Accessions Of Wild Species Research Articles

Tomato early blight (Alternaria solani): the pathogen, genetics, and breeding for resistance

Alternaria solani causes diseases on foliage (early blight), basal stems of seedlings (collar rot), stems of adult plants (stem lesions), and fruits (fruit rot) of tomato. Early blight is the most destructive of these diseases and hence receives considerable attention in breeding. For over 60 years, breeding for early blight resistance has been practiced, but the development of cultivars with high levels of resistance has been hampered by the lack of sources of strong resistance in the cultivated tomato and by the quantitative expression and polygenic inheritance of the resistance. In some accessions of wild species, high levels of early blight resistance have been found, but breeding lines still have unfavorable horticultural traits from the donor parent. Recently, the first linkage maps with loci controlling early blight resistance have been developed based on interspecific crosses. These maps may facilitate marker-assisted selection. This overview presents the current knowledge about the A. solani–tomato complex with respect to its biology, genetics, and breeding.

Screening of Lycopersicon sp. accessions for resistance to Pepper yellow mosaic virus

The tomato is a crop of great economical importance, however it is susceptible to a large number of pests and diseases, including viral disease for which the best control strategy is genetic resistance. The disease, caused by Pepper yellow mosaic virus (PepYMV) has become a recent problem. Consequently, the idea of this work was to screen 376 accessions of Lycopersicon sp. to find possible sources of resistance to PepYMV. Out of 355 accessions of L. esculentum inoculated with PepYMV, 52 did not express symptoms. However, the virus reached high concentration in the tissues as measured by indirect ELISA, and therefore they were not considered as safe sources of resistance. Among 21 accessions of wild Lycopersicon species, one of L. hirsutum was shown to be resistant, with no observed symptoms. A low concentration of the virus was detected as measured by indirect ELISA. This accession seems to be suitable for breeding programs aiming at incorporating resistance for this disease into commercial tomato cultivars.

Genetic diversity in ber (Ziziphus spp.) revealed by AFLP markers

SummaryBer (Ziziphus spp.), belonging to the buckthorn family Rhamnaceae, is an important fruit crop that grows naturally in harsh environmental conditions. Commercial cultivars have been developed in India, mostly through selection of productive natural variants propagated by budding on wild rootstocks and used for orchard plantations. There is a complete lack of information on the extent of genetic diversity in this fruit crop. In the present study, 33 accessions of cultivated Z. mauritiana (Lam.) and eight accessions of its wild relative Z. nummularia (Burm.f.) Wight & Arn. were investigated using AFLP. Eleven primer pairs used in AFLP analysis detected 952 fragments, of which 789 (83.8%) were polymorphic. Similarity coefficients ranging from 0.14 – 0.86 suggested that the 41 Ziziphus accessions used in this study were divergent. The wild relative being used as a rootstock showed greater diversity than the cultivated species. The range (0.37 – 0.86) and average (0.62) similarity among the 33 cultivated species accessions indicated the broad genetic base of the commercial cultivars. Cluster analysis revealed the complete separation of the 33 cultivated species accessions and the eight wild species accessions into two distinct groups. Morphologically similar cultivars clustered together with a high degree of similarity. A high correlation of the similarity matrix using a single primer combination with that based on all eleven primer combinations suggested that one or a few primer combinations could be used to estimate diversity in ber. Nine primer combinations detected 86 accession-specific amplified fragments and each primer combination gave a discrimination rate of 1. Any of these primers can thus be used to maintain the quality of both scions and rootstocks to establish productive orchards.

