Fine mapping, functional analysis and pyramiding of genes controlling begomovirus resistance in tomato

. Tomato yellow leaf curl virus (TYLCV), a monopartitebegomovirus, is one of the most devastating viruses of cultivated tomatoes and poses increasing threat to tomato production worldwide. Because all accessions of the cultivated tomato are susceptible to these viruses, wild tomato species have become a valuable resource of resistance genes. QTL controlling resistance to TYLCV and other begomoviruses (Ty loci) were introgressed from several wild tomato species and mapped to the tomato genome. Additionally, a non-isogenic F₁diallel study demonstrated that several of these resistance sources may interact with each other, and in some cases generate hybrid plants displaying lower symptoms and higher fruit yield compared to their parental lines, while their respective resistance genes are not necessarily allelic. This suggests that pyramiding genes originating from different resistance sources can be effective in obtaining lines and cultivars which are highly resistant to begomoviruses. Molecular tools needed to test this hypothesis have been developed by our labs and can thus significantly improve our understanding of the mechanisms of begomovirus resistance and how to efficiently exploit them to develop wider and more durable resistance. Five non-allelic Ty loci with relatively major effects have been mapped to the tomato genome using molecular DNA markers, thereby establishing tools for efficient marker assisted selection, pyramiding of multiple genes, and map based gene cloning: Ty-1, Ty-2, Ty-3, Ty-4, and ty-5. This research focused on Ty-3 and Ty-4 due to their broad range of resistance to different begomoviruses, including ToMoV, and on ty-5 due to its exceptionally high level of resistance to TYLCV and other begomoviruses. Our aims were: (1) clone Ty-3, and fine map Ty-4 and Ty-5 genes, (2)introgress each gene into two backgroundsand develop semi isogenic lines harboring all possible combinations of the three genes while minimizing linkage-drag, (3) test the resulting lines, and F₁ hybrids made with them, for symptom severity and yield components, and (4) identify and functionally characterize candidate genes that map to chromosomal segments which harbor the resistance loci. During the course of this research we have: (1) found that the allelic Ty-1 and Ty-3 represent two alternative alleles of the gene coding DFDGD-RDRP; (2) found that ty-5is highly likely encoded by the messenger RNA surveillance factor PELOTA (validation is at progress with positive results); (3) continued the map-based cloning of Ty-4; (4) generated all possible gene combinations among Ty-1, Ty-3 and ty-5, including their F₁ counterparts, and tested them for TYLCV and ToMoV resistance; (5) found that the symptomless line TY172, carrying ty-5, also carries a novel allele of Ty-1 (termed Ty-1ⱽ). The main scientific and agricultural implications of this research are as follows: (1) We have developed recombination free DNA markers that will substantially facilitate the introgression of Ty-1, Ty-3 and ty-5 as well as their combinations; (2) We have identified the genes controlling TYLCV resistance at the Ty-1/Ty-3 and ty-5 loci, thus enabling an in-depth analyses of the mechanisms that facilitate begomovirus resistance; (3) Pyramiding of Ty resistance loci is highly effective in providing significantly higher TYLCV resistance.