Mapping of QTLs for lycopene and other fruit traits in a Lycopersicon esculentum x L-pimpinellifolium cross and comparison of QTLs across tomato species

Quantitative trait loci (QTLs) for several fruit traits in tomato were mapp ed and characterized in a backcross population of an interspecific cross be tween Lycopersicon esculentum fresh-marker breeding line NC84173 and L. pim pinellifolium accession LA722. A molecular linkage map of this cross that w as previously constructed based on 119 BC1 individuals and 151 RFLP markers was used for the QTL mapping. The parental lines and 119 BC1S1 families (s elf-pollinated progeny of BC1 individuals) were grown under field condition s at two locations, Rock Spring, PA, and Davis, CA, and fruits were scored for weight (FW), polar (PD) and equatorial diameters (ED), shape (FS), tota l soluble solids content (SSC), pH and lycopene content (LYC). For each tra it, between 4 and 10 QTLs were identified with individual effects ranging b etween 4.4% and 32.9% and multilocus QTL effects ranging between 39% and 75 % of the total phenotypic variation. Most QTL effects were predictable from the parental phenotypes, and several QTLs were identified that affected mo re than one trait. A few pairwise epistatic interactions were detected betw een QTL-linked and QTL-unlinked markers. Despite great differences between PA and CA growing conditions, the majority of FW QTLs (78%) and SSC QTLs (7 5%) in the two locations shared similar genomic positions. Almost all of th e QTLs that were identified in the present study for FW and SSC were previo usly identified in six other studies that used different interspecific cros ses of tomato; this indicates conservation of QTLs for fruit traits across tomato species. Altogether, the seven studies identified at least 28 QTLs f or FW and 32 QTLs for SSC on the 12 tomato chromosomes. However, for each t rait a few major QTLs were commonly identified in 4 or more studies; such ` popular' QTLs should be of considerable interest for breeding purposes as w ell as basic research towards cloning of QTLs. Notably, a majority of QTLs for increased SSC also contributed to decreased fruit size. Therefore, to s ignificantly increase SSC of the cultivated tomato, some compromise in frui t size may be unavoidable.