N. Kedar

Mapping and introgression of a tomato yellow leaf curl virus tolerance gene, TY-1.

The whitefly-transmitted tomato yellow-leaf curl gemini-virus (TYLCV) is a major pathogen of tomatoes. The wild tomato species Lycopersicon chilense, which is resistant to the virus, was crossed to the cultivated tomato, L. esculentum. The backcross-1 selfed (BC1S1) generation was inoculated and a symptomless plant was selected. This plant was analyzed using 61 molecular markers, which span the tomato genome, to determine which L. chilense chromosome segments were introgressed. A BC2S1 population was cage-inoculated with viroliferous whiteflies (Bemisia tabaci), the natural insect vector of the virus, and subjected to RFLP analysis. Markers on chromosomes 3 and 6 were significantly associated with the level of tolerance; the association of chromosome-6 markers was further substantiated in two additional BC2S1 populations. A tolerant BC2S1 plant which was homozygous for L. chilense introgressions in chromosomes 3, 6 and 7 was crossed to generate a BC3S1 population which was planted in an infested field. A TYLCV-tolerance gene with partial dominance, TY-1, was mapped to chromosome 6; two modifier genes were mapped to chromosomes 3 and 7. Field and whitefly-mediated cage inoculations of nearly-isogenic lines in BC3S3 supported our conclusion that TY-1 is the major TYLCV-tolerance locus.

Screening Lycopersicon accessions for resistance to tomato yellow leaf curl virus: presence of viral DNA and symptom development

Twenty-three Lycopersicon accessions representing five tomato species were screened for resistance to the tomato yellow leaf curl virus (TYLCV). Plants were grown in a field naturally infested with Bemisia tabaci, the natural vector of this geminiviral disease. The screened genotypes were examined for the presence of viral DNA and symptom development at 2-wk intervals. Tomato cultivars harbored the virus and developed symptoms. Accessions of the wild species L. pimpinellifolium, L. hirsutum, and L. peruvianum showed variance in their response to infection (.)

Morphological and physiological characters affecting flower drop and fruit set of tomatoes at high temperatures

SummarySeven tomato cultivars and lines were studied under high temperature conditions. Fruit set varied between 77.3% in the heat tolerant cv. Hotset, 62% in cv. Gamad and 16.3% in the most sensitive cv. Hosen-Eilon. The characters contributing to low fruit set were bud drop, splitting of the antheridial cone, style exsertion and reduction of the quantity and/or functionality of the gametes. Employing the above characters as criteria for selection, fruit set of an F4 line, phenotypically similar to the sensitive parent, was improved to 63.1%. Improved fruit set, 87.6%, was also obtained in an F1 hybrid between ‘Hotset’ and ‘Gamad’. The importance is discussed of various easily recognizable flower components contributing to satisfactory fruit set under high temperatures and their possible use in breeding is elaborated.

Replication of tomato yellow leaf curl virus (TYLCV) DNA in agroinoculated leaf discs from selected tomato genotypes.

The leaf disc agroinoculation system was applied to study tomato yellow leaf curl virus (TYLCV) replication in explants from susceptible and resistant tomato genotypes. This system was also evaluated as a potential selection tool in breeding programmes for TYLCV resistance. Leaf discs were incubated with a head-to-tail dimer of the TYLCV genome cloned into the Ti plasmid ofAgrobacterium tumefaciens. In leaf discs from susceptible cultivars (Lycopersicon esculentum) TYLCV single-stranded genomic DNA and its double-stranded DNA forms appeared within 2–5 days after inoculation. Whiteflies (Bemisia tabaci) efficiently transmitted the TYLCV disease to tomato test plants following acquisition feeding on agroinoculated tomato leaf discs. This indicates that infective viral particles have been produced and have reached the phloem cells of the explant where they can be acquired by the insects. Plants regenerated from agroinfected leaf discs of sensitive tomato cultivars exhibited disease symptoms and contained TYLCV DNA concentrations similar to those present in field-infected tomato plants, indicating that TYLCV can move out from the leaf disc into the regenerating plant. Leaf discs from accessions of the wild tomato species immune to whitefly-mediated inoculation,L. chilense LA1969 andL. hirsutum LA1777, did not support TYLCV DNA replication. Leaf discs from plants tolerant to TYLCV issued from breeding programmes behaved like leaf discs from susceptible cultivars.

