2 in ‘Tortuga’, a Spanish cantalupensis accession, was studied from crosses of ‘Tortuga’ by the susceptible line ‘Piel de Sapo’ and the resistant one ‘Charentais-Fom1’ that carries the resistance gene Fom-1. The segregation patterns observed in the F₂ (‘Tortuga’ × ‘Piel de Sapo’) and the backcross (‘Piel de Sapo’ × (‘Tortuga’ × ‘Piel de Sapo’) populations, suggest that resistance of ‘Tortuga’ to races 0 and 2 of F.o.m. is conferred by two independent genes: one dominant and the other recessive. In the F2 derived from the cross between accessions ‘Tortuga’ and ‘Charentais-Fom1’, the lack of susceptible plants indicated that the two accessions are carrying the same resistance gene (Fom-1). The analysis of 158 F₂ plants (‘Tortuga’ × ‘Piel de Sapo’) with a Cleaved Amplified Polymorphic Sequence marker 618-CAPS, tightly linked to Fom-1 (0.9 cM), confirmed that ‘Tortuga’ also carries a recessive gene, that we propose to symbolize by fom-4.     

Molecular mapping of cereal cyst nematode resistance in <Emphasis Type="Italic">Triticum monococcum</Emphasis> L. and its transfer to the genetic background of cultivated wheat

Triticum monococcum, the diploid A genome species, harbours enormous variability for resistance to biotic stresses. A spring type T. monococcum acc. 14087 was found to be resistant to Heterodera avenae (cereal cyst nematode, CCN). A recombinant inbred line population (RIL) developed by crossing this accession with a CCN susceptible T. boeoticum acc. 5088 was used for studying the inheritance and map location of the CCN resistance. Based on composite interval mapping two QTL, one each on chromosome 1AS and 2AS, were detected. The QTL on 1A, designated as Qcre.pau-1A, appeared to be a major gene with 26% contribution to the overall phenotypic variance whereas the QTL on 2A designated as Qcre.pau-2A contributed 13% to total phenotypic variation. Qcre.pau-1A is novel, being the only CCN resistance gene mapped in any ‘A’ genome species and none of the other known genes have been mapped on chromosome 1A. The QTL Qcre.pau-2A might be allelic to Cre5, a CCN resistance gene transferred from Ae. ventricosa and mapped on 2AS. The Qcre.pau-1A was transferred to cultivated wheat using T. durum cv. PBW114 as the bridging species. Selected CCN resistant F₈ lines showed introgression for the molecular markers identified to be linked with CCN resistance locus Qcre.pau-1A. Thus, this gene alone could impart complete resistance against CCN. These introgression lines can be used for marker-assisted transfer of Qcre.pau-1A to elite wheat cultivars.     

Mapping QTL tolerance to <Emphasis Type="Italic">Phytophthora</Emphasis> root rot in soybean using microsatellite and RAPD/SCAR derived markers

Broad tolerance to phytophthora root rot (PRR) caused by Phytophthora sojae has become an important goal for the improvement of soybean (Glycine max) because of the rapid spread of races that defeat the available resistance genes. The aim of this research was to identify the location of quantitative trait loci (QTL) in ‘Conrad’, a soybean cultivar with broad tolerance to many races of P. sojae. A PRR susceptible breeding line ‘OX760-6-1’was crossed with Conrad. Through single-seed-descent, 112, F₂ derived, F₇ recombinant inbred lines (RILs) were advanced. A total of 39 random amplified polymorphic DNA bands (RAPDs) and 89 type 1 microsatellite (simple sequence repeat; SSR) markers were used to construct a genetic linkage map. In the greenhouse, RILs were inoculated with four P. sojae isolates (three from China and one from Canada). Disease was measured as the percent of dead plants 20 days after germination in P. sojae inoculated vermiculite in the greenhouse. Three QTLs (QGP1, QGP2, QGP3) for PRR tolerance in the greenhouse were detected using WinQTLCart 2.0 with a log-likelihood (LOD) score 27.14 acquired through permutations (1,000 at P ≤ 0.05). QGP1 (near Satt509) was located at linkage group F and explained 13.2%, 5.9%, and 6.7% of the phenotypic variance for tolerance to the JiXi, JianSanJiang and ShuangYaShan isolates, respectively. QGP2 (near Satt334) was located in a different interval on linkage group F and explained 5.1% and 2.4% of the phenotypic variance for JiXi and ShuangYaShan isolates, respectively. QGP3 was located on linkage group D1b + W (near OPL18₈₀₀/SCL18₆₅₉) and explained 10.2% of the phenotypic variance for Woodslee isolate. QGP1 and QGP2 appeared to be associated with PRR tolerance across a range of isolates but QGP3 was active only against the Woodslee isolate. At Woodslee and Weaver (in Ontario) in 2000, the interval associated with QGP3 explained 21.6% and 16.7% of phenotypic variance in resistance to PRR, respectively and was referred as QFP1. The identified QTLs would be beneficial for marker assistant selection of PRR tolerance varieties against both China and North America P. sojae races. Yingpeng Han and Weili Teng have equal contribution to the paper.     

Inheritance of resistance to anthracnose caused by Colletotrichum capsici in Capsicum

Inheritance of resistance to anthracnose caused by Colletotrichum capsici (Syd.) Butler & Bisby was studied in interspecific Capsicum populations derived from a cross between a Thai elite cultivar Capsicum annuum L.‘Bangchang’ and a resistant line C. chinense Jacq.‘PBC932′. The resistance was assessed by measuring lesion area per fruit area (LFA) on detached chili fruits, using a laboratory‐based injection inoculation. Nil symptoms resembling the resistant parent ‘PBC932’ were also identified in the progeny F₂ and BC1 populations. Segregation of resistance (nil LFA) and susceptibility in the F₂ fitted a 1: 3 Mendelian ratio, indicating that resistance was responsible by a single recessive gene. The segregation of the trait in the testcrosses in both BC1s also confirmed the 1: 3 gene segregating model as found in the F₂.     

