Tobacco mosaic virus (TMV ) is the type species of the Tobamoviruses, which have not yet been assigned to a virus family. TMV is closely related to Tomato mosaic virus (ToMV), and although the two species are distinct in protein composition, they have historically been considered synonymously. However, the species that predominates in tomato production worldwide is ToMV (Brunt 1986).
Resistance to these viruses is widely used for disease control and was bred into cultivated tomato from S. habrochaites and S. peruvianum. The source from S. habrochaites is a single dominant gene, Tm-1 ,which was mapped close to the centromere of chromosome 2 with several co-segregating RFLP markers (Tanksley et al. 1992). Ohmoriet al. (1995b) identified six RAPD markers linked to Tm-1, which were subsequently converted to two codominant and four dominant SCAR markers (Ohmori etal. 1996). The Tm-1 gene, however, has been overcome by several naturally occurring ToMV strains, which has reduced its importance in breeding programs (Lanfermeijer et al. 2003).
S. peruvianum was utilized in several cases to breed for resistance to TMV and ToMV. A single dominant gene, Tm-2, was identified from several sources including PI 126926 (Laterrot and Pecaut 1969), while another single dominant gene was identified from PI 128650 (Alexander 1963). The gene from PI 128650 was determined to be allelic to Tm-2, and was named Tm-21 (Pecaut 1965). Because Tm-22 provides resistance to a broader range of pathogenic strains than Tm-2, Cirulli and Alexander (1969) hypothesized that Tm-22 may be a closely linked gene, rather than an allele of Tm-2. Based on this observation, Cirulli and Alexander (1969) suggested that Tm-22 be called Tm-2a, and both names have been used in the literature.
The Tm-2a gene was the first virus resistance gene in tomato to be tagged by molecular markers. Young et al. (1988) utilized NIL to map Tm-2a near the centromere on the long arm of chromosome 9, tightly linked (0.4 cM) to two RFLP markers, TG79 and TG101. Subsequently, Ohmori et al. (1995a) identified a total of 13 RAPD markers linked to the Tm-2 locus, three of which were linked in coupling with the Tm-2a allele and ten that were linked in coupling with the Tm-2 allele. Further cloning and characterization of these markers produced three dominant SCAR markers linked to both Tm-2 and Tm-2a (Sobir et al. 2000). The Tm-22 allele was subsequently cloned using a transposon tagging approach where only individuals with a disrupted Tm-22 gene survived (Lan-fermeijer et al. 2003). Lanfermeijer et al. (2003) determined that the Tm-2 locus contained a single gene for which gene-specific PCR primers were designed. These primers were subsequently utilized to isolate the Tm-2 allele (Lanfermeijer et al. 2005), which veri fied that Tm-2 and Tm-22 are allelic. From the limited sequence polymorphism at the locus, Lanfermeijer et al. (2005) developed CAPS markers to differentiate the Tm-2, Tm-22, and tm-2 (susceptible) alleles. The molecular characterization of this locus has provided valuable insight into resistance to ToMV along with a PCR-based marker to distinguish alleles of resistant material in breeding programs. The determination of allelic composition is critical since the Tm-2 allele has been overcome by some naturally occurring ToMV strains while the Tm-22 allele has provided solid resistance.
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