Dynamic gene action at QTLs for resistance to Setosphaeria turcica in maize

 Cultivars with quantitative resistance are widely used to control Setosphaeria turcica (Luttrell) Leonard & Suggs, the causal organism of northern corn leaf blight (NCLB). Here the effectiveness of quantitative trait loci (QTLs) for NCLB resistance was investigated over the course of host plant development in inoculated field trials. A population of 194–256 F_2:3 lines derived from a cross between a susceptible Italian (Lo951) and a highly resistant African inbred line (CML202) was tested in three environments in Kenya. The traits assessed were the incubation period (IP), the percentage disease severity (DS 1 to 5, taken biweekly), and the area under the disease progress curve (AUDPC). Considering all resistance traits and environments, a total of 19 putative QTLs were detected by composite interval mapping using a linkage map with 110 RFLP markers. In the combined analysis across environments, nine QTLs were significant (LOD >3.0) for DS 3, recorded around flowering time, explaining 71% of the genotypic variance. Four of these nine QTLs displayed significant (P<0.05) QTL×environment (QTL×E) interaction. Most QTLs were already significant in the juvenile stage (IP) and became less effective after flowering. Across environments, three QTLs conditioned adult-plant resistance, in the sense that they were only significant after flowering. Six QTL alleles on chromosomes 2, 4, 5, 8, and 9 of CML202 should be useful for marker-assisted backcrossing. Received: 24 August 1998 / Accepted: 29 September 1998     

Identification of molecular markers linked to quantitative trait loci for soybean resistance to corn earworm

 One hundred and thirty nine restriction fragment length polymorphisms (RFLPs) were used to construct a soybean (Glycine max L. Merr.) genetic linkage map and to identify quantitative trait loci (QTLs) associated with resistance to corn earworm (Helicoverpa zea Boddie) in a population of 103 F₂-derived lines from a cross of ‘Cobb’ (susceptible) and PI229358 (resistant). The genetic linkage map consisted of 128 markers which converged onto 30 linkage groups covering approximately 1325 cM. There were 11 unlinked markers. The F₂-derived lines and the two parents were grown in the field under a plastic mesh cage near Athens, Ga., in 1995. The plants were artificially infested with corn earworm and evaluated for the amount of defoliation. Using interval-mapping analysis for linked markers and single-factor analysis of variance (ANOVA), markers were tested for an association with resistance. One major and two minor QTLs for resistance were identified in this population. The PI229358 allele contributed insect resistance at all three QTLs. The major QTL is linked to the RFLP marker A584 on linkage group (LG) ‘M’ of the USDA/Iowa State University public soybean genetic map. It accounts for 37% of the total variation for resistance in this cross. The minor QTLs are linked to the RFLP markers R249 (LG ‘H’) and Bng047 (LG ‘D1’). These markers explain 16% and 10% of variation, respectively. The heritability (h²) for resistance was estimated as 64% in this population. Received: 15 October 1997 / Accepted: 4 November 1997     

QTL Analysis for Root Protein in a Backcross Family of Cassava Derived from Manihot esculenta ssp flabellifolia

Root protein content of elite cassava is very low, largely due to breeder’s selection for other agronomic traits mainly fresh weight yield and disease resistance. Increased protein content in the root of cassava will improve its usefulness as a more complete food source in the developing world. An inter-specific F₁ hybrid CW 198 - 11 was earlier developed at International Center for Tropical Agriculture (CIAT), Cali, Colombia by genetic crosses of OW 230 - 1 (FLA 441 - 5) and CW 30–65 (an inter-specific hybrid between an improved cassava variety SG 427 - 87 and an accession of Manihot esculenta ssp flabellifolia (MESCFLAX – 80)). The inter-specific cross was ‘backcrossed’, in the sense of another cross to cassava (MTAI – 8) to generate a B₁P₂ family with 225 progenies in which major quantitative trait loci (QTL) for root protein in the backcross population of cassava were identified. A linkage map from the female parent of the backcross population was used for QTL detection. A total of three QTL (protg.7, protg.13 and protg.23) controlling protein were identified in three different environments. One QTL was expressed across all three environments. These results demonstrated high broad sense heritability of 61.6% for protein over 2 years, in two different locations. The individual effects of alleles at these QTL explained from 15% to 25% of the phenotypic variance. The consistency of QTL controlling protein across environments reveals their potential for use in marker-assisted recurrent selection.     

