Inheritance and sources of resistance to bacterial speck of tomato caused by Pseudomonas syringae pv. tomato

Inheritance of resistance to bacterial speck of tomato was determined by analysing F₁ F₂ and backcross progenies of crosses involving a susceptible (VF-198) and a resistant cultivar (Rehovot-13). The results fit the hypothesis that resistance is controlled by a single dominant gene in interaction with minor genes. Cultivar susceptibility to Pseudomonas syringae pv. tomato was tested under greenhouse conditions under high inoculum pressure using infested tomato seeds together with infested soils and spray-inoculated wounded plants. Of 21 species, cultivars and lines, Rehovot-13, Ontario 7710 and Lycopersiconpimpinellifolium P.I. 126927 were found to be resistant to the pathogen. VF-198 and Tropic-VF were the most susceptible. Extra Marmande, Saladette, Acc.339944–3 and the wild type Lycopersicon esculentum var. cerasiforme were moderately resistant.     

High-resolution genetic map of the Lv resistance locus in tomato

 Bulked segregant analysis and high-resolution mapping were used to pinpoint the position of the Lv gene for resistance to Leveillula taurica in tomato. Mapping in an F₂, corresponding to more than 3800 gametes, indicates that Lv is positioned within the 0.84-cM interval defined by the RFLP markers CT121 and CT129, with the closest marker, CT121, being only 0.16 cM from the gene. The tight linkage of these markers demonstrates their usefulness in marker-assisted breeding for Lv, and the high-resolution map provides a starting a starting point for positional cloning of this resistance gene. Received: 25 February 1997 / Accepted: 21 March 1997     

Development of diagnostic PCR markers closely linked to the tomato powdery mildew resistance gene Ol-1 on chromosome 6 of tomato

Lycopersicon hirsutum G1.1560 is a wild accession of tomato that shows resistance to Oidium lycopersicum, a frequently occurring tomato powdery mildew. This resistance is largely controlled by an incompletely dominant gene Ol-1 near the Aps-1 locus in the vicinity of the resistance genes Mi and Cf-2/Cf-5. Using a new F₂ population (n=150) segregating for resistance, we mapped the Ol-1 gene more accurately to a location between the RFLP markers TG153 and TG164. Furthermore, in saturating the Ol-1 region with more molecular markers using bulked segregant analysis, we were able to identify five RAPDs associated with the resistance. These RAPDs were then sequenced and converted into SCAR markers: SCAB01 and SCAF10 were L. hirsutum-specific; SCAE16, SCAG11 and SCAK16 were L. esculentum-specific. By linkage analysis a dense integrated map comprising RFLP and SCAR markers near Ol-1 was obtained. This will facilitate a map-based cloning approach for Ol-1 and marker-assisted selection for powdery mildew resistance in tomato breeding. Received: 21 June 1999 / Accepted: 1 December 1999     

Combining transgenic and natural resistance to obtain broad resistance to tospovirus infection in tomato (Lycopersicon esculentum mill)

This study was undertaken to develop tomato plants with broad resistanceto tospoviruses which are a major limiting factor to tomato productionworldwide. A nontransgenic tomato line Stevens-Rodale (S-R), six transgenictomato lines expressing the nucleocapsid (N) protein gene of the lettuceisolate of tomato spotted wilt virus (TSWV-BL), and progeny of the crosses between S-Rand three of the transgenic lines homozygous for the N gene were evaluated fortheir resistance to tospovirus infection in greenhouse inoculation tests. S-Rhas the Sw-5 gene that confers resistance to several TSWVisolates. The six transgenic lines showed high levels of resistance wheninoculated with either TSWV-BL or a tomato isolate from Hawaii (TSWV-H).However, these same plants were highly susceptible to the Brazilian isolate ofgroundnut ringspot virus (GRSV-BR). Plants with the Sw-5gene were resistant to TSWV-BL and GRSV-BR, but were susceptible to TSWV-H.When inoculated with any of the three viruses, the F₁ progeny of thecrosses exhibited a susceptible, tolerant, or resistant phenotype with a higherproportion of the plants being either tolerant or resistant. When F₂progeny from F₁ resistant plants of each cross were inoculated withany of the three viruses, a higher proportion of tolerant and resistant plantswas observed compared to the F₁ progeny. Our results show thepotential to obtain broad resistance to tospoviruses by combining transgenicand natural resistance in a single plant.     