A Potato Collecting Expedition in the Province of Jujuy, Argentina and Disease Indexing of Virus and Fungus Pathogens in Andean Cultivars

A potato collecting expedition was carried out in the province of Jujuy, Argentina in March 24 to April 15, 2001. The objective was to collect local cultivars of potatoes and wild potato species, covering high mountain valleys not previously collected or areas where germplasm was not available. A total of 247 accessions was collected, 188 cultivated accessions of S. tuberosum subsp. andigenum, four of Solanum tuberosum subsp. tuberosum, two of S. curtilobum and 53 accessions of wild species. The wild species collected were S. acaule subsp. acaule, S. chacoense, S. infundibuliforme, S. megistacrolobum subsp. megistacrolobum, S. microdontum, S. gourlayi, S. × viirsooi and S. oplocense. Herbarium voucher specimens were obtained when possible. For the collection of cultivated potatoes, tubers were gathered from farmer's fields and in a few cases from stores or markets. Seed samples were generally obtained for the wild species. Detailed collection site data were recorded at every site. After breaking dormancy, the accessions of the cultivated species were screened for the presence of Potato virus Y (PVY), Potato virus X (PVX), Potato leafroll virus (PLRV), Potato virus S (PVS), Potato virus M (PVM), Potato virus V (PVV), Andean potato latent virus (APLV), Andean potato mottle virus (APMoV), Potato rough dwarf virus (PRDV), Potato spindle tuber viroid (PSTVd) and Spongospora subterranea f. sp. subterranea. PSTVd, APLV and PRDV were not detected, but different levels of infection are reported for the other pathogens assayed.

Effect of root exudates from different tomato genotypes on broomrape ( O. aegyptiaca) seed germination and tubercle development

Nine accessions of wild species and six cultivated tomato genotypes were evaluated for their resistance to Orobanche aegyptiaca. They differed in the number of tubercles fixed on their roots, emerged shoots of broomrape and broomrape dry weight. The wild species L. pennellii LA 716, L. hirsutum PI 247087, L. pimpinellifolium hirsute and L. chilense LA 1969 were shown to be the most resistant to O. aegyptiaca. Moderate levels of resistance were obtained in L. hirsutum B, L. hirsutum LA 1777 L. peruvianum D4×D5, L. peruvianum CMV sel INRA, Ferum, Cervil, Levovil and UC 82-B. Among the cultivated tomato genotypes, Tresor (hybrid Fl), Levovil and Momor were the most susceptible hosts to Orobanche aegyptiaca. Similar results were also obtained in a hydroponic co-culture system where L. pennellii LA 716 and L. hirsutum PI 247087 were the most resistant, whereas Ferum, Tresor and Momor were the most susceptible. The root exudates of Ferum, Tresor and Momor induced far greater germination of O. aegyptiaca seeds than those of L. hirsutum PI 247087 and L. pennellii LA 716. Moreover, root exudates of L. pennellii LA 716 inhibited the germination of O. aegyptiaca seeds. Their use as an irrigation liquid, in pot experiments, on the susceptible genotypes (Momor and M82) was a relatively efficient control method since both the number of tubercles and the biomass of O. aegyptiaca decreased when watered with L. pennellii LA 716 exudates, while spike emergence was retarded.

Chloroplast DNA Variation in the Most Primitive Cultivated Diploid Potato Species Solanum stenotomum Juz. et Buk. and its Putative Wild Ancestral Species using High-Resolution Markers

Solanum stenotomum Juz. et Buk. (2n = 2x = 24) is considered to be the most primitive diploid cultivated species from which all the other Andean cultivated potatoes were originated (Hawkes 1990). To disclose chloroplast DNA (ctDNA) variability and the maternal origin of S. stenotomum, 36 accessions of S. stenotomum and 86 accessions of putative wild ancestral species were determined for ctDNA types and analyzed by high-resolution markers (seven ctDNA microsatellites and an H3 marker). High-resolution markers discriminated 57 different ctDNAs (haplotypes), which were classified into the W-type ctDNA group and C-, S- and A-type ctDNA group, and within the latter group S- and A-type ctDNAs were distinct from each other among many different haplotypes mostly having C-type ctDNA. This ctDNA relationship supported our previous findings obtained for mostly Andean cultivated species (Sukhotu et al. 2004). Compared with other putative ancestral wild species, S. stenotomum showed somewhat limited ctDNA diversity, having two major haplotypes 1 and 2 also found in different wild species in different places. Therefore, the ctDNA in S. stenotomum was of at least dual origins either by successive domestication from different species or else by introgression after initial S. stenotomum arose.