Identification of QTLs for resistance to powdery mildew and SSR markers diagnostic for powdery mildew resistance genes in melon (Cucumis melo L.)

Powdery mildew caused by Podosphaera xanthii is an important foliar disease in melon. To find molecular markers for marker-assisted selection, we constructed a genetic linkage map of melon based on a population of 93 recombinant inbred lines derived from crosses between highly resistant AR 5 and susceptible ‘Earl’s Favourite (Harukei 3)’. The map spans 877 cM and consists of 167 markers, comprising 157 simple sequence repeats (SSRs), 7 sequence characterized amplified region/cleavage amplified polymorphic sequence markers and 3 phenotypic markers segregating into 20 linkage groups. Among them, 37 SSRs and 6 other markers were common to previous maps. Quantitative trait locus (QTL) analysis identified two loci for resistance to powdery mildew. The effects of these QTLs varied depending on strain and plant stage. The percentage of phenotypic variance explained for resistance to the pxA strain was similar between QTLs (R ² = 22–28%). For resistance to pxB strain, the QTL on linkage group (LG) XII was responsible for much more of the variance (41–46%) than that on LG IIA (12–13%). The QTL on LG IIA was located between two SSR markers. Using an independent population, we demonstrated the effectiveness of these markers. This is the first report of universal and effective markers linked to a gene for powdery mildew resistance in melon.     

Identification of quantitative trait loci for resistance to <Emphasis Type="Italic">Xanthomonas campestris</Emphasis> pv. <Emphasis Type="Italic">campestris</Emphasis> in <Emphasis Type="Italic">Brassica rapa</Emphasis>

Resistance to six known races of black rot in crucifers caused by Xanthomonas campestris pv. campestris (Pammel) Dowson is absent or very rare in Brassica oleracea (C genome). However, race specific and broad-spectrum resistance (to type strains of all six races) does appear to occur frequently in other brassica genomes including B. rapa (A genome). Here, we report the genetics of broad spectrum resistance in the B. rapa Chinese cabbage accession B162, using QTL analysis of resistance to races 1 and 4 of the pathogen. A B. rapa linkage map comprising ten linkage groups (A01–A10) with a total map distance of 664 cM was produced, based on 223 AFLP bands and 23 microsatellites from a F₂ population of 114 plants derived from a cross between the B. rapa susceptible inbred line R-o-18 and B162. Interaction phenotypes of 125 F₂ plants were assessed using two criteria: the percentage of inoculation sites in which symptoms developed, and the severity of symptoms per plant. Resistance to both races was correlated and a cluster of highly significant QTL that explained 24–64% of the phenotypic variance was located on A06. Two additional QTLs for resistance to race 4 were found on A02 and A09. Markers closely linked to these QTL could assist in the transference of the resistance into different B. rapa cultivars or into B. oleracea.     

Mapping of one major gene and of QTLs involved in resistance to clubroot in Brassica napus

Clubroot, caused by Plasmodiophora brassicae, is a damaging disease of Brassica napus. Genetic control and mapping of loci involved in high and partial quantitative resistance expressed against two single spore isolates (Pb137–522 and K92–16) were studied in the F₁ and DH progenies of the cross Darmor-bzh (resistant) x Yudal (susceptible). The high level of resistance expressed by Darmor-bzh to isolate Pb137–522 was found to be mainly due to a major gene, which we have named Pb-Bn1, located on linkage group (LG) DY4. Partial quantitative resistance showed by Darmor-bzh to the K92–16 isolate arose from the association of at least two additive QTLs detected on LGs DY4 and DY15; the QTL on DY4, explaining 19% of the variance, was mapped at the same position as the major gene Pb-Bn1. Epistatic interactions between nine regions with or without additive effects were detected. The total phenotypic variation accounted for by additive and epistatic QTLs ranged from 62% to 81% depending on the isolate. For one isolate, the relative effect due to additivity was similar to that due to epistasis. Received: 10 November 1999 / Accepted:18 February 2000     