Inheritance of potato aphid resistance in hybrids between Lycopersicon esculentum and L. pennellii

Summary The potato aphid, Macrosiphum euphorbiae Thomas, is an important pest of tomato, Lycopersicon esculentum Mill., because it transmits tomato viruses and directly reduces crop yields by its feeding. This study was conducted to determine whether the wild tomato species, Lycopersicon pennellii (Corr.) D'Arcy, would be useful as a source of potato aphid resistance for tomato. Type IV trichome density and aphid resistance were assessed in six generations (P₁, P₂, F₁, F₂, BC₁P₁, and BC₁P₂) from crosses between L. pennellii (LA 716) and two tomato cultivars, New Yorker and VF Vendor. Weighted leastsquares were used in joint scaling tests to estimate the relative importance of gene effects on type IV trichome density and potato aphid resistance of the hybrids. A simple additive-dominance model adequately explained the variation in type IV trichome density. Models which included digenic epistatic effects were required to explain the variation in aphid resistance. Standard unit heritability estimates of aphid resistance in the backcross to L. esculentum were obtained by regression of BC₁F₂ off-spring families on BC₁F₁ parents. Regression coefficients and heritability estimates varied between years with the level and uniformity of the aphid infestation. In the 1985–1986 growing seasons, when aphid infestations were uniform, aphid resistance exhibited a moderate level of heritability (29.8% ± 14.1% and 47.1% ± 11.5% in New Yorker and VF Vendor backcross populations, respectively). The non-uniform aphid infestation of 1984 resulted in lower heritability estimates in the 1984–1985 growing seasons (16.1% ± 15.7% and 21.9% ± 14.8% in the New Yorker and VF Vendor backcross populations, respectively). Selection for potato aphid resistance would probably be most efficient if it were delayed until gene combinations are fixed in later generations, because of the large epistatic effects and the low heritability of this trait in seasons with variable aphid infestations.     

Genetic analysis of larval survival and larval growth of two populations of Leptinotarsa decemlineata on tomato

The genetics of adaptation to tomato in Leptinotarsa decemlineata (Say) were investigated in reciprocal F₁, F₂, and backcross populations generated from crosses between beetles from a tomato adapted population and from a population that was poorly adapted to tomato. Larvae from the parent and test populations were reared on tomato for four days, after which survivorship and larval weights were recorded. Most results indicate that differences in larval growth and survival on tomato between the parent populations are largely determined by autosomal, polygenic mechanisms, the inheritance of which involves a significant dominance component. However, results from F₂ crosses are not consistent with this conclusion. A significant difference in larval weights, but not in survival, between reciprocal F₁ populations in an analysis of combined data from four separate experiments suggests that maternal cytoplasmic effects may contribute to differences in larval performance on tomato between the adapted and unadapted populations. The unusual results obtained from F₂ crosses in this study are not atypical of results from previous studies of the genetics of adaptation to host plants by the Colorado potato beetle. Host plant adaptation by Colorado potato beetles may therefore involve unusual genetic mechanisms that are not easily assessed by classical Mendelian analysis.     

A new semi-determinate tomato hybrid for early production

By using the knowledge of the genetic constitution and the characteristics of the available tomato lines for selection, there were created several dozens of hybrid combinations characterised by semi-determinate habit, with large fruits and early maturing. These hybrids were tested in preliminary and comparative trials with the hybrids Balkan F₁, Balca F₁, Žar F₁, and some others as standard varieties. These tomato hybrids were grown earlier but they are grown even today in the early production, for the market in green houses and in the open field. The hybrid Marco F₁ was the best of all. Its yield was 25 t/ha in the plastic-house (the same as in the standard), up to 32 t/ha in the open field (5 % more than in the standard). This hybrid was a few days later than Balkan, with the fruits weighting 200 grams (fruits were approximately 60 % heavier than in the standard). It possess the genetic V, F and Tm resistance.     