The potential of ripening mutants for extending the storage life of the tomato fruit

SummaryThe effect of the fruit ripening mutants rin, gr, nor and Nr on storage life and pigmentation was investigated in homozygous material, in heterozygous F1 combinations between the mutants with the colour mutant hp and with the normal cv. Kewalo. Crosses with nor showed a 3-to 5-fold increase in storage life in comparison with the normal cv. or with hp. Maximum pigmentation of the fruits of crosses with nor was pale-red with vineripened fruit and pink with fruit harvested at the breaker stage and ripened on the shelf. The ripening inhibitory effect of rin in the different F1 combinations was less pronounced than that of nor, and the colour of the fruits was improved. Fruits of the F1 cross between rin and nor showed greatly improved storage life and developed pink or pale red colour. Most heterozygotes with hp showed improved pigmentation. Problems anticipated in utilizing ripening mutants in breeding for improved keeping quality are discussed.

A random amplified polymorphic DNA (RAPD) molecular marker for the Tm-2 a gene in tomato

SummaryA RAPD marker, linked to the Tm-2agene engendering TMV resistance in tomatoes, was identified. The validity of the RAPD marker was corroborated by screening several tomato varieties, and correctly identifying those which carried Tm-2a, as well as by F2 segregation analysis. All tested resistant varieties descending from a common Lycopersicon peruvianum/esculentum ancestor, LA1791, exhibited this marker.

Non-random segregation of gene I for Fusarium resistance in the tomato

Significant deviations from the ratios expected, according to the single dominant gene hypothesis for resistance to Fusarium wilt, were found in crosses involving several susceptible and resistant tomato lines. The susceptible class was the deficient one in F2 and F3 populations, as well as in backcrosses in which the heterozygotic resistant F1 served as the male parent. The reciprocal backcross, with the F1 as the female and the homozygous susceptible as the male, gave segregations better approximating or consistent with the single gene hypothesis. Reciprocal F1 and F2 generations did not give any evidence of cytoplasmic effects.The results were interpreted assuming preferential fertilization of ovules by pollen grains carrying the dominant I allele for resistance.The practical implications of the phenomenon of preferential fertilization in breeding for Fusarium resistance are discussed.

A SCAR marker linked to the ToMV resistance gene, Tm22, in tomato

A polymerase chain reaction (PCR)-based co-dominant marker was developed which is tightly linked to Tm22. This dominant locus confers resistance to ToMV in tomato. Random-amplified-polymorphic DNA (RAPD) screening was carried out with DNA from ToMV-susceptible and resistant tomato near-isogenic lines. A polymorphic band linked to ToMV resistance was observed. The polymorphic fragment was cloned and the DNA sequences of both ends determined. Specific PCR primers were designed from these sequences. PCR amplification with the specific primers resulted in an amplified band (SCAR) in both susceptible and resistant tomato lines. The amplified band from the susceptible lines could, however, be discerned from that of the resistant ones after cleavage with the restriction enzyme Hind III. In an F2 population of 90, the polymorphic markers co-segregated with susceptibility or resistance, as determined by biological assays for ToMV resistance. The reported SCAR marker is linked to ToMV resistance not only in cultivars derived from American lineage, but also from European lineage. This method enables the distinction of homozygous and heterozygous individual plants in segregating populations, and provides a convenient and rapid assay for both selection and quality control during breeding programs and hybrid seed production, respectively.

Changed sex expression and possibilities for F1-hybrid seed production in some cucurbits by application of Ethrel and Alar (B-995)

SummaryExperiments were performed to study the effects of the ethylene releasing compound Ethrel on sex expression in cucumbers and squash, and of Alar (B-995) plus Ethrel in muskmelons. As a result of foliage sprays with one or both of the above compounds normally monoecious plants produced female flowers only, for the first 2–3 weeks of flowering. The optimum treatments for cucumbers were two foliage sprays with Ethrel 250 ppm or 500 ppm applied at the second and the fourth true leaf stages. The optimum treatments for squash were Ethrel 250 ppm and 500 ppm applied at the first and the third true leaf stages.High doses (1000 ppm) or repeated applications of Ethrel retarded growth of muskmelons and cucumbers. Applications of B-995 (5000 ppm) plus Ethrel (500 ppm) at the second true leaf stage inhibited male flowering for 2–3 weeks of the flowering period. F1-hybrid seeds of muskmelons were experimentally produced in large isolation cages in the field, using two monoecious lines as female parents. The merits and some of the problems associated with the production of F1-hybrid seeds by the above methods are discussed.

Mode of inheritance of Alcobaca, a tomato fruit-ripening mutant

SummaryAlcobaca, an abnormally ripening tomato mutant, was crossed with the normal cultivar Rutgers. Examination of F1, F2 and BC1 populations indicated that a single recessive gene controls the mutant phenotype. No maternal effect was detected.