Molecular mapping for resistance to pea rust caused by <Emphasis Type="Italic">Uromyces fabae</Emphasis> (Pers.) de-Bary

Pea rust caused by Uromyces fabae (Pers.) de-Bary is a major problem in warm humid regions causing huge economic losses. A mapping population of 136 F_6:7 recombinant inbred lines (RILs) derived from the cross between pea genotypes, HUVP 1 (susceptible) and FC 1 (resistant) was evaluated in polyhouse as well as under field conditions during two consecutive years. Infection frequency (IF) and area under disease progress curve (AUDPC) were used for evaluation of rust reaction of the RILs. A linkage map was constructed with 57 polymorphic loci selected from 148 simple sequence repeats (SSRs), 3 sequence tagged sites (STS), and 2 random amplified polymorphic (RAPD) markers covering 634 cM of genetic distance on the seven linkage groups of pea with an average interval length of 11.3 cM. Composite interval mapping (CIM) revealed one major (Qruf) and one minor (Qruf1) QTL for rust resistance on LGVII. The LOD (5.2–15.8) peak for Qruf was flanked by SSR markers, AA505 and AA446 (10.8 cM), explaining 22.2–42.4% and 23.5–58.8% of the total phenotypic variation for IF and AUDPC, respectively. The minor QTL was environment-specific, and it was detected only in the polyhouse (LOD values 4.2 and 4.8). It was flanked by SSR markers, AD146 and AA416 (7.3 cM), and explained 11.2–12.4% of the total phenotypic variation. The major QTL Qruf was consistently identified across all the four environments. Therefore, the SSR markers flanking Qruf would be useful for marker-assisted selection for pea rust (U. fabae) resistance.     

QTL analysis of field resistance to Xanthomonas axonopodis pv. manihotis in cassava

We evaluated cassava bacterial blight (CBB) infection in an pair-cross population of 150 individuals derived from an intra-specific cross between two non-inbred cassava (Manihot esculenta Crantz) lines. The replicated trials were carried out in the field under high disease pressure over two consecutive crop cycles. Evaluations were conducted at 4 and 7 months after planting for the two cycles. Simple regression analysis and the nonparametric Kruskal-Wallis rank-sum test revealed that eight quantitative trait loci (QTLs) were involved in resistance. We detected changes in QTLs from crop cycle to crop cycle. The pathogen population (Xanthomonas axonopodis pv. manihotis) was also monitored over the period, using a restriction fragment length polymorphism probe and pathogenic tests. Changes in QTL detection over the 2 years could be correlated with changes in pathogen population structure. One QTL, located in linkage group D, was conserved over the two crop cycles, and in field to greenhouse evaluations. This study thus identified molecular markers useful for marker assisted-selection, a technique that can accelerate the long, multiple-season process of breeding for CBB resistance. Received: 1 January 2000 / Accepted: 25 June 2000     

Genetic mapping reveals a single major QTL for bacterial wilt resistance in Italian ryegrass (Lolium multiflorum Lam.)