Genetic diversity among varieties and wild species accessions of pea (Pisum sativum L.) based on molecular markers, and morphological and physiological characters.

Random amplified polymorphic DNA, simple sequence repeat, and inter-simple sequence repeat markers were used to estimate the genetic relations among 65 pea varieties (Pisum sativum L.) and 21 accessions from wild Pisum subspecies (subsp.) abyssinicum, asiaticum, elatius, transcaucasicum, and var. arvense. Fifty-one of these varieties are currently available for growers in western Canada. Nei and Li's genetic similarity (GS) estimates calculated using the marker data showed that pair-wise comparison values among the 65 varieties ranged from 0.34 to 1.00. GS analysis on varieties grouped according to their originating breeding programs demonstrated that different levels of diversity were maintained at different breeding programs. Unweighted pair-group method arithmetic average cluster analysis and principal coordinate analysis on the marker-based GS grouped the cultivated varieties separately from the wild accessions. The majority of the food and feed varieties were grouped separately from the silage and specialty varieties, regardless of the originating breeding programs. The analysis also revealed some genetically distinct varieties such as Croma, CDC Handel, 1096M-8, and CDC Acer. The relations among the cultivated varieties, as revealed by molecular-marker-based GS, were not significantly correlated with those based on the agronomic characters, suggesting that the 2 systems give different estimates of genetic relations among the varieties. However, on a smaller scale, a consistent subcluster of genotypes was identified on the basis of agronomic characters and their marker-based GS. Furthermore, a number of variety-specific markers were identified in the current study, which could be useful for variety identification. Breeding strategies to maintain or enhance the genetic diversity of future varieties are proposed.

Preliminary screening and selection for cold tolerance in annual wild Cicer species

The study was aimed to select cold tolerant accessions of annual wild Cicer species in the highlands in the west Mediterranean area of Turkey. A total of 43 accessions of eight annual wild Cicer species was screened for cold tolerance and compared with the best cold tolerant cultivars, ILC 8262 and FLIP 93-53C. After the sensitive check, ILC 533, was killed, accessions were evaluated using a 1–9 scale. Three accessions of Cicer bijugum, 2 accessions of Cicer reticulatum and 2 accessions of Cicer echinospermum were highly cold tolerant, while 3 accessions of C. bijugum, 9 accessions of C. reticulatum, 2 accessions of C. echinospermum and 1 accession of Cicer pinnatifidum were cold tolerant. All accessions of Cicer judaicum were sensitive to cold. Whereas Cicer chorassanicum and Cicer cuneatum were killed, Cicer yamashitae was recorded as highly sensitive to cold. The cold tolerant accessions of annual wild Cicer species were superior to the best cultivar as far as hardiness is concerned.

Prospects for celeriac (Apium graveolens var. rapaceum) improvement by using genetic resources of Apium, as determined by AFLP markers and morphological characterization

