Pyramiding of genes conferring resistance to Tomato yellow leaf curl virus from different wild tomato species.

Tomato (Solanum lycopersicum) production in tropical and subtropical regions of the world is limited by the endemic presence of Tomato yellow leaf curl virus (TYLCV). Breeding programmes aimed at producing TYLCV-resistant tomato cultivars have utilized resistance sources derived from wild tomato species. So far, all reported breeding programmes have introgressed TYLCV resistance from a single wild tomato source. Here, we tested the hypothesis that pyramiding resistances from different wild tomato species might improve the degree of resistance of the domesticated tomato to TYLCV. We have crossed TYLCV-resistant lines that originated from different wild tomato progenitors, Solanum chilense, Solanum peruvianum, Solanum pimpinellifolium, and Solanum habrochaites. The various parental resistant lines and the F 1 hybrids were inoculated in the greenhouse using viruliferous whiteflies. Control, non-inoculated plants of the same lines and hybrids were exposed to non-viruliferous whiteflies. Following inoculation, the plants were scored for disease symptom severity, and transplanted to the field. Resistance was assayed by comparing yield of inoculated plants to those of the control non-inoculated plants of the same variety. Results showed that the F 1 hybrids between the resistant lines and the susceptible line suffered major yield reduction because of infection, but all hybrids were more resistant than the susceptible parent. All F 1 hybrids resulting from a cross between two resistant parents, showed a relatively high level of resistance, which in most cases was similar to that displayed by the more resistant parent. In some cases, the hybrids displayed better levels of resistance than both parents, but the differences were not statistically significant. The F 1 hybrid between a line with resistance from S. habrochaites and a line with resistance from S. peruvianum (HAB and 72-PER), exhibited the lowest yield loss and the mildest level of symptoms. Although the resistance level of this F 1 hybrid was not statistically different from the level of resistance displayed by the 72-PER parent itself, it was statistically better than the level of resistance displayed by the F 1 hybrids between 72-PER and any other resistant or susceptible line.