Identification of two AFLP markers linked to bacterial wilt resistance in tomato and conversion to SCAR markers

Tomato bacterial wilt (BW) incited by Ralstonia solanacearum is a constraint on tomato production in tropical, subtropical and humid regions of the world. In this paper, we present the results of a research aimed at the identification of PCR-based markers amplified fragment length polymorphism (AFLP) linked to the genes that confer resistance to tomato BW. To this purpose, bulked segregant analysis was applied to an F₂ population segregating for the BW resistant gene and derived from the pair-cross between a BW resistant cultivar T51A and the susceptible cultivar T9230. Genetic analysis indicated that tomato BW was conferred by two incomplete dominant genes. A CTAB method for total DNA extraction, developed by Murray and Thompson with some modifications was used to isolation the infected tomato leaves. Thirteen differential fragments were detected using 256 primer combinations, and two AFLP markers were linked to the BW resistance. Subsequently, the AFLP markers were converted to co-dominant SCAR markers, named TSCAR_AAT/CGA and TSCAR_AAG/CAT. Linkage analysis showed that the two markers are on the contralateral side of TRSR-1. Genetic distance between TSCAR_AAT/CGA and TRS-1 was estimated to 4.6 cM, while 8.4 cM between TSCAR_AAG/CAT and TRS-1.     

Identification and mapping on chromosome 9 of RAPD markers linked to Sw-5 in tomato by bulked segregant analysis

Bulked segregant analysis was used to identify random amplified polymorphic DNA (RAPD) markers linked to the Sw-5 gene for resistance to tomato spotted wilt virus (TSWV) in tomato. Using two pools of phenotyped individuals from one segregating population, we identified four RAPD markers linked to the gene of interest. Two of these appeared tightly linked to Sw-5, whereas another, linked in repulsion phase, enabled the identification of heterozygous and susceptible plants. After linkage analysis of an F₂ population, the RAPD markers were shown to be linked to Sw-5 within a distance of 10.5 cM. One of the RAPD markers close to Sw-5 was used to develop a SCAR (sequence characterized amplified region) marker. Another RAPD marker was stabilized into a pseudo-SCAR marker by enhancing the specificity of its primer sequence without cloning and sequencing. RAPD markers were mapped to chromosome 9 on the RFLP tomato map developed by Tanksley et al. (1992). The analysis of 13 F₃ families and eight BC₂ populations segregating for resistance to TSWV confirmed the linkage of the RAPD markers found. These markers are presently being used in marker-assisted plant breeding.     

Identification of I‐7 expands the repertoire of genes for resistance to Fusarium wilt in tomato to three resistance gene classes

The tomato I‐3 and I‐7 genes confer resistance to Fusarium oxysporum f. sp. lycopersici (Fol) race 3 and were introgressed into the cultivated tomato, Solanum lycopersicum, from the wild relative Solanum pennellii. I‐3 has been identified previously on chromosome 7 and encodes an S‐receptor‐like kinase, but little is known about I‐7. Molecular markers have been developed for the marker‐assisted breeding of I‐3, but none are available for I‐7. We used an RNA‐seq and single nucleotide polymorphism (SNP) analysis approach to map I‐7 to a small introgression of S. pennellii DNA (c. 210 kb) on chromosome 8, and identified I‐7 as a gene encoding a leucine‐rich repeat receptor‐like protein (LRR‐RLP), thereby expanding the repertoire of resistance protein classes conferring resistance to Fol. Using an eds1 mutant of tomato, we showed that I‐7, like many other LRR‐RLPs conferring pathogen resistance in tomato, is EDS1 (Enhanced Disease Susceptibility 1) dependent. Using transgenic tomato plants carrying only the I‐7 gene for Fol resistance, we found that I‐7 also confers resistance to Fol races 1 and 2. Given that Fol race 1 carries Avr1, resistance to Fol race 1 indicates that I‐7‐mediated resistance, unlike I‐2‐ or I‐3‐mediated resistance, is not suppressed by Avr1. This suggests that Avr1 is not a general suppressor of Fol resistance in tomato, leading us to hypothesize that Avr1 may be acting against an EDS1‐independent pathway for resistance activation. The identification of I‐7 has allowed us to develop molecular markers for marker‐assisted breeding of both genes currently known to confer Fol race 3 resistance (I‐3 and I‐7). Given that I‐7‐mediated resistance is not suppressed by Avr1, I‐7 may be a useful addition to I‐3 in the tomato breeder's toolbox.     

