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- Author or Editor: Samuel F. Hutton x
Mechanization of farm work is increasingly demanded for the current system of fresh-market tomato (Solanum lycopersicum) production. One essential element for the adoption of mechanical harvest of fresh-market tomatoes is modification of plant architecture so that the crop can be grown without staking. To address this in the current production system, the stem length should be reduced. The tomato brachytic (br) locus has been shown to be a primary source of reducing stem length. To improve the effectiveness of marker-assisted selection (MAS) for the br-mediated trait and to provide resources for cloning this gene, we fine-mapped br to the tomato genome. Fine mapping of br to chromosome 1 was initiated by a survey of genome-wide single-nucleotide polymorphisms (SNPs) shown to be polymorphic between the br phenotype and normal using the tomato array, identifying the interval that harbors br. Genetic markers that flank the locus further permitted saturation of the interval. Twenty-six fixed homozygous recombinant lines were identified together in two different populations and tested with those markers. These efforts resulted in the first report that the br is fine-mapped to a 763-kb physical interval of tomato reference genome. The identified markers close to the br in the present study will be significant resources for MAS and gene cloning research.
Bacterial spot of tomato (Solanum lycopersicum), caused by several Xanthomonas species, is one of the most important diseases of the crop in humid production regions of the world. Conventional breeding approaches for resistance to bacterial spot previously identified race-specific resistances, but current efforts also seek to use quantitative trait loci (QTLs) effecting broad-spectrum resistance. Resistance QTLs and candidate QTLs have been reported on several chromosomes, including a major QTL on chromosome 11. Fusarium wilt (Fusarium oxysporum f. sp. lycopersici) race 3 resistance gene, I-3, is associated with smaller fruit size and has been implicated in other associations with negative characteristics. We evaluated four F2 populations involving the bacterial spot-tolerant breeding lines Fla. 8517, Fla. 8233, and Fla. 8326 across two field seasons to validate and quantify previously identified loci and to test for an effect of I-3 on bacterial spot sensitivity. The chromosome 11 QTL and the I-3 locus were each consistently positively and negatively associated with resistance, respectively, and together explained from 44% to 47% of the variation in each population. The chromosome 11 QTL displayed a dominant to incompletely dominant effect, reducing infection by 14% to 25%. This QTL is distinct from the X. perforans race T3 hypersensitivity loci, Rx-4 and Xv3. The I-3 locus contributed to as much as a 20% increase in infection in I-3/I-3 plants vs. i-3/i-3 plants, and heterozygosity for I-3 generally resulted in an intermediate susceptible response. Significant effects for QTLs on chromosomes 3, 5, and 12 were also observed, but these effects were not consistent in all populations or seasons in which they were segregating. Implications of these findings toward breeding strategies are discussed.
Tomato yellow leaf curl virus (TYLCV), a Begomovirus in the family Geminiviridae, is an important disease of cultivated tomato (Solanum lycopersicum L.) in many parts of the world. Disease is managed primarily by chemical control of the vector, the sweetpotato whitefly, Bemisia tabaci (Genn.), and by growing resistant varieties. Resistance derived from the cultivar Tyking is being used in many breeding programs, but the location of resistance factors has not been reported. The breeding lines Fla. 8753 and Fla. 344 both have high levels of resistance to TYLCV derived from ‘Tyking’ and from S. chilense accession LA 1938, but none of their parent lines contain any of the known genes Ty-1 to Ty-4. An additional resistance locus, Ty-5, was recently identified, and to determine if this locus controls TYLCV resistance in Fla. 8753 and Fla. 344, appropriate segregating populations were analyzed using the Ty-5 marker, SlNAC1. Results show that SlNAC1 cosegregates with a recessive allele derived from ‘Tyking’. We suggest the gene symbol ty-5 be used to describe this gene. Mean disease severity of progeny homozygous for either the resistant or susceptible alleles did not equal parental levels of resistance and susceptibility, respectively, suggesting the involvement of an additional gene that is likely derived from LA1938.
Resistance of tomato (Solanum lycopersicum) to bacterial spot race T4 (Xanthomonas perforans) was characterized by generation means analysis (GMA) in three advanced breeding lines: Fla. 8326, Fla. 8233, and Fla. 8517. GMA of Fla. 8326 for two of three seasons (Fall 2006 and Summer 2007) indicated that resistance is mostly dominant with significant additive and epistatic effects. GMA of Fla. 8233 in Spring 2007 and of Fla. 8517 in Summer 2007 also showed dominance to be the main effect in addition to additive and epistatic effects. Duplicate dominance or recessive suppressor type epistasis was indicated in each breeding line. Transgressive segregation was not clearly observed in F2 populations of crosses between resistant parents, suggesting that these lines have quantitative trait loci in common.
The jointless pedicel trait of tomato conferred by the j-2 gene is widely used in processing markets for stem-free removal of fruit to accommodate mechanized harvest. Although current utilization of j-2 for fresh-market tomato breeding is limited, interest in this trait may increase as breeders seek to address high labor costs through the development of mechanically harvestable cultivars for the fresh market. Yet, the introduction of this trait into new market classes heavily relies on phenotypic selection because there are presently no high-throughput methods available to genotype j-2. Reliable, high-throughput molecular markers to genotype the presence/absence of j-2 for selective breeding were developed. The molecular markers described here use the high-resolution DNA melting analysis (HRM) genotyping with single-nucleotide polymorphism (SNP) and derived cleaved amplified polymorphic sequence (dCAPS)–based genotyping. Two separate HRM-based markers target the j-2 on chromosome 12 or a linked sequence region 3.5 Mbp apart from the gene, and a dCAPS marker resides on the latter. We demonstrate the association between each marker and the jointless pedicel phenotype using segregating populations of diverse filial generations in multiple genetic backgrounds. These markers provide a useful resource for marker-assisted selection of j-2 in breeding populations.