Differentiation of Meloidogyne incognita and M. arenaria novel resistance phenotypes in Lycopersicon peruvianum and derived bridge-lines

Lycopersicon peruvianum PI 270435 clone 2R2 and PI 126443 clone 1MH were crossed reciprocally with three L. esculentum-L. peruvianum bridge-lines. The incongruity barrier between the two plant species was overcome; F₁ progeny were obtained from crosses between four parental combinations without embryo-rescue culture. Hybridity was confirmed by leaf and flower morphology and by the production of nematode-resistant F₁ progeny on homozygous susceptible parents. Clones of the five F₁ bridgeline hybrids were highly resistant to Mi-avirulent root-knot nematode (Meloidogyne incognita) at both 25°C and 30°C soil temperatures. However, only clones from PI 270435-3MH and PI 126443-1MH, and hybrids from PI 126443-1MH, were resistant to Mi-virulent M. incognita isolates at high soil temperature. Clones and hybrids from PI 270435-2R2 were not resistant to two Mi-virulent M. incognita isolates at high soil temperature. A source of heat-stable resistance was identified in bridge-line EPP-2, and was found to be derived from L. peruvianum LA 1708. Accessions of the L. peruvianum Maranon races, LA 1708 and LA 2172, and bridge-line EPP-2, segregated for heat-stable resistance to Mi-avirulent M. incognita, but were susceptible to Mi-virulent M. incognita isolates. Clone LA 1708-I conferred heat-stable resistance to M. arenaria isolate W, which is virulent to heat-stable resistance genes in L. peruvianum PI 270435-2R2, PI 270435-3MH, and PI 126443-1MH. Clone LA 1708-I has a distinct heat-stable factor for resistance to Mi-avirulent M. arenaria isolate W, for which the gene symbol Mi-4 is proposed. A Mi-virulent M. arenaria isolate Le Grau du Roi was virulent on all Lycopersicon spp. accessions tested, including those with novel resistance genes.     

Relationships between Meloidogyne incognita resistance genes in Lycopersicon peruvianum differentiated by heat sensitivity and nematode virulence

Resistance to Meloidogyne incognita (Kofoid and White) Chitwood in clones of Lycopersicon peruvianum (L.) Mill. PI 126443-1MH, 270435-2R2 and 2704353MH, their F₁, a field-produced F₂, and their test-cross (TC₁) populations, was evaluated based on egg masses and eggs produced on root systems. Reactions to M. incognita isolates differing in virulence to gene Mi were determined at 25°C (Mi expressed) and 32°C (Mi not expressed). PI 126443-1MH, 270435-2R2, 270435-3MH, and their F₁ progenies were resistant to Mi-virulent and Mi-avirulent isolates. At 32°C with a Mi-avirulent isolate and at 25°C with a Mi-virulent isolate, four TC₁ generations segregated into resistant: susceptible (RS) ratios close to 31. These results indicated resistance to Mi-(a)virulent M. incognita isolates is conferred by different non-allelic dominant genes in PI 126443-1MH, 270435-2R2 and 270435-3MH. The F₂ progeny of PI 126443-1MH x EPP-1, challenged with Mi-avirulent M. incognita at 32°C and with Mi-virulent M. incognita at both 25°C and 32°C, segregated with a ratio of 31 (RS), indicating expression of a single dominant resistance gene in PI 126443-1MH in each case. In dual screenings on clones of the same individual plants from the TC₁ and F₂ segregating populations, some individual plants were susceptible at 32°C to a Mi-avirulent isolate but resistant to the Mi-virulent isolate, and vice versa, suggesting that different but linked genes confer heat-stable resistance to Mi-avirulent M. incognita and resistance to Mi-virulent M. incognita. We propose the symbol Mi-5 for the gene in PI 126443 clone 1MH and the symbol Mi-6 for the gene in PI 270435 clone 3MH which both confer resistance to Mi-avirulent M. incognita isolates at high temperature. We propose the symbol Mi-7 for the gene in PI 270435 clone 3MH and the symbol Mi-8 for the gene in PI 270435 clone 2R2 that both confer resistance to the Mi-virulent M. incognita isolate 557R at moderate (25°C) temperature. The novel resistance genes are linked and reside in a genomic region in each parental clone that is independent from the Mi locus.     