Bacterial wilt caused by Xanthomonas translucens pv. graminis (Xtg) is a major disease of economically important forage crops such as ryegrasses and fescues. Targeted breeding based on seedling inoculation has resulted in cultivars with considerable levels of resistance. However, the mechanisms of inheritance of resistance are poorly understood and further breeding progress is difficult to obtain. This study aimed to assess the relevance of the seedling screening in the glasshouse for adult plant resistance in the field and to investigate genetic control of resistance to bacterial wilt in Italian ryegrass (Lolium multiflorum Lam.). A mapping population consisting of 306 F₁ individuals was established and resistance to bacterial wilt was assessed in glasshouse and field experiments. Highly correlated data (r = 0.67–0.77, P < 0.01) between trial locations demonstrated the suitability of glasshouse screens for phenotypic selection. Analysis of quantitative trait loci (QTL) based on a high density genetic linkage map consisting of 368 amplified fragment length polymorphism (AFLP) and simple sequence repeat (SSR) markers revealed a single major QTL on linkage group (LG) 4 explaining 67% of the total phenotypic variance (Vp). In addition, a minor QTL was observed on LG 5. Field experiments confirmed the major QTL on LG 4 to explain 43% (in 2004) to 84% (in 2005) of Vp and also revealed additional minor QTLs on LG 1, LG 4 and LG 6. The identified QTLs and the closely linked markers represent important targets for marker-assisted selection of Italian ryegrass.     

QTL analysis of resistance to Fusarium head blight in the novel wheat germplasm CJ 9306. I. Resistance to fungal spread

Fusarium head blight (FHB or scab) caused by Fusarium species is a destructive disease in wheat and barley worldwide. The objectives of our study were to identify quantitative trait loci (QTLs) for resistance to FHB spread (Type II resistance) and to quantify the magnitude of their effects in a novel highly resistant wheat germplasm, CJ 9306. A set of 152 F₇ recombinant inbred lines (RILs) derived from a cross Veery/CJ 9306 and two parents were evaluated for FHB resistance by single-floret inoculation in three greenhouse experiments in 2002 and 2004. Percentage (PSS) and number (NSS) of scabby spikelets at 25 days post-inoculation were analyzed. In total 682 simple sequence repeat (SSR) markers were screened for polymorphism between the two parents, and a genetic linkage map was constructed with 208 polymorphic markers. Ten QTLs associated with FHB resistance were detected, five from CJ 9306 and five from Veery. The major QTL on 3BS (QFhs.ndsu–3BS) was validated in CJ 9306, exhibiting greatest additive effects and explained 30.7% of phenotypic variation for PSS on the overall average of three experiments. Another major QTL on 2DL (QFhs.nau–2DL) from CJ 9306 explained 9.9–28.4% of phenotypic variation, with a significant QTL × environment interaction. QFhs.nau–1AS and QFhs.nau-7BS showed lower additive effects and explained lower variance (4.5–9.5%). A QTL on 5AS, decreasing PSS by 10.3% on average, was validated by simple marker analysis and joint trait/experiment IM/CIM analysis despite insignificance for single-experiment IM and CIM analyses. Likewise, QFhs.nau-2BL and QFhs.nau-1BC from Veery could reduce PSS by 13.2 and 11.4%, respectively. The effects of other three minor QTLs from Veery were significant for one experiment and combined analysis. Comparisons of two- and three-locus combinations suggested that the effects of FHB resistance QTLs/genes could be accumulated, and the resistance could be feasibly enhanced by selection of favorable alleles for multiple loci. Four two-locus combinations and two three-locus combinations were suggested as the preferential choices in practical marker-assisted selection program.     

Identification of quantitative trait loci contributing to Fusarium wilt resistance on an AFLP linkage map of flax (Linum usitatissimum)

