MAP BASED CLONING OF BRIX9-2-5: A WILD SPECIES QTL THAT INCREASES SUGAR CONTENT IN TOMATO

The Hebrew University of Jerusalem, Faculty of Agriculture, P.O. Box 12, Rehovot 76100, Israel

With the development of marker-saturated complete linkage maps it became possible to map QTLs, however the underlying genes behind the continuous phenotypes remain unknown. To conduct fine analysis of yield associated QTLs in tomato we bred a whole genome coverage population consisting of introgression lines (ILs) each containing a single, RFLP defined, chromosome segment from the green-fruited species L. pennellii in a genetic background of a processing tomato variety. A major breeding objective in tomatoes is to increase the total soluble solids content (TSS or brix: mainly sugars and acids) of the fruits. Processing tomato varieties with high brix are more efficient for the production of the concentrated products and, in fresh market tomatoes TSS is a dominant component of taste. Field trials of the ILs facilitated the mapping of at least 23 QTLs that affect brix. The L. pennellii introgression IL9-2-5 improves brix by 22% by increasing the content of fructose and glucose compared to the nearly isogenic controls. This partially dominant QTL (designated as Brix9-2-5) enhanced TSS of the fruit in different years, environments and genetic backgrounds. Recombinant mapping of the QTL based on analysis of 14,000 gametes placed Brix9-2-5 on a BAC of 100 KB with 29 recombinants between the BAC ends. Sequencing of the recombinant individuals revealed that 12 of the recombination events occurred in a 1 kb interval within a single ORF. Progeny test phenotypic analysis (in replicated glasshouse trials) of all the recombinants mapped the QTL to a 487 bp segment. The relationship of the ORF to the phenotype and to the high recombination rates will be discussed. This research highlights the potential of wild germplasm for yield improvement and of nearly isogenic populations to facilitate the goal of QTL cloning.