Mapping of a Heat-Stable Gene for Resistance to Southern Root-Knot Nematode in Solanum lycopersicum

Root-knot nematodes (Meloidogyne spp.) can cause severe problems in tomato production in warm climates. To date, Mi-1 is the only gene that has been used widely to develop cultivars for controlling disease caused by nematodes around the world. However, Mi-1 does not provide resistance to the pest when the soil temperature is above 28 °C. Tomato breeding line ZN17 has been reported to possess resistance to Meloidogyne incognita at high temperatures (32 °C). To identify markers linked tightly to resistance, an F₂ population was developed by crossing the resistance line ZN17 to susceptible line 09C84. Analysis of F₂ individuals by inoculating M. incognita suggested that resistance in ZN17 is conditioned by a single dominant gene temporarily designated as Mi-HT. Linkage analysis suggested that Mi-HT is located on the short arm of chromosome 6. One marker, W737, co-segregated with Mi-HT. Comparisons of map positions of Mi-HT, Mi-1, and Mi-9, as well as marker genotypes in LA2157, Motelle, and ZN17 suggested that Mi-HT might be a homologue of Mi-1 and Mi-9 or a new gene. The results obtained in this study will facilitate fine-mapping and cloning of the gene as well as marker-assisted breeding for heat-stable resistance to southern root-knot nematodes in tomato.     

The heat-stable root-knot nematode resistance gene <Emphasis Type="Italic">Mi-9</Emphasis> from Lycopersicon peruvianum is localized on the short arm of chromosome 6

The tomato gene Mi-1 confers resistance to three species of root-knot nematodes, Meloidogyne spp. However, the resistance mediated by Mi-1 is inactive at soil temperatures above 28 degrees C. Previously, we identified and mapped a novel heat-stable nematode resistance gene from the wild species Lycopersicon peruvianum accession LA2157 on to chromosome 6. Here we report further characterization of this heat-stable resistance against three Mi-1-avirulent biotypes of Meloidogyne javanica, Meloidogyne arenaria and Meloidogyne incognita. Screening segregating F(2) and F(3) progenies, derived from an intraspecific cross between susceptible LA392 and resistant LA2157, for nematode resistance at 25 degrees C and 32 degrees C, revealed a simple dominant monogenic inheritance with all the biotypes tested. We designate this gene as Mi-9. As a first step towards cloning of Mi-9, we constructed a linkage map around this gene. A total of 216 F(2) progeny from the cross between LA392 and LA2157 were screened with M. javanica at 32 degrees C and with CT119 and Aps-1, markers that flank the genetic interval that contains the Mi-1 gene. DNA marker analysis indicated that these markers also flank Mi-9. Further mapping of recombinants with both RFLP and PCR-based markers localized Mi-9 to the short arm of chromosome 6 and within the same genetic interval that spans the Mi-1 region.     

Identification of resistance to Meloidogyne javanica in the Lycopersicon peruvianum complex

