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Yuanfu Ji and John W. Scott

Resistance to begomoviruses tomato mottle virus (ToMoV) and tomato yellow leaf curl virus (TYLCV) has been introgressed to tomato (Lycopersicon esculentum) from L. chilense accessions LA 1932, LA 2779, and LA 1938. Resistance genes have been mapped to three regions on chromosome 6 using randomly amplified polymorphic DNA (RAPD) markers. We call these regions 1, 2, and 3. To facilitate breeding by marker assisted selection, advanced breeding lines with resistance from the above sources were assayed for the presence of RAPD markers to determine which were most tightly linked to begomovirus resistance. The best RAPD markers were then converted to sequence characterized amplified region (SCAR) markers or cleaved amplified polymorphic sequence (CAPS) markers. In addition, selected restriction fragment length polymorphism (RFLP) markers near the three regions were converted into CAPS markers, which were tested for association with the advanced breeding lines. Only LA 2779 derivatives have the L. chilense introgression in region 1, which is near the location of the Ty-1 gene and spans across CAPS markers 32.5Cla and TG118. Two region 1 RAPD markers UBC197 and UBC621 were converted co-dominant SCAR or CAPS markers, which were present in all 16 resistant breeding lines tested. Derivatives from all three accessions have introgressions in region 2. Further assays with more markers in this region are under way to determine the lengths and locations of the introgressions. No tightly linked RAPD markers have been found for the resistance gene from LA 1932 in region 3. RFLP and CAPS markers are being used to more precisely locate the region 3 gene.

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Yuanfu Ji, John W. Scott, and David J. Schuster

The whitefly-transmitted Tomato yellow leaf curl virus (TYLCV) is a major pathogen of tomatoes grown in tropical and subtropical regions of the world. Several genes of different origins conferring resistance to TYLCV have been introgressed to the cultivated tomato (Solanum lycopersicum), including the single dominant gene, Ty-2, that originated from S. habrochiates and was previously mapped to a 19-cM region on the long arm of chromosome 11 delimited by restriction fragment length polymorphism markers TG36 and TG393. In the present study, we confirmed the dominant inheritance of the Ty-2 gene from TYLCV evaluation and molecular marker analysis of an F2 segregating population derived from a commercial hybrid that carries the Ty-2 gene. Evaluating recombinants recovered from the F2 progeny for TYLCV resistance localized the Ty-2 gene to a marker interval of 5.5 cM between C2_At1g07960 (82.5 cM) and C2_At4g32930 (88 cM). Additional recombinants were identified for the target region carrying the Ty-2 gene. TYLCV evaluation of the progeny from these recombinants further delimited the Ty-2 gene to a 4.5-cM interval between C2_At1g07960 (82.5 cM) and cLEN-11-F24 (87 cM). The smaller introgressions no longer include the fusarium wilt race 2 resistance locus (I-2), which should facilitate combining the two resistance genes in cis configuration. The polymerase chain reaction-based markers developed from the present study can be used to precisely monitor the introgression of the Ty-2 gene, thus offering the opportunity to pyramid TYLCV resistance genes from different sources as well as resistance genes for other pathogens into elite tomato cultivars.

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Yuanfu Ji, Jay W. Scott, David J. Schuster, and Douglas P. Maxwell

Resistance to begomoviruses, including bipartite tomato mottle virus (ToMoV) and monopartite tomato yellow leaf curl virus (TYLCV), has been introgressed to cultivated tomato (Solanum lycopersicum) from Solanum chilense accessions LA1932 and LA2779. A major gene, Ty-3, responsible for resistance to ToMoV and TYLCV was previously mapped on the long arm of chromosome 6. In the present study, we identified a 14-cM S. chilense introgression on the long arm of chromosome 3 in some resistant breeding lines derived from LA1932. A new begomovirus resistance locus, Ty-4, was mapped to the 2.3-cM marker interval between C2_At4g17300 and C2_At5g60160 in the introgression. Analysis of a population segregating for Ty-3 and Ty-4 demonstrated that Ty-3 accounted for 59.6% of the variance, while Ty-4 only accounted for 15.7%, suggesting that Ty-4 confers a lesser effect on TYLCV resistance. Recombinant inbred lines (RILs) with Ty-3 and Ty-4 had the highest level of TYLCV resistance. The PCR-based markers tightly linked to the Ty-4 locus as well as the Ty-3 locus have been recently used in our breeding program for efficient selection of high-levels of begomovirus resistance and now allow for efficient breeding by marker-assisted selection.