Recent advances in molecular breeding:: The example of tomato breeding for flavor traits

With the development of molecular markers, genetic maps have been constructed in plant species which allow for the localization of major loci and QTLs controlling agronomical trait variation. Molecular markers have been widely used for the introgression of major loci, but marker-assisted selection for quantitative trait breeding is used less. Both theoretical and applied aspects of marker-assisted back-crossing of quantitative traits are presented, with special emphasis on a program of tomato breeding for flavor traits. Improving organoleptic quality of fresh market tomato fruit has become an important objective for tomato breeders. The detection of QTLs controlling the variation of tomato quality traits was performed in the progeny of a cross between a cherry tomato chosen for its good flavor and a line with bigger but less tasty fruits. Both physical traits (fruit weight, color and firmness) and chemical traits (dry matter weight, titratable acidity, pH, and the contents of soluble solids, sugars, lycopene, carotene and 12 aroma volatiles) were evaluated. The lines were also evaluated by descriptive sensory profiling (taste, texture and aroma). A number of QTLs were detected for all the traits, some with major effects. Colocalizations of QTLs controlling several traits were found. Most of the favorable alleles came from the cherry tomato parent for chemical and sensory traits, showing the potential usefulness of this line for tomato organoleptic quality improvement. A marker-assisted selection scheme was thus initiated in order to transfer into elite lines the five regions carrying the most important QTLs involved in fruit quality. The backcross scheme was first optimised taking into account both theoretical and practical aspects. Three recurrent lines were chosen in order to study the effect of genetic background on QTL expression. Applications of the scheme are presented.