Clones of Lycopersicon peruvianum PI 2704352R2, PI 270435-3MH and PI 126443-1MH expressed novel resistance to three Mi-avirulent M. javanica isolates in greenhouse experiments. Clones from PI 126443-1MH were resistant to the three M. javanica isolates at 25°C. The three isolates were able to reproduce on one embryorescue hybrid of PI 126443-1MH, but not on three L. peruvianum-L. esculentum bridge-line hybrids of PI 1264431MH when screened at 25°C (Mi-expressed temperature). Clones of PI 270435-2R2 and all its hybrids with susceptible genotypes were resistant to the three M. javanica isolates at 25°C. The bridge-line hybrid EPP-2xPI 2704352R2 was susceptible to M. javanica isolate 811 at 32°C, whereas PI 270435-2R2 and all other hybrids of PI 27043 5-2R2 crossed with susceptible genotypes were resistant at 32°C. At 32°C, one F₂ progeny of PI 126443-IMHxEPP-1, and three test-cross progenies of PI 1264409MHx[PI 270435-3MHxPI 126443-1MH], and reciprocal test-cross progenies of [PI 270435-3MHxPI 2704352R2]xPI 126440-9MH, each segregated into resistant: susceptible (RS) ratios close to 31. The results from the F₂ progeny indicated that heat-stable resistance to Mi-avirulent M. javanica in PI 126443 -1MH is conferred by a single dominant gene. The results from the test-crosses indicated that this gene in PI 126443-1MH is different from the resistance gene in PI 270435-3MH. The resistance gene in PI 270435-3MH was also shown to differ from the resistance factor in PI 270435-2R2. The expression of differential susceptibility and resistance to M. javanica and M. incognita in individual plants of the bridge-line hybrid, embryo-rescue hybrid, F₂, and test-crosses indicated that at least some genes governing resistance to M. javanica differ from the genes conferring resistance to M. incognita. A new source of heat-stable resistance to M. javanica was identified in Lycopersicon chilense.     

Spectrum of resistance to root-knot nematodes and inheritance of heat-stable resistance in in pepper (Capsicum annuum L.)

Capsicum annuum L. has resistance to root-knot nematodes (RKN) (Meloidogyne spp.), severe polyphagous pests that occur world-wide. Several single dominant genes confer this resistance. Some are highly specific, whereas others are effective against a wide range of species. The spectrum of resistance to eight clonal RKN populations of the major Meloidogyne species, M. arenaria (2 populations), M. incognita (2 populations), M. javanica (1 population), and M. hapla (3 populations) was studied using eight lines of Capsicum annuum. Host susceptibility was determined by counting the egg masses (EM) on the roots. Plants were classified into resistant (R; EM ≤ 5) or susceptible (H; EM >5) classes. The french cultivar Doux Long des Landes was susceptible to all nematodes tested. The other seven pepper lines were highly resistant to M. arenaria, M. javanica and one population of M. hapla. Variability in resistance was observed for the other two populations of M. hapla. Only lines PM687, PM217, Criollo de Morelos 334 and Yolo NR were resistant to M. incognita. To investigate the genetic basis of resistance in the highly resistant line PM687, the resistance of two progenies was tested with the two populations of M. incognita: 118 doubled-haploid (DH) lines obtained by androgenesis from F₁ hybrids of the cross between PM687 and the susceptible cultivar Yolo Wonder, and 163 F₂ progenies. For both nematodes populations, the segregation patterns 69 R / 49 S for DH lines and 163 R / 45 S for F₂ progenies were obtained at 22°C and at high temperatures (32°C and 42°C). The presence of a single dominant gene that totally prevented multiplication of M. incognita was thus confirmed and its stability at high temperature was demonstrated. This study confirmed the value of C. annuum as a source of complete spectrum resistance to the major RKN. Received: 2 July 1998 / Accepted: 11 March 1999     

Retention of Resistance to Meloidogyne incognita in Lycopersicon Genotypes at High Soil Temperature

Lycopersicon glandulosum and L. peruvianum clones and L. esculentum cultivars ''VFN8'' (resistant) and ''Rutgers'' (susceptible) were tested for their resistance to Meloidogyne incognita (race l) at soil temperatures of 25 and 32 C. L. esculentum cv. VFN8 and L. peruvianum Acc. No. 128657, both of which possess the Mi gene, were resistant at 25 C but were susceptible at 32 C. L. glandulosum Acc. No. 126443 and L. peruvianum Acc. No. 270435, with combined resistance to M. hapla and M. incognita, and L. peruvianum Acc. Nos. 129152 and LA2157, with resistance to M. incognita, were highly resistant at both temperatures. In a second experiment three of these accessions under heat stress simulated by 32 C ambient and soil temperature retained a high level of resistance. Two clones of L. glandulosum Acc. No. 126440, with resistance to M. hapla, were moderately susceptible to M. incognita at 25 and highly susceptible at 32 C. M. incognita produced significantly (P = 0.01) more eggs on L. esculentum cv. Rutgers at 32 than at 25 C. This study supports the existence of genes other than the Mi gene that confer resistance to M. incognita and are functional at high soil temperatures.     