 An AFLP genetic linkage map of flax (Linum usitatissimum) was used to identify two quantitative trait loci (QTLs) on independent linkage groups with a major effect on resistance to Fusarium wilt, a serious disease caused by the soil pathogen Fusarium oxysporum (lini). The linkage map was constructed using a mapping population from doubled-haploid (DH) lines. The DH lines were derived from the haploid component of F₂ haploid-diploid twin seed originating from a cross between a polyembryonic, low-linolenic-acid genotype (CRZY8/RA91) and the Australian cultivar ‘Glenelg’. The AFLP technique was employed to generate 213 marker loci covering approximately 1400 cM of the flax genome (n=15) with an average spacing of 10 cM and comprising 18 linkage groups. Sixty AFLP markers (28%) deviated significantly (P<0.05) from the expected segregation ratio. The map incorporated RFLP markers tightly linked to flax rust (Melamspora lini) resistance genes and markers detected by disease resistance gene-like sequences. The study illustrates the potential of the AFLP technique as a robust and rapid method to generate moderately saturated linkage maps, thereby allowing the molecular analysis of traits, such as resistance to Fusarium wilt, that show oligogenic patterns of inheritance. Received: 8 December 1997 / Accepted: 7 April 1998     

Genetic dissection of sex determinism, inflorescence morphology and downy mildew resistance in grapevine

A genetic linkage map of grapevine was constructed using a pseudo-testcross strategy based upon 138 individuals derived from a cross of Vitis vinifera Cabernet Sauvignon × Vitis riparia Gloire de Montpellier. A total of 212 DNA markers including 199 single sequence repeats (SSRs), 11 single strand conformation polymorphisms (SSCPs) and two morphological markers were mapped onto 19 linkage groups (LG) which covered 1,249 cM with an average of 6.7 cM between markers. The position of SSR loci in the maps presented here is consistent with the genome sequence. Quantitative traits loci (QTLs) for several traits of inflorescence and flower morphology, and downy mildew resistance were investigated. Two novel QTLs for downy mildew resistance were mapped on linkage groups 9 and 12, they explain 26.0–34.4 and 28.9–31.5% of total variance, respectively. QTLs for inflorescence morphology with a large effect (14–70% of total variance explained) were detected close to the Sex locus on LG 2. The gene of the enzyme 1-aminocyclopropane-1-carboxylic acid synthase, involved in melon male organ development and located in the confidence interval of all QTLs detected on the LG 2, could be considered as a putative candidate gene for the control of sexual traits in grapevine. Co-localisations were found between four QTLs, detected on linkage groups 1, 14, 17 and 18, and the position of the floral organ development genes GIBBERELLIN INSENSITIVE1, FRUITFULL, LEAFY and AGAMOUS. Our results demonstrate that the sex determinism locus also determines both flower and inflorescence morphological traits.     

Genetic dissection of vegetative propagation traits in Eucalyptus tereticornis and E. globulus

We have detected quantitative trait loci (QTLs) affecting vegetative propagation traits in Eucalyptus tereticornis and Eucalyptus globulus. Using amplified fragment length polymorphism (AFLP) genetic linkage maps, the inheritance of 199 markers was assessed in 94 F₁ individuals with extreme adventitious rooting response, and in 221 randomly chosen F₁ individuals. Phenotypes were scored in 1995 and 1996. QTL analyses were performed using chi-square tests (χ²), single-marker analysis (SMA), interval mapping (IM) and composite interval mapping (CIM). All approaches yielded similar QTL detection results. Three QTLs are hypothesized for mortality (MORT=% dead cuttings), nine for adventitious rooting (ROOT, RCT=% rooted cuttings relative to the surviving or total cuttings, respectively), four for petrification (PETR=% surviving unrooted cuttings), one for sprouting ability (SPR=number of stump sprout cuttings harvested in 1995) and four for the stability of adventitious rooting (STAB=absolute value of the difference ROOT95-ROOT96). All putative QTLs for MORT and PETR were located on the E. tereticornis map, and for SPR and STAB on the E. globulus map. We found different QTLs for MORT, ROOT, RCT, SPR and STAB. Putative QTLs accounted for 2.6–17.0% of the phenotypic variance of a trait (R²). Estimated standardized gene substitution effects varied between 0.13 and 0.49 phenotypic standard deviations (σ_p). These results indicate that the phenotypic variation in these traits has a meaningful genetic component and that stable QTLs can be found in a family of reasonable size where no previous knowledge of the trait was available. Received: 1 September 1998 / Accepted: 24 February 1999     