AbstractGenetic relationships among elite celeriac varieties and celeriac accessions conserved in genebanks are generally unknown. The objective of this study was to use amplified fragment length polymorphism (AFLP) markers and morphological characterization to identify material that could be of use in celeriac breeding. Genetic relationships were estimated in 34 elite celeriac varieties bred in Europe and 28 celeriac accessions conserved at the German genebank. Two varieties of celery, two varieties of leaf celery and three genebank accessions of wild Apium species were additionally analysed. Fifteen EcoRI/MseI-based AFLP primer combinations were used. Polymorphic AFLP fragments were scored for calculation of Jaccard's coefficient of genetic similarity (GS). Morphological distances (MD) were determined based on 11 morphological traits. Average GS estimate in elite germplasm (GS=0.90) was higher than in exotic germplasm (GS=0.80). An AMOVA (analysis of molecular variance) revealed that a high proportion of variation was due to variation within elite celeriac varieties and genebank accessions. Although GS and MD matrices were poorly correlated (r=0.22), UPGMA (unweighted pair group method using arithmetic averages) cluster analyses revealed clear genetic groupings of celeriac germplasm, which was supported by morphological traits. Elite, moderately bred and exotic varieties formed distinct clusters, indicating that only a part of the available genetic diversity in celeriac germplasm has been exploited in breeding. Distinct Apium species might be useful for the introgression of new genes into cultivated celeriac material. Broadening of celeriac collections in genebanks and detection of new genetic resources are vital for improvements in celeriac breeding.

Genetic resources of the genus Cucumis and their morphological description (English-Czech version)

Czech collections of Cucumis spp. genetic resources includes 895 accessions ofcultivated C. sativus and C. melo species and 89 accessions of wild species. Knowledge of their morphological and biological features and a correct taxonomical ranging serve a base for successful use of germplasm in modern breeding. List of morphological descriptors consists of 65 descriptors and 20 of them are elucidated by figures. It provides a tool for Cucumis species determination and characterization and for a discrimination of an infraspecific variation. Obtained data can be used for description of genetic resources and also for research purposes.  

Comparison of resistance to Tomato leaf curl virus (India) and Tomato yellow leaf curl virus (Israel) among Lycopersicon wild species, breeding lines and hybrids

The objective of this study was to screen wild and domesticated tomatoes for resistance to Tomato yellow leaf curl virus, Israel (TYLCV-Is) and Tomato leaf curl virus from Bangalore isolate 4, India (ToLCV-[Ban4]) to find sources of resistance to both viruses. A total of 34 tomato genotypes resistant/tolerant to TYLCV-Is were screened for resistance to ToLCV-[Ban4] under glasshouse and field conditions at the University of Agricultural Sciences, Bangalore, India. Resistance was assessed by criteria like disease incidence, symptom severity and squash-blot hybridization. All the tomato genotypes inoculated with ToLCV-[Ban4] by the whitefly vector Bemisia tabaci (Gennadius) produced disease symptoms. In some plants of the lines 902 and 910, however, the virus was not detected by hybridization. The tomato genotypes susceptible to ToLCV-[Ban4] by whitefly-mediated inoculation were also found susceptible to the virus under field conditions. However, there were substantial differences between genotypes in disease incidence, spread, symptom severity and crop yield. Despite early disease incidence, many genotypes produced substantially higher yields than the local hybrid, Avinash-2. Sixteen tomato genotypes from India resistant/tolerant to ToLCV-[Ban4] were also tested for TYLCV-Is resistance at the Hebrew University of Jerusalem, Rehovot, Israel. Accessions of wild species, Lycopersicon hirsutum LA 1777 and PI 390659 were the best sources of resistance to both viruses. Lines 902 and 910, which were, resistant to TYLCV-Is were only tolerant to ToLCV-[Ban4] and accession Lycopersicon peruvianum CMV Sel. INRA, resistant to ToLCV-[Ban4], was only tolerant to TYLCV-Is. Implications of using the resistant lines in breeding programme is discussed.

