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Tomato yellow leaf curl disease, a devastating disease of Solanum lycopersicum (tomato), is caused by a complex of begomoviruses generally referred to as Tomato yellow leaf curl virus (TYLCV). Almost all breeding for TYLCV resistance has been based on the introgression of the Ty-1 resistance locus derived from Solanum chilense LA1969. Knowledge about the exact location of Ty-1 on tomato chromosome 6 will help in understanding the genomic organization of the Ty-1 locus. In this study, we analyze the chromosomal rearrangement and recombination behavior of the chromosomal region where Ty-1 is introgressed. Nineteen markers on tomato chromosome 6 were used in F2 populations obtained from two commercial hybrids, and showed the presence of a large introgression in both. Fluorescence in situ hybridization (FISH) analysis revealed two chromosomal rearrangements between S. lycopersicum and S. chilense LA1969 in the Ty-1 introgression. Furthermore, a large-scale recombinant screening in the two F2 populations was performed, and 30 recombinants in the Ty-1 introgression were identified. All recombination events were located on the long arm beyond the inversions, showing that recombination in the inverted region was absent. Disease tests on progenies of informative recombinants with TYLCV mapped Ty-1 to the long arm between markers MSc05732-4 and MSc05732-14, an interval overlapping with the reported Ty-3 region, which led to the indication that Ty-1 and Ty-3 may be allelic. With this study we prove that FISH can be used as a diagnostic tool to aid in the accurate mapping of genes that were introgressed from wild species into cultivated tomato.

The necessity to develop potato and tomato crops that possess durable resistance against the oomycete pathogen Phytophthora infestans is increasing as more virulent, crop-specialized and pesticide resistant strains of the pathogen are rapidly emerging. Here, we describe the positional cloning of the Solanum bulbocastanum-derived Rpi-blb2 gene, which even when present in a potato background confers broad-spectrum late blight resistance. The Rpi-blb2 locus was initially mapped in several tetraploid backcross populations, derived from highly resistant complex interspecific hybrids designated ABPT (an acronym of the four Solanum species involved:S. acaule, S. bulbocastanum, S. phureja and S. tuberosum), to the same region on chromosome 6 as the Mi-1 gene from tomato, which confers resistance to nematodes, aphids and white flies. Due to suppression of recombination in the tetraploid material, fine mapping was carried out in a diploid intraspecific S. bulbocastanum F1 population. Bacterial artificial chromosome (BAC) libraries, generated from a diploid ABPT-derived clone and from the resistant S. bulbocastanum parent clone, were screened with markers linked to resistance in order to generate a physical map of the Rpi-blb2 locus. Molecular analyses of both ABPT- and S. bulbocastanum¿derived BAC clones spanning the Rpi-blb2 locus showed it to harbor at least 15 Mi-1 gene homologs (MiGHs). Of these, five were genetically determined to be candidates for Rpi-blb2. Complementation analyses showed that one ABPT- and one S. bulbocastanum-derived MiGH were able to complement the susceptible phenotype in both S. tuberosum and tomato. Sequence analyses of both genes showed them to be identical. The Rpi-blb2 protein shares 82% sequence identity to the Mi-1 protein. Significant expansion of the Rpi-blb2 locus compared to the Mi-1 locus indicates that intrachromosomal recombination or unequal crossing over has played an important role in the evolution of the Rpi-blb2 locus. The contrasting evolutionary dynamics of the Rpi-blb2/Mi-1 loci in the two related genomes may reflect the opposite evolutionary potentials of the interacting pathogens