The Alfalfa mosaic virus (AMV; genus Alfamovirus; family Bromoviridae) infects over 600 species from 250 genera belonging to 70 plant families (Parrella et al. 2004). Although this disease is generally not considered a problem in tomato, it is distributed worldwide and has caused infection in tomato fields grown close to alfalfa (Zitter et al. 1991). The disease can be spread from 22 species of aphid vectors and some evidence suggests that the virus can be seed transmitted. These observations, combined with the discovery of necrotic strains of the virus have led to research on resistance in tomato (Finetti-Sialer et al. 1997; Parrella et al. 2000,2004).
Accessions from S. lycopersicum var. cerasiforme, S. pimpinellifolium, S. habrochaites, S. peruvianum, and S. pennellii were screened for resistance to AMV (Parrella et al. 1997) and resistance was identified in three accessions of S. habrochaites. Additional work identified a single dominant gene, Am (Parrella et al. 1998), which confers resistance to a necrotic strain of the virus. Initially, Am was tagged using AFLP markers in conjunction with bulked segregant analysis (BSA) of resistant and susceptible segregates of 120 plants from a BC population derived from S. habrochaites parents (Parrella etal. 2000). Parrella et al. (2004) identified five AFLP markers linked to Am from 109 AFLP primer combinations. Subsequently, S. habrochaites ILs developed by Monforte and Tanksley (2000a) were employed to map Am to the short arm of chromosome 6 near the centromere. Curiously, Am is found in a disease resistance hotspot where Mi (nematode resistance; Vos et al. 1998),
Cf-2/Cf-5 (resistance to Cladosporium fulvum; Jones etal. 1993), Ty-1 (resistance to TYLCV Tomato yellow leafcurl virus; Zamir et al. 1994), Ty-3 (resistance to both TYLCV and ToMoV Tomato mottle virus; Ji and Scott 2006), Ol-1 (resistance to Oidium lycopersicum; Van der Beek et al. 1994), and Bw-5 (resistance to Ralstonia solanacearum; Thoquet et al. 1996a) are located.
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