Evaluation of Genetic Diversity in Rice Subspecies Using Microsatellite Markers

Molecular markers are useful tools for evaluating genetic diversity and determining cultivar identity. The purpose of this study was to evaluate the genetic diversity within a diverse collection of rice (Oryza sativa L.) accessions, and to determine differences in the patterns of diversity within the two rice subspecies indica and japonica Thirty‐eight rice cultivars of particular interest to U.S. breeding programs and two wild species accessions (O. rufipogon Griffithand O. nivara Sharma et Shastry) were evaluated by means of 111 microsatellite markers distributed over the whole rice genome. A total of 753 alleles were detected, and the number of alleles per marker ranged from 1 to 17, with an average of 6.8. A positive correlation was found between the number of alleles per locus and the maximum number of repeats within a microsatellite marker. Compared to indica cultivars, the japonica group showed significantly higher genetic diversity on chromosomes 6 and 7, and considerably lower diversity on chromosome 2. All rice cultivars and lines could be uniquely distinguished, and the resulting groups corresponded exactly to the indica and japonica subspecies, with japonica divided into temperate and tropical types. With stepwise discrimination, two subsets of approximately 30 markers were identified that produced genetic distance matrices and dendrograms that were the same as those produced by means of all 111 markers. The results suggested that a relatively small number of microsatellite markers could be used for the estimation of genetic diversity and the identification of rice cultivars.

Using the collection of wild species in sunflower breeding

We started to collect wild sunflower (Helianthus L) species in 1998. The main task of this collection is to promote the development of new breeding material. In particular, it was planned to use the wild species as resistance sources to the major diseases of sunflower and as Rf genes sources to the new CMS types. The collection consists presently of 140 accessions of annual wild species and 110 samples of 27 perennial ones. The assessment of the wild sunflowers collection for resistance to Sclerotinia (Sclerotinia sclerotiorum) Phomopsis {Phomopsis helianthi) and broomrape (Orobanche cumana) was carried out on artificial infectious backgrounds. Resistance to other diseases was estimated simultaneously under natural infection. The immunologic potential of the collection was assessed three times during the vegetative period: at early stages of ontogeny (first week of June), at full flowering (second week of August) and at the end of the vegetation period (fourth week of September). A group of 101 samples showed various types of resistance to a complex of diseases. F1 and F2 generations were produced from more than 100 cross combinations of sunflower inbred lines and wild annual species. The tolerance to Phomopsis in field conditions was recorded in the F1 and F2 interspecific hybrids. Also, several sources of fertility restoration for new CMS types (ANN1, ARG1, PEF1, GIG1 and RIG1) have been found among the accessions of wild annual sunflower species. Thus, certain samples from the collection may be useful for breeding to disease resistance. The development of new initial material for sunflower breeding has already started.

RFLP analysis of genetic variation in species of section Arachis , genus Arachis (Leguminosae)

Four A-genome species of the genus Arachis (A. cardenasii, A. correntina, A. duranensis, A. kempff-mercadoi), three B genomes species (A. batizocoi, A. ipaensis and A. magna),the AABB allotetraploid A. hypogaea (cultivated peanut) and introgression lines resulting from a cross between A. hypogaea and A. cardenasii were analyzed by RFLP. The A genome species (cytologically characterized by the presence of a small chromosome pair ‘A’) were closely similar to each other and shared a large number of restriction fragments. In contrast, the B genome species differed more from one another and shared few fragments. The results of this study indicate that the absence of the small chromosome pair is not a good criterion for grouping species of section Arachis as B genome species, since their genome might be quite distinct from the B genome of A. hypogaea.The lowest genetic variation was detected within accessions of A. duranensis (17 accessions), followed by A. batizocoi (4 accessions) and A. cardenasii (9 plants of accession GKP 10017).The high level of genetic variation found in A. cardenasii might indicate that not all accessions of wild species of Arachis are autogamous, as reported for A. hypogaea.

Resistance of Wild Arachis Species to Late Leaf Spot and Rust in Greenhouse Trials.

