Development and Testing of a Method for the Systematic Discovery and Utilization of Novel QTLs in the Production of Improved Crop Varieties: Tomato as a Model System

Modern cultivated varieties carry only a small fraction of the variation present in the gene pool. The narrow genetic basis of modern crop plants is a result of genetic bottlenecks imposed during early domestication and modern plant breeding. The wild ancestors of most crop plants can still be found in their natural habitats and Germplasm Centers have been established to collect and maintain this material. These wild and unadapted resources can potentially fuel crop plant improvement efforts for many years into the future (Tanksley and McCouch 1997). Unfortunately, scientists have been unable to exploit the majority of the genetic potential warehoused in germplasm repositories. This is especially true as regards to the improvement of quantitative traits like yield and quality. One of the major problems is that much of the wild germplasm is inferior to modern cultivars for many of the quantitative traits that breeders would like to improve. Our research, focusing on the tomato as a model system, has shown that despite their inferior phenotypes, wild species are likely to contain QTLs that can substantially increase the yield and quality of elite cultivars (de Vicente and Tanksley 1992, Eshed and Zamir 1994, Eshed et al. 1996). Using novel population structures of introgression lines (ILs; Eshed and Zamir 1995) and advanced backcross lines (AB; Tanksley et al. 1996) we identified and introduced valuable QTLs from unadapted germplasm into elite processing tomato varieties. Populations involving crosses with five Lycopersicon species (L. pennellii (Eshed and Zamir 1994; Eshed et al. 1996; Eshed and Zamir 1996), L. hirsutum (Bernacchi et al. 1998), L. pimpinellifolium (Tanksley et al. 1996), L. parviflorum (unpub.), L. peruvianum (Fulton et al. 1997) have been field and laboratory tested in a number of locations around the world. QTLs from the wild parent were identified that improve one or more of the key quantitative traits for processing tomatoes (yield, brix, sugar and acid composition and earliness) by as much as 10-30%. Nearly isogenic lines (QTL-NILs) have been generated for a subset of these QTLs. Each QTL-NIL contains the entire genome of the elite cultivated parent except for a segment (5-40 cM) of the wild species genome corresponding to a specific QTL. The genetic material and information that was developed in this program is presently used by American and Israeli seed companies for the breeding of superior varieties. We expect that in the next few years these varieties will make a difference in the marketplace.