Histological response of resistant tomato cultivars to infection of virulent Tunisian root-knot nematode (Meloidogyne incognita) populations

The response of a susceptible tomato cultivar (Solanum lycopersicum cv. Rio Grande) to infection by three populations of root-knot nematode (Meloidogyne incognita) was compared histologically with that of Lycopersicon esculentum cv. Monita, L. esculentum cv. VFN8 and Solanum lycopersicum cv. Nemador possessing the Mi-1 resistance gene and accession PI126443 of L. peruvianum possessing the Mi-3 gene. The resistant cultivars showed susceptibility to the Tunisian Meloidogyne populations. Feeding sites were characterised by the development of giant cells that contained granular cytoplasm and several hypertrophied nuclei. The cytoplasm of giant cells was aggregated along their thickened cell walls and consequently the vascular tissues within galls appeared disrupted and disorganised. Feeding site formed on resistant L. esculentum lines and susceptible cultivar Rio Grande are similar according to cell and nucleus number, and the nurse superficies. Resistant accession L. peruvianum PI126443, known to possess heat-stable nematode resistance, also showed susceptible reaction to Tunisian Meloidogyne incognita populations; however, nematode development was reduced in comparison with susceptible plants and less developed feeding cells were observed.     

Three QTLs from Lycopersicon peruvianum confer a high level of resistance to Clavibactermichiganensis ssp. michiganensis

Lycopersicon peruvianum LA2157 originates from 1650 m above sea level and harbours several beneficial traits for cultivated tomatoes such as cold tolerance, nematode resistance and resistance to bacterial canker (Clavibacter michiganensis ssp. michiganensis). In order to identify quantitative trait loci (QTLs) for bacterial canker resistance, a QTL mapping approach was carried out in an F₂ population derived from the interspecific F₁ between Lycopersicon esculentum cv Solentos and L. peruvianum LA2157. Three QTLs for resistance mapped to chromosomes 5, 7 and 9 respectively. The resistance loci were additive and co-dominant with the QTL on chromosome 7 explaining the largest part of the variation for resistance in the F₂ population. The combination of this QTL with either of the other two QTLs conferred a resistance similar to the level in the resistant parent L. peruvianum. Some RFLP markers flanking this QTL on chromosome 7 were converted into SCAR markers allowing efficient marker-assisted selection of plants with high resistance to bacterial canker. Received: 26 February 1999 / Accepted: 12 March 1999     

Inheritance of heat-stable resistance to Meloidogyne incognita in Lycopersicon peruvianum and its relationship to the Mi gene

Summary The inheritance of heat-stable resistance to the root-knot nematode, Meloidogyne incognita (Kofoid and White) Chitwood, was studied in crosses between different accessions and clones of Lycopersicon peruvianum L. F₁, F₂ and BC₁ generations were evaluated for their index of resistance based on numbers of eggs and infective second-stage juveniles (J₂) per gram of root, and the segregation ratios were determined in experiments carried out at constant soil temperatures of 25 °C and 30 °C. L. peruvianum P.I. 270435 clones 3 MH and 2R2 and P.I. 126443 clone 1 MH, all heatstable resistant, were crossed with L. peruvianum P.I. 126440 clone 9 MH, which is susceptible at both 25 °C and 30 °C. All F₁ progeny were resistant at 25 °C and 30 °C; F₂ and BC₁ generations at 25 °C gave resistant: susceptible (RS) ratios of 151 and 31, respectively, which suggests that resistance is conditioned by two independently assorting genes. However, at 30 °C, RS ratios of 31 and 11 were observed for the F₂ and BC₁ generations, respectively. These results indicate that heat-stable resistance is conferred by a single dominant gene expressed at 30 °C, while the second resistance gene is heat unstable and not expressed at 30 °C. P.I. 270435 clones 2R2 and 3 MH and P.I. 126443 clone 1 MH were crossed with P.I. 128657 clone 3 R4 (source of gene Mi), which is resistant at 25 °C but susceptible at 30 °C. All of the F₁ progeny were resistant at 25 °C and 30 °C.TC₁ progeny of 270435-2 R2 x 128657-3 R4, 270435-3 MH x 128657-3 R4 and 126443-1 MH x 128657-3 R4 crossed with susceptible 126440-9 MH were all resistant at 25 °C and segregated in a 11 ratio at 30 °C. These results also suggest that the heat-stable resistance is monogenic and that it is non-allelic to gene Mi. The non-segregation of TC₁ progenies at 25 °C, suggests that the heat-unstable resistance factor in L. peruvianum P.I. 270435 clones 2 R2 and 3 MH and in P.I. 126443 clone 1 MH is allelic to or the same as gene Mi. We propose the symbol Mi-2 for the gene in P.I. 270435 that confers heat-stable resistance to M. incognita.     