Resistance to <Emphasis Type="Italic">Meloidogyne incognita</Emphasis> race 1 in the lettuce cultivars Grand Rapids and Salinas-88

The objective of this work was to check the possible allelism between two sources of resistance to the root-knot nematode Meloidogyne incognita race 1 in lettuce (‘Grand Rapids’ and ‘Salinas-88’). The experiments were carried out in greenhouses, in expanded 128-cell polystyrene trays filled with commercial substrate. Lettuce cultivars ‘Salinas 88’ and ‘Grand Rapids’ were tested along with the populations F₁ (‘Grand Rapids’ × ‘Salinas-88’), F₂ (‘Grand Rapids’ × ‘Salinas-88’), F₃ (‘Grand Rapids’ × ‘Salinas-88’), and with F₄ families derived from the latter population. Seedlings were inoculated 15 days after sowing with a nematode egg suspension equivalent to 30 eggs ml⁻¹ of substrate. Plants were evaluated for apparent gall incidence, gall scores, egg mass scores and extracted egg numbers 45 days after the inoculation date. There was evidence that two different genes are involved in control of resistance to M. incognita race 1 in lettuce cultivars Grand Rapids and Salinas-88. Lines with higher levels of nematode resistance than either Grand Rapids or Salinas-88 could be selected in the F4 generation of the cross between these resistant parental lines.     

Quantitative trait loci analysis of citrus leprosis resistance in an interspecific backcross family of (<Emphasis Type="Italic">Citrus reticulata</Emphasis> Blanco × <Emphasis Type="Italic">C. sinensis</Emphasis> L. Osbeck) × <Emphasis Type="Italic">C. sinensis</Emphasis> L. Osb

Leprosis, caused by citrus leprosis virus (CiLV) and transmitted by the tenuipalpid mite Brevipalpus phoenicis, is one of the most important viruses of citrus in the Americas. Sweet oranges (Citrus sinensis L. Osb.) are highly susceptible to CiLV, while mandarins (C. reticulata Blanco) and some of their hybrids have higher tolerance or resistance to this disease. The mechanisms involved in the resistance and its inheritance are still largely unknown. To study the quantitative trait loci (QTL; quantitative trait loci) associated with the resistance to CiLV, progeny analyses were established with 143 hybrid individuals of ‘Pêra’ sweet orange (C. sinensis L. Osb.) and ‘Murcott’ tangor (C. reticulata Blanco × C. sinensis L. Osb.) from controlled crossings. Disease assessment of the hybrid individuals was conducted by infesting the plants with viruliferous mites in the field. The experiment consisted of a randomized completely block design with ten replicates. The evaluated phenotypic traits were incidence and severity of the disease on leaves and branches, for a period of 3 years. The MapQTL™ v.4.0 software was used for the identification and location of possible QTL associated with resistance to CiLV on a genetic map obtained from 260 AFLP and 5 RAPD markers. Only consistent QTLs from different phenotypic traits and years of evaluation, with the critical LOD scores to determine the presence or absence of each QTL calculated through the random permutation test, were considered. A QTL was observed and had a significant effect on the phenotypic variation, ranging from 79.4 to 84% depending on which trait (incidence or severity) was assessed. This suggests that few genes are involved in the genetic resistance of citrus to CiLV.     

Inheritance of adult-plant resistance to Phytophthora capsici in pepper

Summary Inheritance studies were conducted to determine the genetic basis of adult-plant resistance in pepper (Capsicum annuum L.) to Phytophthora capsici. F₁, backcrosses and F₂ populations were developed using the resistant parent Criollo de Morellos 334 and susceptible parents Agronômico 10-G and Yolo Wonder. Pepper plants, at 36 days post-emergence, were inoculated near the base of the stem with an inoculum suspension of 5×10⁴ zoospores/ml. Segregation ratios in the F₂ generation of 13 resistant to 3 susceptible plants fit a 2-gene model for resistance with dominant and recessive epistasis.     

Discovery of single nucleotide polymorphism in <Emphasis Type="Italic">Capsicum</Emphasis> and SNP markers for cultivar identification

Molecular markers based on single nucleotide polymorphisms (SNPs) are abundant and evenly distributed in a whole genome enough to distinguish individuals in a population. In recent years, sets of SNP markers have been designed and applied for cultivar identification in various crop species. This paper is the first to report the development of a panel of SNP markers for variety identification in peppers. We used conserved ortholog set II (COSII) markers developed from conserved unigenes between tomato and Arabidopsis to identify SNPs in peppers. We tested 438 COSII primer sets amplified as single PCR products out of a total 600 COSII primer sets. Among the 438 COSII primers, 170 primer sets (38.8%) showed polymorphisms between Capsicum annuum ‘RNaky (RN)’and C. chinense ‘PI 159234 (234)’. In contrast, only 48 primer sets (11.0%) out of 438 primers sets were polymorphic between C. annuum ‘Perennial (PER), and ‘Dempsey (DEMP)’. The average frequency of SNPs plus InDels between C. annuum and C. chinense was 1/189 bp and between C. annuum spp. was 1/948 bp. Primer sets showing SNP between C. annuum PER and DEMP were re-designed to Allele Specific PCR (AS-PCR) primers and we finally selected a total of 40 SNP markers for cultivar identification. As the result, we were able to discriminate 97.5% of the 81 commercial hot cultivars and 100% of the 17 sweet pepper cultivars. We conclude the paper by discussing the use of the SNP marker set for cultivar identification and other applications.