Three QTLs for <Emphasis Type="Italic">Botrytis cinerea</Emphasis> resistance in tomato

Tomato (Solanum lycopersicum) is susceptible to grey mold (Botrytis cinerea). Partial resistance to this fungus was identified in accessions of wild relatives of tomato such as S. habrochaites LYC4. In order to identify loci involved in quantitative resistance (QTLs) to B. cinerea, a population of 174 F₂ plants was made originating from a cross between S. lycopersicum cv. Moneymaker and S. habrochaites LYC4. The population was genotyped and tested for susceptibility to grey mold using a stem bioassay. Rbcq1, a QTL reducing lesion growth (LG) and Rbcq2, a QTL reducing disease incidence (DI) were identified. Rbcq1 is located on Chromosome 1 and explained 12% of the total phenotypic variation while Rbcq2 is located on Chromosome 2 and explained 15% of the total phenotypic variation. Both QTL effects were confirmed by assessing disease resistance in two BC₂S₁ progenies segregating for either of the two QTLs. One additional QTL, Rbcq4 on Chromosome 4 reducing DI, was identified in one of the BC₂S₁ progenies. F₂ individuals, homozygous for the Rbcq2 and Rbcq4 alleles of S. habrochaites showed a reduction of DI by 48%. QTLs from S. habrochaites LYC4 offer good perspectives for breeding B. cinerea resistant tomato cultivars. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

A PCR-based marker tightly linked to the nematode resistance gene,Mi, in tomato

A PCR-based codominant marker has been developed which is tightly linked to Mi, a dominant genetic locus in tomato that confers resistance to several species of root-knot nematode. DNA from tomato lines differing in nematode resistance was screened for random amplified polymorphic DNA markers linked to Mi using decamer primers. Several markers were identified. One amplified product, REX-1, obtained using a pair of decamer primers, was present as a dominant marker in all nematode-resistant tomato lines tested. REX-1 was cloned and the DNA sequences of its ends were determined and used to develop 20-mer primers. PCR amplification with the 20-mer primers produced a single amplified band in both susceptible and resistant tomato lines. The amplified bands from susceptible and resistant lines were distinguishable after cleavage with the restriction enzyme Taq I. The linkage of REX-1 to Mi was verified in an F₂ population. This marker is more tightly linked to Mi than is Aps-1, the currently-used isozyme marker, and allows screening of germplasm where the linkage between Mi and Aps-1 has been lost. Homozygous and heterozygous individuals can be distinguished and the procedure can be used for rapid, routine screening. The strategy used to obtain REX-1 is applicable to obtaining tightly-linked markers to other genetic loci. Such markers would allow rapid, concurrent screening for the segregation of several loci of interest.     

Inheritance of resistance to the two-spotted spider mite from L. pimpinellifolium (Jusl.) Mill. accession ‘TO-937’