Seventy-four accessions of wild Arachis species along with a susceptible line TMV 2 belonging to A. hypogaea were evaluated for resistance to late leaf spot and rust under greenhouse environment. The experiment was conducted twice to confirm the resistance of the accessions. Percentage of leaf area damaged and disease score on a 1 to 9 scale for late leaf spot and rust were recorded for all accessions in both experiments. Additionally, percentage of defoliation was recorded for late leaf spot. One accession, ICG 8190, of A. hoehnei and one accession, ICG 13199, of A. duranensis were asymptomatic to late leaf spot in both experiments. In addition, 26 accessions were classified as resistant, 10 accessions were moderately resistant, and the remaining 36 accessions exhibited a susceptible reaction to late leaf spot. Only one accession, ICG 8954 of A. kuhlmannii, remained asymptomatic to rust in both experiments. All other accessions, except ICG 8206 of A. ipaensis, ICG 8197, ICG 8198, ICG 11549, and ICG 13178 of A. monticola, and ICG 13171 of A. stenosperma were classified as highly resistant to rust. Four accessions of A. monticola were susceptible to late leaf spot and rust.

Nuclear- and chloroplast-microsatellite variation in A-genome species of rice

Simple sequence length polymorphism analysis was carried out to reveal microsatellite variation and to clarify the phylogenetic relationships among A-genome species of rice. Total DNA from 29 cultivars (23 Oryza sativa and 6 O. glaberrima) and 30 accessions of wild A-genome species (12 O. rufipogon, 5 O. glumaepatula, 2 O. longistaminata, 6 O. meridionalis, and 5 O. barthii) was used as a template for PCR to detect 24 nuclear and 10 chloroplast microsatellite loci. Microsatellite allelic diversity was examined based on amplified banding patterns. Microsatellites amplified clearly in all 59 accessions, with an average of 18.4 alleles per locus. The polymorphism information content (PIC) value ranged from 0.85 to 0.94, with an average of 0.89. At the species level, high average PIC values were observed in O. sativa (0.79) and O. rufipogon (0.80). For chloroplast microsatellites, the average number of alleles per locus and the average PIC value were 2.9 and 0.38, respectively. While the magnitude of diversity was much greater for nuclear microsatellites than for chloroplast microsatellites, they showed parallel patterns of differentiation for each taxonomic group. Using the ratio of common alleles (estimated as size of amplified fragments) as a similarity index, the average percentages of common microsatellite alleles were calculated between taxa. For both nuclear and chloroplast microsatellites, O. sativa showed the highest similarity values to O. rufipogon, and O. glaberrima was most similar to O. barthii. These data support previous evidence that these cultivars originated from the corresponding wild ancestral species.

Rice improvement at IRRI: An example of International Collaboration.

The rice improvement program of International Rice Research Institute (IRRI) is an international and interdisciplinary effort. It has five components, i.e.(1) Germplasm collection, conservation and evaluation, (2) Methodology development, (3) Development of improved germplasm, (4) Exchange and evaluation of germplasm, and (5) Training and staff development. More than 91, 000 accessions of cultivated rice and wild species of Oryza are being maintained in germplasm bank at IRRI. These are being evaluated for various morphological traits, grain quality and resistance to biotic and abiotic stresses. Methods for accelerated rice improvement have been developed. Superior germplasm has been bred for all the rice growing ecologies with a focus on yield enhancement, shorter growth duration, superior grain quality, resistance to diseases and insects and tolerance to abiotic stresses. More than 600 improved varieties have been selected from the IRRI bred materials and released in rice growing countries of Asia, Africa and Latin America. Rice production more than doubled in a 30-year period and most of the rice growing countries became self-sufficient. The germplasm is shared freely with the national programs on the basis of requests from scientists or through international nurseries. More than 1, 000 rice breeders have been trained at IRRI and are leading the rice improvement programs in respective countries.

