Tomato chlorotic mottle virus (TCMV), ToMoV, and TYLCV are members of the Begomovirus genus (family Geminiviridae). These three viruses, for which resistance genes in tomato have been described, are among at least 35 viruses in this genus (Fauquet et al. 2003). These whitefly vectored viruses can be divided into two groups, those with a monopartite or bipartite genome (Ji etal. 2007). The cultivated tomato has provided little resistance to these viruses (Pico et al. 1998; Ji etal. 2007). The cultivar "Tyking" is the only reported S. lycopersicum source of resistance (believed to have been derived from a landrace); however, there is some evidence that the resistance was derived from a wild tomato species (Giordano et al. 2005; Ji et al. 2007). To date, there are five wild relatives of tomato (S. habrochaites, S. chilense, S. pimpinelli-folium, S. cheesmaniae, S. peruvianum) that are purported to have resistance to various Begomoviruses (Zamir et al. 1994; Chague et al. 1997; Giordano et al. 2005; Ji and Scott 2005,2006; Agrama and Scott 2006; Hanson et al. 2006; Ji et al. 2007). Molecular markers have been identified that are linked to each of the four single genes/QTLs (Ty-1, Ty-2, Ty-3, tcm-1, Table 11) that reportedly provide resistance to Bego-moviruses.
Ty-1 was derived from S. chilense and positioned on the short arm of chromosome 6, tightly linked to the RFLP marker TG97 (Zamir et al. 1994) and is part of the Am, Mi, Cf-2/Cf-5, Ty-3, Ol-1 and Bw-5 complex. The Ty-2 gene, derived from S. habrochaites, was originally mapped between the RFLP markers TG36 and TG393 on the long arm of chromosome 11 (Hanson et al. 2006). Additional research refined the position of Ty-2 to the telomeric end of the long arm of chromosome 11 and tightly linked to the CAPS marker TG105A (Ji et al. 2007). Ty-3 is also part of the aforementioned resistance gene complex on chromosome 6, and is the most recent of Begomovirus resistance genes described (Ji and Scott 2006). This gene is flanked by the CAPS markers cLET-31-P16 and C2_At5g41480, which are separated by 0.5 centiMor-gans (cM) (Ji and Scott 2006; Ji et al. 2007). Initial work to identify the location of Ty-3 was accomplished with RAPD markers, which were later converted to SCAR markers.
The only recessive single gene (tcm-1) conferring resistance to a Begomovirus identified to date, was found using an F6 line derived from "Tyking" with resistance to TCMV (Giordano et al. 2005). However, neither the genetic map location nor molecular markers linked to this gene was reported. "Tyking" has shown resistance to Begomoviruses in several studies (Fargette et al. 1996; Lapidot et al. 1997; Pico et al. 1998; Ji et al. 2007). Interestingly, a genomic region in the vicinity of Ty-1 and Ty-3 possibly originated from wild tomato species (Ji et al. 2007).
Resistance to ToMoV and TYLCV has been reported from several QTLs (Chague et al. 1997; Griffiths 1998; Griffiths and Scott 2001). In each of these studies, RAPD markers were linked to some of the QTLs, all of which were located on chromosome 6. It is not likely that these QTLs are identical between each report since the studies were done on germplasm derived from two different species (S. chilense and S. pimpinellifolium), although they could be allelic.
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