Molecular Mapping of the Tomato Spotted Wilt Virus Resistance Gene "Sw-7" In Tomato

Tomato spotted wilt virus (TSWV), vectored by several thrips species, is the causal agent of devastating tomato crop losses in many areas of the world. Recently, field tests have demonstrated that there are TSWV isolates that overcome the resistance gene Sw-5, derived from Lycopersicon peruvianum L. However, Sw-7, a new source of TSWV resistance, has been introgressed from L. chilense Dun. This new gene has demonstrated field resistance to various isolates of this disease including greenhouse trials utilizing isolates that overcome Sw-5. In order to determine the genetic location of Sw-7, we screened over 200 SSR and InDel molecular markers from across the tomato genome. We used the homozygous (Sw-7/Sw-7) Ck12 line and one of the susceptible (Sw-7⁺/ Sw-7⁺) recurrent backcross parents (Fla 7482B) along with six BC₁ and F₂ plants segregating for Sw-7. The results of this screening suggested that Sw-7 resided near SSR20 on chromosome 12. To confirm the linkage of Sw-7 and SSR20 we tested this SSR marker and others on chromosome 12 using 94 BC₁ and F₂ progeny segregating for Sw-7. Additionally, we screened 47 lines segregating for Sw-7 under high natural TSWV field pressure conditions in northern Florida. As a result of these studies we have narrowed the location of the L. chilense-derived DNA with Sw-7.  This region resides between markers T1263 (45.0 cM) and SSR20 (58.2 cM), according to the “Tomato EXPEN-2000” map found on the Sol Genomics Network. Our efforts are now focused on determining the precise position of Sw-7 in relationship with the markers available in this region and the AFLP markers previously reported.