Long-Term Chilling of Young Tomato Plants under Low Light. IV. Differential Responses of Chlorophyll Fluorescence Quenching Coefficients in Lycopersicon Species of Different Chilling Sensitivity

Temperature dependences of chlorophyll fluorescence quenchingcoefficients were studied in the cultivated tomato (Lycopersiconesculentum) and three lines of the chilling-tolerant L.peruvianumfrom different altitudes, i.e. LA 1373 (20 m a.s.l.), LA 2157(1,650 m a.s.l.) and LA 385 (2,400 m a.s.l.). At actinic lightintensity near light saturation of photosynthesis (370 µEm^–2 s^7minus;1), photochemical quenching (qP) increasedwith increasing temperature between 5 and 30°C. The temperature,at which qP reached the numerical value 0.5 [T (qP=0.5)] decreasedby 2.5–4.5°C after a chilling treatment of 14 daysat 10°C in L. peruvianum, indicating acclimation of thephotosynthetic dark reactions in this species. The final T (qP=0.5)attained after chilling could be arranged in the order L.esculentum>LA1373>LA 2157>LA 385. The fast relaxing non-photochemicalquenching (qN) component (qf, consisting mainly of energy-dependentquenching, qE) exhibited minima near the optimum temperaturefor photosynthesis. These minima shifted to lower temperaturesupon chilling in L. peruvianum. Photoinhibitory quenching (ql)was unaffected by chilling in the high altitude lines, but-increasedstrongly in LA 1373 and L. esculentum. Under low actinic light(40 µE m^–2 s^–1), temperature dependences ofqP and qN were nearly identical in L. esculentum and LA 385and revealed abrupt changes at approx. 8°C. It is concludedthat qP and ql, measured after defined chilling treatments,are valuable screening parameters for chilling tolerance inearly growth stages of Lycopersicon plants. (Received November 2, 1993; Accepted February 28, 1994)     

Mapping <Emphasis Type="Italic">Ol-4</Emphasis>, a gene conferring resistance to <Emphasis Type="Italic">Oidium neolycopersici</Emphasis> and originating from <Emphasis Type="Italic">Lycopersicon peruvianum</Emphasis> LA2172, requires multi-allelic,single-locus markers

Lycopersicon peruvianum LA2172 is completely resistant to Oidium neolycopersici, the causal agent of tomato powdery mildew. Despite the large genetic distance between the cultivated tomato and L. peruvianum, fertile F₁ hybrids of L. esculentum cv. Moneymaker × L. peruvianum LA2172 were produced, and a pseudo-F₂ population was generated by mating F₁ half-sibs. The disease tests on the pseudo-F₂ population and two BC₁ families showed that the resistance in LA2172 is governed by one dominant gene, designated as Ol-4. In the pseudo-F₂ population, distorted segregation was observed, and multi-allelic, single-locus markers were used to display different marker-allele configurations per locus. Parameters for both distortion and linkage between genetic loci were determined by maximum likelihood estimation, and the necessity of using multi-allelic, single-locus markers was illustrated. Finally, a genetic linkage map of chromosome 6 around the Ol-4 locus was constructed by using the pseudo-F₂ population.     

