Tomato yellow leaf curl virus (TYLCV) hampers tomato production worldwide. Our previous studies have focussed on mapping and ultimately cloning of the TYLCV resistance genes Ty-1 and Ty-3. Both genes are derived from Solanum chilense and were shown to be allelic. They code for an RNA-dependent RNA polymerase (RDR) belonging to the RDRγ type defined by a DFDGD catalytic domain. In this study, we first fine-mapped the TYLCV resistance in S. chilense LA1932, LA1960 and LA1971. Results showed that chromosomal intervals of the causal genes in these TYLCV-resistant accessions overlap and cover the region where Ty-1/Ty-3 is located. Further, virus-induced gene silencing was used to silence Ty-1/Ty-3 in tomato lines carrying TYLCV resistance introgressed from S. chilense LA1932, LA1938 and LA1971. Results showed that silencing Ty-1/Ty-3 compromised the resistance in lines derived from S. chilense LA1932 and LA1938. The LA1971-derived material remained resistant upon silencing Ty-1/Ty-3. Further, we studied the allelic variation of the Ty-1/Ty-3 gene by examining cDNA sequences from nine S. chilense-derived lines/accessions and more than 80 tomato cultivars, landraces and accessions of related wild species. The DFDGD catalytic domain of the Ty-1/Ty-3 gene is conserved among all tomato lines and species analysed. In addition, the 12 base pair insertion at the 5-prime part of the Ty-1/Ty-3 gene was found not to be specific for the TYLCV resistance allele. However, compared with the susceptible ty-1 allele, the Ty-1/Ty-3 allele is characterized by three specific amino acids shared by seven TYLCV-resistant S. chilense accessions or derived lines. Thus, Ty-1/Ty-3-specific markers can be developed based on these polymorphisms. Elevated transcript levels were observed for all tested S. chilenseRDR alleles (both Ty-1 and ty-1 alleles), demonstrating that elevated expression level is not a good selection criterion for a functional Ty-1/Ty-3 allele.Electronic supplementary materialThe online version of this article (doi:10.1007/s11032-015-0329-y) contains supplementary material, which is available to authorized users.

Tomato yellow leaf curl disease (TYLCD) is caused by a complex of begomovirus. Breeding for resistance to this disease has mainly been based on Ty-1 gene, derived from Solanum chilense LA1969. Commercial varieties available to date still develop symptoms and suffer yield losses with high inoculum pressure and early infections. It is of interest to incorporate in breeding programs resistance from the different available sources. Lines with resistance to TYLCD derived from S. chilense accessions LA1932, LA1960 and LA1971 were previously developed. The objectives of this work were to study the genetic control of the resistance derived from these accessions and to map the resistance loci. Response to viral infection was assayed in segregating generations derived from these sources. Results obtained were compatible with a monogenic control of resistance. A total of 94 markers were used to locate the S. chilense introgressions in each of the lines. Only the presence of a large introgression in chromosome 6 was common to all lines. Analysis of recombinants allowed localizing the resistance loci in an interval of approximately 25 cM, also common to all five families. This interval includes the region to which two previously S. chilense-derived TYLCD resistance loci have been mapped, the Ty-1/Ty-3 region. This is the first report of LA1960 and LA1971-derived TYLCV resistance loci to be located on chromosome 6. Further work will be done to fine map the loci found in the present work, in order to determine if they are indeed located in the Ty-1/Ty-3 region.

A set of introgression lines from S. peruvianum PI 126944 into the genetic background of cultivated tomato (S. lycopersicum) is being developed. Several generations were derived from three interspecific hybrids previously obtained. A lot of crosses and embryo rescue were required to obtain until the third backcross, due to the high incompatibility existing between tomato and PI 126944. Crosses between F1 plants allowed the obtaining of a pseudo-F2 generation. The same procedure was followed until pseudo-F6 generation. Additional crosses between plants of different generations were made in order to increase progeny. Among a total number of 263 molecular markers tested, 105 resulted polymorphic between tomato and PI 126944. This set of polymorphic markers consisted in 90 Simple Sequence Repeats and 15 Cleaved Amplified Polymorphic Sequences. Generations available were genotyped with these markers, observing a progressive reduction in the S. peruvianum genome in the most advanced. A reduction of incompatibility was achieved as a consequence of the S. peruvianum genome reduction. In addition, S. peruvianum genome was almost completely represented considering the different plants of the most advanced generations, so the set of ILs will be basically developed from them. Tomato yellow leaf curl virus (TYLCV) and Tomato spotted wilt virus (TSWV) resistance was evaluated in some generations, having been successfully introgressed and expressed into tomato background.