Identification of molecular markers linked to a new Tomato spotted wilt virus resistance source in tomato

David L. Price¹, Frederic D. Memmott¹, Jay W. Scott², Steve M. Olson³, Mikel R. Stevens¹

¹Brigham Young University, Department of Plant and Animal Sciences, 287 Widstoe Bldg. Box 25183, Provo, UT 84602-5183, email: mikel_stevens@byu.edu

²University of Florida, IFAS, Gulf Coast Research & Education Center, 14625 CR 672, Wimauma, FL 33598, email: jwsc@ifas.ufl.edu

³University of Florida, IFAS, North Florida Research & Education Center, 30 Research Road, Quincy, FL 32351-5684, email: smolson@ifas.ufl.edu

Tomato spotted wilt virus (TSWV) is one of the most damaging pathogens in tomatoes (Solanum lycopersicum L.).  In some areas of the world TSWV has become a limiting factor in tomato production (Canady et al., 2001).  Several TSWV resistance genes have been identified (Sw₁^a, Sw₁^b, sw₂, sw₃, sw₄, and Sw-5); however, Sw-5 is the only gene that has been broadly utilized in tomato breeding because of its durability to multiple tospoviruses (Boiteux and Giordano, 1992; Stevens et al, 1992). Additionally, we have tested UPV 32, which has TSWV resistance gene identified as Sw-6 (Roselló et al. 2001), with typical TSWV isolates and were unable identify this resistance in greenhouse screening conditions (Stevens, unpublished data).  Although rare, there have been new TSWV isolates identified that overcome Sw-5 (Latham. and Jones, 1998; McMichael et al. 2002).

Tospovirus resistance from S. chilense has been identified by Stevens et al. (1994) and introgression from this species has demonstrated to be useful under field conditions (Canady et al., 2001).  Furthermore, this germplasm has demonstrated resistance to isolates that overcome Sw-5 (Stevens, unpublished data).  In a preliminary study, Scott et al., (2005) reported that resistance was controlled by one or two dominant genes. More recently we concluded that this resistance was conferred by a single dominant gene not linked to Sw-5 (Scott, Olson, and Stevens, unpublished data). This gene will tentatively be referred to as Sw-7.  It is essential that linked molecular markers be identified to facilitate the deployment of Sw-7 in coordination with Sw-5.  The objective in our BYU lab has been to identify molecular markers linked to this new source of tospovirus resistance.

Thirty-seven sister lines putatively containing Sw-7 (developed from F₂ and BC₁ plants suggesting TSWV resistance) are being used to identify the molecular markers.  Eight of these selected F₂ and BC₁ plants have undergone preliminary P³³ AFLP analysis.  Candidate markers suggesting linkage to Sw-7 have been tested using all 37 of the above mentioned lines in a second screening using the AFLP LI-COR 4300.  Additionally, we have included seven Florida lines isogenic except for Sw-7. These Florida lines have undergone further crossing to elite tomato lines and thus have unique and more advanced genetic backgrounds compared to the lines used in our lab at BYU.

Our initial AFLP screenings of eight plants containing resistant/susceptible plants, along with the two parents, were screened with 256 primer combinations. Of the 256 primer combinations 16 resulted in the identification of 30 candidate markers. These 30 putative Sw-7 markers are being examined on our carefully screened 37 F₂ and BC₁ lines along with the seven field selected Florida lines. 

One strong candidate AFLP marker has been identified (~200bp) in both the 37 greenhouse selected lines and the seven Florida field selected lines. This candidate marker is currently being prepared to be cloned and sequenced.  We are hoping to use this marker to identify where in the tomato genome Sw-7 is located.  Additionally, analysis with this marker is currently being conducted on a much larger TSWV resistant population derived from the same germplasm.  Furthermore, we are continuing to evaluate the 29 additional AFLP markers identified in our initial marker screening.  Several of these markers suggest looser linkage to Sw-7; however, our analyses of the complete set of these markers is incomplete. Development of these markers may allow us to stack Sw-7 with Sw-5 which may allow development of germplasm with broader and more stable resistance to tospoviruses. 

Literature Cited

Boiteux, L.S., and L.deB. Giordano. 1992. Screening Lycopersicon germplasm for resistance to a Brazilian isolate of spotted wilt virus (TSWV). Tom. Genet. Coop. Rep. 42:13-14.

Canady, M.A., M.R. Stevens, M.S. Barineau, and J.W. Scott. 2001. Tomato spotted wilt virus (TSWV) resistance in tomato derived from Lycopersicon chilense Dun. LA 1938. Euphytica. 117:19-25.

Latham, L.J., and R.A.C. Jones. 1998. Selection of resistance breaking strains of Tomato spotted wilt tospovirus. Ann. Appl. Biol. 133:385-402.

McMichael, L. A., D.M. Persley, and J.E. Thomas. 2000. The first record of a serotype IV Tospovirus in Australia. Australas. Plant Pathol. 29:149.

Roselló, S., B. Ricarte, M.J. Díez, and F. Nuez.  2001. Resistance to Tomato spotted wilt virus introgressed from Lycopersicon peruvianum in line UPV 1 may be allelic to Sw-5 and can be used to enhance the resistance of hybrids cultivars. Euphytica 119:357-367.

Scott, J.W., M.R. Stevens, and S.M. Olson. 2005. An alternative source of resistance to Tomato spotted wilt virus. Tom. Genet. Coop. Rep. 55:40-41.

Stevens, M.R., S. J. Scott, and R.C. Gergerich. 1992. Inheritance of a gene for resistance to Tomato spotted wilt virus (TSWV) from Lycopersicon peruvianum Mill. Euphytica 59:9-17.

Stevens, M.R., S.J. Scott, and R.C. Gergerich. 1994. Evaluation of seven Lycopersicon species for resistance to Tomato spotted wilt virus (TVSW). Euphytica, 80:79-84.