Mi-1-Mediated Nematode Resistance in Tomatoes is Broken by Short-Term Heat Stress but Recovers Over Time

Tomato (Solanum lycopersicum L.) is among the most valuable agricultural products, but Meloidogyne spp. (root-knot nematode) infestations result in serious crop losses. In tomato, resistance to root-knot nematodes is controlled by the gene Mi-1, but heat stress interferes with Mi-1-associated resistance. Inconsistent results in published field and greenhouse experiments led us to test the effect of short-term midday heat stress on tomato susceptibility to Meloidogyne incognita race 1. Under controlled day/night temperatures of 25°C/21°C, ‘Amelia’, which was verified as possessing the Mi-1 gene, was deemed resistant (4.1 ± 0.4 galls/plant) and Rutgers, which does not possess the Mi-1 gene, was susceptible (132 ± 9.9 galls/plant) to M. incognita infection. Exposure to a single 3 hr heat spike of 35°C was sufficient to increase the susceptibility of ‘Amelia’ but did not affect Rutgers. Despite this change in resistance, Mi-1 gene expression was not affected by heat treatment, or nematode infection. The heat-induced breakdown of Mi-1 resistance in ‘Amelia’ did recover with time regardless of additional heat exposures and M. incognita infection. These findings would aid in the development of management strategies to protect the tomato crop at times of heightened M. incognita susceptibility.     

Fine mapping of the nematode resistance gene <Emphasis Type="Italic">Mi-3</Emphasis> in <Emphasis Type="Italic">Solanum peruvianum</Emphasis> and construction of a <Emphasis Type="Italic">S. lycopersicum</Emphasis> DNA contig spanning the locus

Currently, the only genetic resistance against root-knot nematodes in the cultivated tomato Solanum lycopersicum (Lycopersicon esculentum) is due to the gene Mi-1. Another resistance gene, Mi-3, identified in the related wild species Solanum peruvianum (Lycopersicon peruvianum) confers resistance to nematodes that are virulent on tomato lines that carry Mi-1, and is effective at temperatures at which Mi-1 is not effective (above 30°C). Two S. peruvianum populations segregating for Mi-3 were used to develop a high-resolution map of the Mi-3 region of chromosome 12. S. lycopersicum BACs carrying flanking markers were identified and used to construct a contig spanning the Mi-3 region. Markers generated from BAC-end sequences were mapped in S. peruvianum plants in which recombination events had occurred near Mi-3. Comparison of the S. peruvianum genetic map with the physical map of S. lycopersicum indicated that marker order is conserved between S. lycopersicum and S. peruvianum. The 600 kb contig between Mi-3-flanking markers TG180 and NR18 corresponds to a genetic distance of about 7.2 cM in S. peruvianum. We have identified a marker that completely cosegregates with Mi-3, as well as flanking markers within 0.25 cM of the gene. These markers can be used to introduce Mi-3 into cultivated tomato, either by conventional breeding or cloning strategies.     

Localization of genes for bacterial canker resistance in Lycopersicon peruvianum using RFLPs

A backcross population of the L. peruvianum accession LA 2157, which is resistant to bacterial canker caused by Clavibacter michiganensis ssp. michiganensis, with the susceptible L. peruvianum accession LA 2172 was evaluated for the segregation of C. michiganenis resistance and of RFLP markers in order to map the loci involved in this resistance. The development of symptoms of the disease was scored using an ordinal scale. The mapping of the disease resistance was hampered by distorted segregation ratios of a large number of markers and unexpected quantitative inheritance of the resistance. By means of the Kruskal-Wallis rank-sum test, five regions on chromosomes 1, 6, 7, 8 and 10 were identified that may be involved in C. michiganensis resistance.     