Identification of a new major QTL associated with resistance to soybean cyst nematode (Heterodera glycines)

Resistance of soybean [Glycine max (L.) Merr.] to cyst nematode (SCN) (Heterodera glycines Ichinohe), one of the most destructive pathogens affecting soybean, involves a complex genetic system. The identification of QTLs associated with SCN resistance may contribute to the understanding of such system. The objective of this work was to identify and map QTLs for resistance to SCN Race 14 with the aid of molecular markers. BC₃F_2:3 and F_2:3 populations, both derived from an original cross between resistant cv. Hartwig and the susceptible line BR-92–31983 were screened for resistance to SCN Race 14. Four microsatellite (Satt082, Sat_001, Satt574 and Satt301) and four RAPD markers (OPAA-11₇₉₅, OPAE-08₈₃₇, OPR-07₅₄₈ and OPY-07₂₀₃₀) were identified in the BC₃F_2:3 population using the bulked segregant analysis (BSA) technique. These markers were amplified in 183 F_2:3 families and mapped to a locus that accounts for more than 40% of the resistance to SCN Race 14. Selection efficiency based on these markers was similar to that obtained with the conventional method. In the case of the microsalellite markers, which identify homozygous resistant genotypes, the efficiency was even higher. This new QTL has been mapped to the soybean linkage group D2 and, in conjunction with other QTLs already identified for SCN resistance, will certainly contribute to our understanding of the genetic basis of resistance of this important disease in soybean. Received: 12 October 1999 / Accepted: 14 April 2000     

Molecular mapping and location of QTLs for drought-resistance traits in <Emphasis Type="Italic">indica</Emphasis> rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) lines adapted to target environments

Drought is a major limitation for rice production in rainfed ecosystems. Identifying quantitative trait loci (QTLs) linked to drought resistance provides opportunity to breed high yielding rice varieties suitable for drought-prone areas. Although considerable efforts were made in mapping QTLs associated with drought-resistance traits in rice, most of the studies involved indica × japonica crosses and hence, the drought-resistance alleles were contributed mostly by japonica ecotypes. It is desirable to look for genetic variation within indica ecotypes adapted to target environment (TE) as the alleles from japonica ecotype may not be expressed under lowland conditions. A subset of 250 recombinant inbred lines (RILs) of F₈ generation derived from two indica rice lines (IR20 and Nootripathu) with contrasting drought-resistance traits were used to map the QTLs for morpho-physiological and plant production traits under drought stress in the field in TE. A genetic linkage map was constructed using 101 polymorphic PCR-based markers distributed over the 12 chromosomes covering a total length of 1,529 cM in 17 linkage groups with an average distance of 15.1 cM. Composite interval mapping analysis identified 22 QTLs, which individually explained 4.8–32.2% of the phenotypic variation. Consistent QTLs for drought-resistance traits were detected using locally adapted indica ecotypes, which may be useful for rainfed rice improvement.     

Identification of QTLs for partial resistance to leaf rust (Puccinia hordei) in barley

 The partial resistance to leaf rust in barley is a quantitative resistance that is not based on hypersensitivity. To map the quantitative trait loci (QTLs) for partial resistance to leaf rust, we obtained 103 recombinant inbred lines (RILs) by single-seed descent from a cross between the susceptible parent L94 and the partially resistant parent Vada. These RILs were evaluated at the seedling and adult plant stages in the greenhouse for the latent period (LP) of the rust fungus, and in the field for the level of infection, measured as area under the disease progress curve (AUDPC). A dense genetic map based on 561 AFLP markers had been generated previously for this set of RILs. QTLs for partial resistance to leaf rust were mapped using the “Multiple Interval Mapping” method with the putative QTL markers as cofactors. Six QTLs for partial resistance were identified in this population. Three QTLs, Rphq1, Rphq2 and Rphq3, were effective at the seedling stage and contributed approximately 55% to the phenotypic variance. Five QTLs, Rph2, Rphq3, Rphq4, Rphq5, and/or Rphq6 contributed approximtely. 60% of the phenotypic variance and were effective at the adult plant stage. Therefore, only the QTLs Rphq2 and Rhpq3 were not plant-stage dependent. The identified QTLs showed mainly additive effects and only one significant interaction was detected, i.e. between Rphq1 and Rphq2. The map positions of these QTLs did not coincide with those of the race-specific resistance genes, suggesting that genes for partial resistance and genes for hypersensitive resistance represent entirely different gene families. Also, three QTLs for days to heading, of which two were also involved in plant height, were identified in the present recombinant inbred population. These QTLs had been mapped previously on the same positions in different populations. The perspectives of these results for breeding for durable resistance to leaf rust are discussed. Received: 15 July 1997 / Accepted: 30 December 1997     