The two-spotted spider mite (Tetranychus urticae Koch) is an important pest of tomato (Lycopersicon esculentum Mill.) crops in temperate regions as this spider mite has a very large capacity for population increase and causes severe tomato yield losses. There is no described tomato cultivar fully resistant to this pest, although resistant accessions have been reported within the green-fruited tomato wild species L. pennellii (Corr.) D’Arcy and L. hirsutum Humb. & Bonpl. We observed a L. pimpinellifolium (Jusl.) Mill. accession, ‘TO-937’, which seemed to be completely resistant to mite attacks and we crossed it with the susceptible L. esculentum cultivar. ‘Moneymaker’ to obtain a family of generations consisting of the two parents, the F₁, the F₂, the BC₁ to L. esculentum, and the BC₁ to L. pimpinellifolium. This family was evaluated for mite resistance in a polyethylene greenhouse using an experimental design in 60 small complete blocks distributed along 12 double rows. Each block consisted of five F₂ plants in one row and one plant of each of the two parents, the F₁, the BC₁ to L. esculentum, and the BC₁ to L. pimpinellifolium in the adjacent row. Plants at the 10–15 leaf stage were artificially infested by putting on them two pieces of French bean leaf heavily infested with T. urticae. After two months, evaluations of infestation were made by visual observation of mite nets and leaf damage. Plants that were free of signs of mite reproduction on the top half were considered as resistant, plants with silky nets only on their basal leaves, intermediate, and plants with mite reproduction on both basal and top canopies were scored as susceptible. Dominance for resistance appeared because all the ‘To-937’, BC₁ to L. pimpinellifolium, and F₁ plants were resistant. Not all ‘Moneymaker’ plants behaved as susceptible because 35% of plants were intermediate. In the BC₁ to L. pimpinellifolium and the F₂, most plants were scored as resistant, only 7 % BC₁ and 3 % F₂ plants were intermediate, and a single F₂ plant (0.3 %) was susceptible. With these figures, resistance seemed to be controlled by either four or two genes according to whether segregation in the BC₁ or in the F₂, respectively, were considered. These results could in part be explained because of appearance of negative interplot interference due to the high frequency of resistant genotypes within most of the generations. Therefore, the family was evaluated again but using a different experimental design. In the new experiment, 16 ‘TO-937’, 17 ‘Moneymaker’, 17 F₁, 37 BC₁ to L. pimpinellifolium, 38 BC₁ to L. esculentum, and 125 F₂ plants were included. Each of these test plants was grown besides a susceptible ‘Moneymaker’ auxilliary plant that served to keep mite population high and homogeneous in the greenhouse. Negative interplot interference was avoided with this design and all the ‘TO-937’, F₁, and BC₁ to L. pimpinellifolium plants were resistant, all ‘Moneymaker’ test plants were susceptible, and 52 % BC₁ to L. esculentum and 25 % F₂ plants were susceptible, which fitted very well with the expected for resistance governed by a single dominant gene. The simple inheritance mode found will favour sucessful introgression of mite resistance into commercial tomatoes from the very close relative L. pimpinellifolium.     

Linkage between Frl (Fusarium oxysporum f.sp. radicis-lycopersici resistance) and Tm-2 (tobacco mosaic virus resistance-2) loci in tomato (Lycopersicon esculentum)

A study was carried out on the linkage relationship between the Frl locus carrying resistance to Fusarium oxysporum f.sp. radicis-lycopersici and the Tm-2 locus carrying resistance to several races of tobacco mosaic virus in the tomato inbred line IRB-301-31. The inbred line Motelle (Frl⁺/Frl⁺, Tm-2⁺/Tm-2⁺) was crossed with the inbred line IRB-301-31 (Frl/Frl, Tm-2/Tm-l). The resulting 222 F₂ plants were selfed, and from each F₃ family groups of 15–60 seedlings were tested for resistance to either F. oxysporum f.sp. radicis-lycopersici or tobacco mosaic virus race 0. Segregation data indicated a very tight linkage between Frl and Tm-2, equal to 5.1 ± 1.07 map units.     