Biodiversity and ecogeography of wild Lactuca spp. in some European countries

Staff members of the Department of Botany of Palacký University in Olomouc and Gene Bank Department – Workplace Olomouc, Research Institute of Crop Production in Prague, Czech Republic, conducted an expedition in seven European countries (Austria, Czech Republic, France, Germany, Italy, the Netherlands, Switzerland) in August/September 1999 to collect wild Lactuca spp. germplasm and study its geographic distribution, ecology and biodiversity. During the mission, more than 600 locations were visited resulting in the collection of 602 seed samples (accessions) of wild Lactuca species and 13 seed samples of related genera (Chondrilla and Mycelis). Lactuca serriola f. serriola, L. serriola f. integrifolia, L. saligna and L. viminea subsp. chondrilliflora were prevalent in southern Europe (Italy, France), however, only L. serriola was common in central and western Europe (Austria, Czech Republic, Germany, Netherlands, Switzerland). The greatest diversity of Lactuca species was found in France, where also the most seed samples (165) were collected. The most characteristic habitats with a high density of Lactuca spp. populations were observed along roads and highways, grassy ditches, ruderal communities, and dust-heaps. Natural infections by powdery mildew (Erysiphe cichoracearum) and downy mildew (Bremia lactucae) on some wild Lactuca spp. were observed. Recent observations concerning the geographic distribution, population structure, habitats, and natural occurrence of diseases of Lactuca spp. are discussed. This assemblage of genetic resources of Lactuca spp. can serve as the basis of future studies of species diversification, spatial population structure, plant microevolution, domestication processes, and genetic variability of host-parasite interactions.

Status of the Arachis Germplasm Collection at ICRISAT

Abstract ICRISAT maintains a substantial Arachis germplasm collection of 14,723 accessions, comprising 14,310 accessions of cultivated peanut (Arachis hypogaea L.) from 92 countries and 413 accessions of wild species representing 43 taxa. All germplasm is freely available for distribution. Forty-five percent of the cultivated peanut collection is of var. hypogaea, followed by 35.7% var. vulgaris and 16.1% var.fastigiata. Varieties hirsuta and aequatoriana are represented by 20 and 15 accessions, respectively. All passport and characterization data are accessible through the internet. To enhance the utilization of the collection and understand the diversity it contains, efforts have focused on characterization and documentation of the collection and the formation of a core of 1704 A. hypogaea accessions. These are representative of the genetic diversity in the entire collection. The core provides an entry point into the collection and is currently being evaluated for maturity, biotic, and abiotic stress resistance and quality parameters, including aflatoxin contamination. A subset of the core is used in prescreening for polymorphic molecular markers. Evaluation of the wild Arachis collection to major abiotic stresses is a continuing process. Future efforts in both the wild and cultivated collections will focus on germplasm exchange and acquisition, and specific regions for future collections are identified. The development of molecular markers for diversity assessments in all Arachis taxa and alternative strategies for utilization of the wild species are also important areas of research.

Characterization and mapping of resistance to Oidium lycopersicum in two Lycopersicon hirsutum accessions: evidence for close linkage of two Ol-genes on chromosome 6 of tomato.

The cultivated tomato is susceptible to powdery mildew (Oidium lycopersicum). Several accessions of wild species are resistant. In this study we describe (i) the genetics and mapping of resistance to O. lycopersicum in G1.1290, one of the resistant accessions in Lycopersicon hirsutum, (ii) fine mapping of Ol-1 originated from L. hirsutum G1.1560, another resistant accession of L. hirsutum, and (iii) tests of allelism for resistance in G1.1290 and G1.1560. Initially, it is demonstrated that the resistance in G1.1290 to O. lycopersicum is controlled by an incompletely dominant gene, designated Ol-3. By using an advanced breeding line (ABL) containing introgression fragment(s) from G1.1290, Ol-3 was found to be associated with several RFLP and SCAR markers on chromosome 6. By using these markers, Ol-3 was mapped between markers TG25/SCAF10 and H9A11 on chromosome 6. Secondly, after testing some F3 lines and their progenies from the cross between L. esculentum cv Moneymaker and L. hirsutum G1.1560, we provided more evidence for the map position of Ol-1 to lie between SCAF10 and H9A11, indicating that Ol-1 and Ol-3 are in the same chromosome region. Thirdly, although allelism tests could not discriminate between Ol-1 and Ol-3, (indirect) evidence suggested that these two genes are not identical. They might instead represent functional genes of a cluster of Ol-homologues.