Advanced backcross QTL analysis: Results and perspectives

The advent of molecular maps and the derived quantitative trait locus (QTL) mapping technology has provided strong evidence that despite the inferior phenotype, exotic germplasm is likely to contain QTLs that can increase the yield and quality of elite breeding lines. These results have motivated the development of a new molecular breeding strategy, referred to as advanced backcross (AB) QTL method which integrates QTL analysis with variety development, by simultaneously identifying and transferring favorable QTL alleles from unadapted to cultivated germplasm. The AB-QTL strategy has so far been tested in tomato, rice, barley, maize and pepper, and in most of the cases favorable exotic QTL alleles for important agronomic traits have been identified. The most extensive experiments have been conducted in tomato, where populations involving crosses with five wild Lycopersicon species have been genotyped and field tested in a number of locations around the world for numerous traits important for the tomato processing industry. These studies have shown that on average, for approximately 30% of QTLs for any given trait, the wild species allele is predicted to be superior (from an agricultural viewpoint) to the cultivated parent allele. From the AB populations, near-isogenic lines (NILs) have been created that contain specific QTLs from the wild donors that are able to significantly improve the performance of the elite variety. Marker-assisted selection can be used to combine favorable QTL alleles from the same or from different wild donor species to obtain multi-QTL NILs with even higher performance. Currently, in the tomato project, a multi-QTL NIL has been obtained by pyramiding four wild QTL alleles that together produce an effect on brix and brix ✕ yield that goes far beyond any other commercial cultivar and nearly doubles the brix ✕ yield over the original starting material (cv E6203). QTL-NILs provide a starting point for fine-mapping QTLs, thus elimi