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In tomato, Lycopersi conesculentum Mill., currently there are >285 known morphological, physiological and disease resistance markers, 36 isozymes, and >1000 RFLPs, which have been mapped onto the 12 tomato chromosomes. In addition, currently there are >162,000 ESTs, of which ∼3.2% have been mapped. Several tomato genetic maps have been developed, mainly based on interspecific crosses between the cultivated tomato and its related wild species. The markers and maps have been used to locate and tag genes or QTLs for disease resistance and other horticultural characteristics. Such information can be used for various purposes, including marker-assisted selection (MAS) and map-based cloning of desirable genes or QTLs. Many seed companies have adopted using MAS for manipulating genes for a few simple morphological characteristics and several vertical disease resistance traits in tomato. However, MAS is not yet a routine procedure in seed companies for manipulating QTLs although it has been tried for a few complex disease resistance and fruit quality characteristics. In comparison, the use of MAS is less common in public tomato breeding programs, although attempts have been made to transfer QTLs for resistances to a few complex diseases. The potential benefits of marker deployment to plant breeding are undisputed, in particular for pyramiding disease resistance genes. It is expected that in the near future MAS will be routine in many breeding programs, taking advantage of high-resolution markers such as SNPs. For quantitative traits, QTLs must be sought for components of genetic variation before they are applicable to marker-assisted breeding. However, MAS will not be a “silver bullet” solution to every breeding problem or for every crop species.

Lycopene is the red pigment and a major carotenoid in tomato (Lycopersicon esculentum Mill.) fruit. It is a potent natural antioxidant, and the focus of many tomato genetics and breeding programs. Crop improvement for increased fruit lycopene content requires a rapid and accurate method of lycopene quantification. Among the various available techniques, high-performance liquid chromatography (HPLC) can be accurate, however, it is laborious and requires skilled labor and the use of highly toxic solvents. Similarly, spectrophotometric methods, although easier than HPLC, also require time-consuming extractions and may not be as accurate as HPLC, as they often overestimate fruit lycopene content. Colorimetric estimation of fruit lycopene using chromaticity values has been proposed as an alternative rapid method. Previous studies that examined the utility of this technique, however, were confined to the evaluation of only one or few cultivars and, therefore, lacked broad applicability. The purpose of the present study was to examine the utility of chromaticity values for estimating lycopene and β-carotene contents in tomato across diverse genetic backgrounds. Measurements of the chromaticity values (L^*, a^*, b^*, C^*, h^*) were taken on whole fruit and purée of 24 tomato genotypes and were compared with HPLC measurements of fruit lycopene and β-carotene. Examination of different regression models indicated that a model based on the transformed value a^*4 from purée measurements explained up to 94.5% of the total variation in fruit lycopene content as measured by HPLC. When this model was applied to a second set of fruit harvested at a later date from the same 24 genotypes, it explained more than 90% of the total variation in lycopene, suggesting its reliability. The best estimation for β-carotene content was obtained by using the b^* chromaticity value from whole fruit measurements or the transformed a^*2 value from purée measurements. Neither model, however, could explain more than 55% of the variation in β-carotene content, suggesting that chromaticity values may not be appropriate for estimating tomato β-carotene content. The overall results indicated that fruit lycopene content could be measured simply and rather accurately across a wide range of tomato genotypes using chromaticity values taken on fruit purée.

Early blight (EB), caused by the fungus Alternaria solani, is a destructive disease of tomato (Lycopersicon esculentum) worldwide. Sources of genetic resistance have been identified within related wild species, including green-fruited L. hirsutum and red-fruited L. pimpinellifolium. We have employed traditional protocols of plant breeding and contemporary molecular markers technology to discern the genetic basis of EB resistance and develop tomatoes with improved resistance. Backcross breeding has resulted in the development of germplasm with improved resistance; however, linkage drag has been a major obstacle when using L. hirsutum as a donor parent. To identify and map QTLs for EB resistance, we used several filial and backcross populations derived from interspecific crosses between L. esculentum and either L. hirsutum or L. pimpinellifolium. In each population, an average of seven resistance QTLs were detected. While similar QTLs were detected in different generations of the same cross, generally different QTLs were identified in populations derived from different crosses. The results suggested stability of QTLs across environments and generations but variation in QTLs in different interspecific populations. It is expected that marker-assisted pyramiding of QTLs from different sources results in development of germplasm with strong and durable resistance. Further inspection of the results led to the identification and selection of six QTLs with stable and independent effects for use in marker–assisted selection (MAS). However, to facilitate “clean” transfer and pyramiding of these QTLs, near-isogenic lines (NILs) containing individual QTLs in a L. esculentum background should be developed.