Tomato spotted wilt virus (TSWV), which is vectored by at least ten species of thrips (Whitfield et al. 2005), is the type species of the genus Tospovirus (family Bunyaviridae) and was the only tospovirus name recognized until the early 1990s. Related tospo species that infect tomato include Groundnut bud necrosis virus (GBNV; Folkertsma et al. 1999), Groundnut ringspot virus (GRSV; Boiteux and Giordano 1993), and Tomato chlorotic spot virus (TCSV; Boiteux and Giordano 1993). Tospoviruses infect over 1,000 species of plants in both monocots and dicots (Parrella et al. 2003) and have been identified in all major tomato growing areas of the world.

Resistance to TSWV was first reported from wild tomato relatives in 1930 (Samuel etal. 1930). Early studies of the inheritance of resistance in tomato identified five resistant genes/alleles summarized by Stevens et al. (1992; Table 11). All five of these genes were strain-specific and had little resistance application beyond the area in which they were found. None are broadly used in present day tomato breeding programs for international markets (Paterson et al. 1989; Stevens etal. 1992; Gordillo etal. 2007). However, a single dominant TSWV resistance gene (Sw-5) was reported in a line from South Africa (van Zijl et al. 1986; Stevens et al. 1992). Sw-5 is derived from S. peru-vianum, which has since been shown to be resistant to a number of species of tospoviruses (Table 11). Since the identification of Sw-5, an additional allele of Sw-5 and two more genes (Sw-6 and Sw-7) have been reported (Rosello et al. 1998; Gordillo et al. 2007). To date only Sw-5 has had molecular markers associated with it, although work is underway to find markers linked to Sw-7 (MR Stevens, unpublished data).

Sw-5 is on the long arm of chromosome 9, very close to the telomere, and very tightly linked to the RFLP marker CT220. Several RAPD markers have been linked to this gene (Stevens 1993; Stevens et al. 1995,1996a, b; Brommonschenkel 1996; Chague et al. 1996). In addition, CAPS and SCAR-like markers linked to Sw-5 have been developed (Stevens et al. 1996a; Folkertsma et al. 1999). Sw-5 has been cloned by two separate teams using map-based cloning techniques (Brommonschenkel and Tanksley 1997; Folkertsma etal. 1999; Brommonschenkel etal. 2000). Since the cloning of Sw-5, a gene-specific PCR marker was developed based on its DNA sequence (Garland et al. 2005). According to representatives from private companies (MR Stevens, pers. comm.) and reports in the literature (Smiech et al. 2000; Langella et al. 2004; Garland et al. 2005; Anfoka et al. 2006), Sw-5 associated molecular markers are actively used for MAS.

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