The results obtained from the tomato QTL mapping studies along with those obtained for other crops suggest that substantial improvements in yield-associated phenotypes as well as other agriculturally important traits will unlikely be achieved with the introgression of a single QTL (Lawson et al. 1997; Ashikari and Matsuoka 2006). On the other hand, MAS pyramiding of newly discovered favorable wild QTL alleles from the same or from different wild donor species to obtain multiple-introgression lines could greatly improve crop performance.
From the tomato AB-QTL project, four favorable wild QTLs were identified that had positive effects on Brix° and Brix° x yield; these QTLs were pyramided into a single multi-QTL NIL whose performance in Brix° and Brix° x yield was far beyond any other commercial cultivar and nearly doubled the Brix° x yield over the original starting material (cv. E6203) (SD Tanksley, pers. comm.). A MAS pyramiding approach was pursued by Gur and Za-mir (2004) to develop a multiple-introgression line (IL789) carrying three independent yield-promoting genomic regions derived from the drought-tolerant green-fruited wild species S. pennellii (Table 1) into the genetic background of the cultivated recipient genotype M82. The IL789 was crossed with four inbred tester lines, whose hybrids with M82 exhibit the highest Brix° x yield values, which permitted the assessment of the potential of the wild QTL in the context of high-yield genetic backgrounds -those close to the "yield barrier"- (Gur and Za-mir 2004). The hybrids between IL789 and the four testers yielded 50% more than the leading commercial tomato hybrid (BOS3155) that was used as a control. This higher performance was observed under both wet and dry (10% irrigation) field conditions. The observation that the wild introgressions exerted the expected effects in diverse genetic backgrounds suggested that alleles similar to those of the wild species are not present in the cultivated tomato gene pool.
Overall these results indicate that QTL pyramiding represents a successful approach for producing new varieties; it also provides concrete evidence for the value of exotic germplasm as a rich source of new valuable QTL alleles. As more wild accessions are screened by means of the AB and "exotic libraries" breeding strategies, it will be these new combinations of QTLs, from various accessions, that will greatly improve the performance of elite germplasm.
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