Selection of resistance breaking strains of tomato spotted wilt tospovirus

In glasshouse tests, sap from plants infected with 15 different isolates of tomato spotted wilt tospovirus (TSWV) from three Australian states was inoculated to nine genotypes of tomato carrying TSWV resistance gene Sw-5 or one of its alleles. A further two resistant tomato genotypes were inoculated with four isolates each. The normal response in resistant genotypes was development of necrotic local lesions in inoculated leaves without systemic invasion, but 22/752 plants also developed systemic reactions in addition to local hypersensitive ones. Using cultures from two of these systemically infected plants and following four cycles of subculture in TSWV resistant tomato plants, two isolates were obtained that gave susceptible type systemic reactions but no necrotic spots in inoculated leaves of resistant tomatoes. When these two isolates, Da_WA-1d and To_TAS-1d, were maintained by repeated subculture for 10 successive cycles in Nicotiana glutinosa or a susceptible tomato genotype, they still induced susceptible type systemic reactions when inoculated to resistant tomato plants. They were therefore stable resistance breaking isolates as regards overcoming gene Sw-5. When resistance-breaking isolate Da_WA^-1ld multiplied together with original isolate Da_WA-l in susceptible tomato, it was fully competitive with the original isolate. However, when Da_WA-ld and To_TAS-ld were inoculated to TSWV resistant Lycopersicon peruvianum lines PI 128660R and PI 128660S and to TSWV resistant Capsicum chinense lines PI 152225, PI 159236 and AVRDC CO0943, they failed to overcome the resistance, producing only necrotic local lesions without systemic infection. Thus, although the ease of selection, stability and competitive ability of resistance breaking isolates of TSWV is cause for concern, L. peruvianum and C. chinense lines are available which are effective against them. The effectiveness of the resistance to TSWV in nine tomato genotypes was examined in a field experiment. Spread was substantial in the susceptible control genotype infecting 42% of plants. Resistance was ineffective in cv. Bronze Rebel, 26% of plants developing infection. In contrast, it held up well in the other eight resistant genotypes with only 1–3 or no plants of each becoming infected. Accumulated numbers of Thrips tabaci, Frankliniella occidentalis and F. schultzei were closely correlated with TSWV spread.     

Response of tomato plants to chilling stress in association with nutrient or phosphorus starvation

The experiments were conducted on two tomato cultivars: Garbo and Robin. Mineral starvation due to plant growth in 20-fold diluted nutrient solution (DNS) combined with chilling reduced the rate of photosynthesis (P _N) and stomatal conductance (g) to a greater extent than in plants grown in full nutrient solution (FNS). In phosphate-starved tomato plants the P _N rate and stomatal conductance decreased more after chilling than in plants grown on FNS. In low-P plants even 2 days after chilling the recovery of CO₂ assimilation rate and stomatal conductance was low. A resupply of phosphorus to low-P plants (low P + P) did not improve the rate of photosynthesis in non-chilled plants (NCh) but prevented P_N inhibition in chilled (Ch) plants. The greatest effect of P resupply was expressed as a better recovery of photosynthesis and stomatal conductance, especially in non-chilled low P + P plants. The F _v/F _m (ratio of variable to maximal chlorophyll fluorescence) decreased more during P starvation than as an effect of chilling. Supplying phosphorus to low-P plants caused the slight increase in the F _v/F _mratio. In conclusion, after a short-term chilling in darkness a much more drastic inhibition of photosynthesis was observed in nutrient-starved or P-insufficient tomato plants than in plants from FNS. This inhibition was caused by the decrease in both photochemical efficiency of photosystems and the reduction of stomatal conductance. The presented results support the hypothesis that tomato plants with limited supply of mineral nutrients or phosphorus are more susceptible to chilling.     

RFLP linkage analysis of the Cf-4 and Cf-9 genes for resistance toCladosporium fulvum in tomato

Four different populations segregating for one of the two closely linked (possibly allelic) tomato disease resistance genes to the fungusCladosporium fulvum,Cf-4 andCf-9, were generated and analysed for recombination frequencies between theCf-genes and restriction fragment length polymorphism (RFLP) loci. The population consisting of F₂ progeny from the interspecific crossLycopersicon esculentum carryingCf-9 ×L. pennellii was identified as the most useful for RFLP mapping of theCf-4/9 locus and an RFLP map around this locus was constructed mainly using this population. The two closest markers identified were CP46, 2.6 cM distal, and a group of 11 markers including TG236, 3.7 cM proximal toCf-4/9. A polymerase chain reaction (PCR)-based procedure for the rapid identification of recombination events between these two markers was developed. The regions of foreign DNA introgression surroundingCf-4 andCf-9 in near-isogenic lines were delimited.     