QTL analysis for aphid resistance and growth traits in apple

The rosy apple aphid (Dysaphis plantaginea), the leaf-curling aphid (Dysaphis cf. devecta) and the green apple aphid (Aphis pomi) are widespread pest insects that reduce growth of leaves, fruits and shoots in apple (Malus × domestica). Aphid control in apple orchards is generally achieved by insecticides, but alternative management options like growing resistant cultivars are needed for a more sustainable integrated pest management (IPM). A linkage map available for a segregating F₁-cross of the apple cultivars ‘Fiesta’ and ‘Discovery’ was used to investigate the genetic basis of resistance to aphids. Aphid infestation and plant growth characteristics were repeatedly assessed for the same 160 apple genotypes in three different environments and 2 consecutive years. We identified amplified fragment length polymorphism (AFLP) markers linked to quantitative trait loci (QTLs) for resistance to D. plantaginea (‘Fiesta’ linkage group 17, locus 57.7, marker E33M35–0269; heritability: 28.3%), and to D. cf. devecta (‘Fiesta’ linkage group 7, locus 4.5, marker E32M39–0195; heritability: 50.2%). Interactions between aphid species, differences in climatic conditions and the spatial distribution of aphid infestation were identified as possible factors impeding the detection of QTLs. A pedigree analysis of simple sequence repeat (SSR) marker alleles closely associated with the QTL markers revealed the presence of the alleles in other apple cultivars with reported aphid resistance (‘Wagener’, ‘Cox’s Orange Pippin’), highlighting the genetic basis and also the potential for gene pyramiding of aphid resistance in apple. Finally, significant QTLs for shoot length and stem diameter were identified, while there was no relationship between aphid resistance and plant trait QTLs. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Genetics and molecular mapping of genes for race-specific all-stage resistance and non-race-specific high-temperature adult-plant resistance to stripe rust in spring wheat cultivar Alpowa

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the most widespread and destructive wheat diseases worldwide. Growing resistant cultivars is the preferred control of the disease. The spring wheat cultivar ‘Alpowa’ has both race-specific, all-stage resistance and non-race-specific, high-temperature adult-plant (HTAP) resistances to stripe rust. To identify genes for the stripe rust resistances, Alpowa was crossed with ‘Avocet Susceptible’ (AVS). Seedlings of the parents, and F₁, F₂ and F₃ progeny were tested with races PST-1 and PST-21 of P. striiformis f. sp. tritici under controlled greenhouse conditions. Alpowa has a single partially dominant gene, designated as YrAlp, conferring all-stage resistance. Resistance gene analog polymorphism (RGAP) and simple sequence repeat (SSR) techniques were used to identify molecular markers linked to YrAlp. A linkage group of five RGAP markers and two SSR markers was constructed for YrAlp using 136 F₃ lines. Amplification of a set of nulli-tetrasomic Chinese Spring lines with RGAP markers Xwgp47 and Xwgp48 and the two SSR markers indicated that YrAlp is located on the short arm of chromosome 1B. To map quantitative trait loci (QTLs) for the non-race-specific HTAP resistance, the parents and 136 F₃ lines were tested at two sites near Pullman and one site near Mount Vernon, Washington, under naturally infected conditions. A major HTAP QTL was consistently detected across environments and was located on chromosome 7BL. Because of its chromosomal location and the non-race-specific nature of the HTAP resistance, this gene is different from previously described genes for adult-plant resistance, and is therefore designated Yr39. The gene contributed to 64.2% of the total variation of relative area under disease progress curve (AUDPC) data and 59.1% of the total variation of infection type data recorded at the heading-flowering stages. Two RGAP markers, Xwgp36 and Xwgp45 with the highest R ² values were closely linked to Yr39, should be useful for incorporation of the non-race-specific resistance gene into new cultivars and for combining Yr39 with other genes for durable and high-level resistance.     