The nematode-resistance gene,<Emphasis Type="Italic"> Mi-1</Emphasis>, is associated with an inverted chromosomal segment in susceptible compared to resistant tomato

The gene Mi-1 confers effective resistance in tomato (Lycopersicon esculentum) against root-knot nematodes and some isolates of potato aphid. This locus was introgressed from L. peruvianum into the corresponding region on chromosome 6 in tomato. In nematode-resistant tomato, Mi-1 and six homologs are grouped into two clusters separated by 300 kb. Analysis of BAC clones revealed that the Mi-1 locus from susceptible tomato carried the same number and distribution of Mi-1 homologs, as did the resistant locus. Molecular markers flanking the resistant and susceptible loci were in the same relative orientation, but markers between the two clusters were in an inverse orientation. The simplest explanation for these observations is that there is an inversion between the two clusters of homologs when comparing the Mi-1 loci from L. esculentum and L. peruvianum. Such an inversion may explain previous observations of severe recombination suppression in the region. Two Mi-1 homologs identified from the BAC library derived from susceptible tomato are not linked to the chromosome 6 locus, but map to chromosome 5 in regions known to contain resistance gene loci in other solanaceous species.Communicated by J.S. Heslop-Harrison     

Ectopic Expression of the Tomato <Emphasis Type="Italic">Mi-1</Emphasis> Gene Confers Resistance to Root Knot Nematodes in Lettuce (<Emphasis Type="Italic">Lactuca sativa</Emphasis>)

The full genomic region of the root knot nematode (Meloidogyne spp.) resistance gene Mi-1 was cloned from tomato and transformed into lettuce to investigate its function in a heterologous system. Transgenic lettuce lines containing the Mi-1 gene were developed using Agrobacterium-mediated transformation. Ectopic expression of the Mi-1 gene was observed in transgenic lines, and resistance to root knot nematode was improved.     

RAPD linkage map of the genomic region encompassing the root-knot nematode (Meloidogyne javanica) resistance locus in carrot

Inheritance studies have indicated that resistance to the root-knot nematode (Meloidogyne javanica) in carrot inbred line ’Brasilia-1252’ is controlled by the action of one or two (duplicated) dominant gene(s) located at a single genomic region (designated the Mj-1 locus). A systematic search for randomly amplified polymorphic DNA (RAPD) markers linked to Mj-1 was carried out using bulked segregant analysis (BSA). Altogether 1000 ten-mer primers were screened with 69.1% displaying scorable amplicons. A total of approximately 2400 RAPD bands were examined. Four reproducible markers (OP-C2₁₇₀₀, OP-Q6₅₀₀, OP-U12₇₀₀, and OP-AL15₅₀₀) were identified, in coupling-phase linkage, flanking the Mj-1 region. The genetic distances between RAPD markers and the Mj-1 locus, estimated using an F₂ progeny of 412 individuals from ’Brasilia 1252’×’B6274’, ranged from 0.8 to 5.7 cM . The two closest flanking markers (OP-Q6₅₀₀ and OP-AL15₅₀₀) encompassed a region of 2.7 cM . The frequency of these RAPD loci was evaluated in 121 accessions of a broad-based carrot germplasm collection. Only five entries (all resistant to M. javanica and genetically related to ’Brasilia 1252’) exhibited the simultaneous presence of all four markers. An advanced line derived from the same cross, susceptible to M. javanica but relatively resistant to another root-knot nematode species (M. incognita), did not share three of the closest markers. These results suggest that at least some genes controlling resistance to M. incognita and M. javanica in ’Brasilia 1252’ reside at distinct loci. The low number of markers suggests a reduced amount of genetic divergence between the parental lines at the region surrounding the target locus. Nevertheless, the low rate of recombination indicated these markers could be useful landmarks for positional cloning of the resistance gene(s). These RAPD markers could also be used to increase the Mj-1 frequency during recurrent selection cycles and in backcrossing programs to minimize ’linkage drag’ in elite lines employed for the development of resistant F₁ hybrids. Received: 22 June 1999 / Accepted: 6 July 1999