Fine mapping and analysis of a candidate gene in tomato accession PI128216 conferring hypersensitive resistance to bacterial spot race T3

Bacterial spot caused by Xanthomonas euvesicatoria, X. vesicatoria, X. perforans and X. gardneri is one of the most destructive diseases in tomatoes (Solanum lycopersicum L.) growing in tropical and subtropical regions. Exploring resistance genes from diverse germplasm and incorporating them into cultivated varieties are critical for controlling this disease. The S. pimpinellifolium accession PI128216 was reported to carry the Rx4 gene on chromosome 11 conferring hypersensitivity and field resistance to race T3. To facilitate the use of marker-assisted selection in breeding and map-based cloning of the gene, an F₂ population derived from a cross between the susceptible variety OH88119 and the resistant accession PI128216 was created for fine mapping of the Rx4 gene. Using 18 markers developed through various approaches, we mapped the gene to a 45.1-kb region between two markers pcc17 and pcc14 on chromosome 11. A NBS-LRR class of resistance gene was identified as the candidate for the Rx4 gene based on annotation results from the International Tomato Annotation Group. Comparison of the genomic DNA sequences of the Rx4 alleles in PI128216 and OH88119 revealed a 6-bp insertion/deletion (InDel) and eight SNPs. The InDel marker was successfully used to distinguish resistance and susceptibility in 12 tomato lines. These results will facilitate cloning the Rx4 gene and provide a useful tool for marker-assisted selection of this gene in tomato breeding programs.     

A comparative analysis into the genetic bases of morphology in tomato varieties exhibiting elongated fruit shape

Fruit shape is a quantitatively inherited character. In tomato, two major loci, sun and ovate, control fruit shape index, which is the ratio of fruit height over width. In this study, we measured many additional fruit shape features in three inter-specific F₂ populations using the software application Tomato Analyzer. These populations were derived from varieties carrying elongated fruit but for which the major shape loci differed. We compared the effect of the major fruit shape loci with overall shape, as well as with the distal and proximal end shape features in each population. sun and ovate represented the largest effect on fruit shape in the Howard German and Sausage F₂ populations, respectively. The largest effect QTL in the Rio Grande population carrying neither sun nor ovate, were fs8.1 on chromosome 8 and tri2.1/dblk2.1 on chromosome 2. These latter loci were also segregating in the other two populations, thus indicating common regions that control shape across the three populations. The phenotypic analyses showed that sun and ovate contributed to almost all aspects of shape such as the distal and proximal end features. In Rio Grande however, the largest effect QTL did not control all aspects of shape and the distal and proximal features were distinctly controlled in that population. Combined, our results implied that within the cultivated tomato germplasm pool the largest effect on elongated fruit shape was controlled by a combination of the loci sun, ovate, fs8.1 and tri2.1/dblk2.1. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Inheritance and genetic mapping of cucumber mosaic virus resistance introgressed from Lycopersicon chilense into tomato

Cucumber mosaic virus (CMV) infects a wide variety of crop plants and in tomato (Lycopersicon esculentum Mill.) causes significant economic losses in many growing regions, particularly the Mediterranean. The objective of the present study was to identify the number and map locations of genes controlling resistance to CMV in breeding lines (BC₁–inbreds) derived from the related wild species L. chilense. These lines also carried the gene Tm-2 ^a for resistance to ToMV, which facilitated the interpretation of disease symptoms. The segregation for CMV resistance in the BC₂F₁ and BC₂F₂ generations, following mechanical inoculation with subgroup-I isolates, was consistent with expectations for a single dominant gene, for which the symbol Cmr (cucumber mosaic resistance) was given. Resistant and susceptible BC₁-inbreds were analyzed with RFLP and isozyme markers to identify genomic regions introgressed from L. chilense. The only L. chilense-specific markers found were on chromosome 12; some resistant lines contained a single introgression comprising the entire short arm and part of the long arm of this chromosome, while others contained a recombinant derivative of this introgression. The chromosome 12 markers were significantly associated with CMV resistance in both qualitative and quantitative models of inheritance. The qualitative analysis, however, demonstrated that CMV resistance was not expressed as a reliable monogenic character, suggesting a lack of penetrance, significant environmental effects, or the existence of additional (undetected) resistance factors. In the quantitative analysis, the marker interval TG68 – CT79 showed the most significant association with CMV resistance. No association between CMV resistance and the Tm-2 ^a gene was observed. These breeding lines are potentially useful sources of CMV resistance for tomato improvement, in which context knowledge of the map location of Cmr should accelerate introgression by marker-assisted selection. Received: 9 August 1999 / Accepted: 22 December 1999