QTLs for a component of partial resistance to cucumber mosaic virus in pepper: restriction of virus installation in host-cells

 Ninety four doubled-haploid (DH) lines obtained from the F₁ between Perennial, a cucumber mosaic virus (CMV)-partially resistant Capsicum annuum line, and Yolo Wonder, a CMV-susceptible C. annuum line, were analysed with 138 markers including mostly RFLPs and RAPDs. Clustering of RAPD markers was observed on five linkage groups of the intraspecific linkage map. These clusters could correspond to the centromeric regions of pepper chromosomes. The same progenies were evaluated for restriction of CMV installation in pepper cells in order to map quantitative trait loci (QTLs) controlling CMV resistance. This component of partial resistance to CMV was quantitatively assessed using a CMV strain that induced necrotic local lesions on the inoculated leaves. The number of local lesions gave an estimation of the density of the virus-infection sites. Genotypic variance among the DH lines was highly significant for the number of local lesions, and heritability was estimated to be 0.94. Using both analysis of variance and non-parametric tests, three genomic regions significantly affecting CMV resistance were detected on chromosomes Noir, Pourpre and linkage group 3, together explaining 57% of the phenotypic variation. A digenic epistasis between one locus that controlled significant trait variation and a second locus that by itself had no demonstrable effect on the trait was found to have an effect on CMV resistance. For each QTL, the allele from Perennial was associated with an increased resistance. Implications of QTL mapping in marker-based breeding for CMV resistance are discussed. Received: 16 September 1996     

Two genetic linkage maps of tetraploid roses

A tetraploid F₂ progeny segregating for resistance to black spot, growth habit, and absence of prickles on the stem and petioles was used to construct genetic linkage maps of rose. The F₁ of the progeny, 90–69, was created by crossing a black spot-resistant amphidiploid, 86–7, with a susceptible tetraploid, 82–1134. The F₁ was open-pollinated to obtain 115 seedlings. AFLP and SSR markers were used to eliminate seedlings produced through cross-fertilization. The remaining progeny set of 52 F₂ plants was used to study the inheritance of 675 AFLPs, one isozyme, three morphological and six SSR markers. AFLP markers were developed with three combinations of restriction enzymes, EcoRI/MseI, KpnI/MseI and PstI/MseI. Most of the markers appear to be in simplex or single-dose and segregated 3:1 in the progeny. One linkage map was constructed for each parent using only the single-dose markers. The map of 86–7 consists of 171 markers assigned to 15 linkage groups and covering more than 902 cM of the genome. The map of 82–1134 consists of 167 markers assigned to 14 linkage groups and covering more than 682 cM of the genome. In the AFLP analysis, EcoRI/MseI generated nearly twice as many markers per run than PstI/MseI. Markers developed with three restriction enzyme combinations showed a mixed distribution throughout the maps. A gene controlling the prickles on the petiole was located at the end of linkage group 7 on the map of 86–7. A gene for malate dehydrogenase locus 2 was located in the middle of linkage group 4 on the map of 86–7. These first-generation maps provide initial tools for marker- assisted selection and gene introgression for the improvement of modern tetraploid roses. Received: 20 June 2000 / Accepted: 13 January 2001