RFLP mapping of QTLs conferring salt tolerance during germination in an interspecific cross of tomato

Most cultivars of tomato (Lycopersicon esculentum) are sensitive to salinit y during seed germination and at later stages. Genetic resources for salt t olerance have been identified within the related wild species of tomato. Th e purpose of the present study was to identify quantitative trait loci (QTL s) for salt tolerance during germination in an inbred backcross (BC1S1) pop ulation of an interspecific cross between a salt-sensitive tomato breeding line (NC84173, maternal and recurrent parent) and a salt-tolerant Lycopersi con pimpinellifolium accession (LA722). One-hundred and nineteen BC1 indivi duals were genotyped for 151 restriction fragment length polymorphism (RFLP ) markers and a genetic linkage map was constructed. The parental lines and 119 BC1S1 families (self-pollinated progeny of 119 BC1 individuals) were e valuated for germination at an intermediate salt-stress level (150 mM NaCl + 15 mM CaCl2, water potential approximately -850kPa). Germination was scor ed visually as radicle protrusion at 8-h intervals for 28 consecutive days. Germination response was analyzed by survival analysis and the time to 25, 50, and 75% germination was determined. In addition, a germination index ( GI) was calculated as the weighted mean of the time from imbibition to germ ination for each family/line. Interval mapping, single-marker analysis and distributional extreme analysis, were used to identify QTLs and the results of all three mapping methods were generally similar. Seven chromosomal loc ations with significant effects on salt tolerance were identified. The L. p impinellifolium accession had favorable QTL alleles at six locations. The p ercentage of phenotypic variation explained (PVE) by individual QTLs ranged from 6.5 to 15.6%. Multilocus analysis indicated that the cumulative actio n of all significant QTLs accounted for 44.5 % of the total phenotypic vari ance. A total of 12 pairwise epistatic interactions were identified, includ ing four between QTL-linked and QTL-unlinked regions and eight between QTL- unlinked regions. Transgressive phenotypes were observed in the direction o f salt sensitivity. The graphical genotyping indicated a high correspondenc e between the phenotypes of the extreme families and their QTL genotypes. T he results indicate that tomato salt tolerance during germination can be im proved by marker-assisted selection using interspecific variation.