Mapping of the Tomato Begomovirus Resistance Gene Ty-6 and the Resistance Provided to Two Viruses by Ty-6 Alone and in Combination with Ty-3 or ty-5

Whitefly-transmitted begomoviruses are a major disease problem for many tropical and sub-tropical tomato production regions of the world. The bipartite Tomato mottle virus (ToMoV) was the first begomovirus identified in Florida in the early 1990s, but by the late 1990s ToMoV was displaced by the monopartite Tomato yellow leaf curl virus (TYLCV). Multiple resistance genes (termed Ty genes) have been introgressed into cultivated tomato, primarily from wild tomato species, and these provide an effective means of disease control. Some of these genes, however, are ineffective against certain begomovirus species and/or strains. A better understanding of each gene’s efficacy against various begomoviruses, along with availability of linked molecular markers for each, is necessary for the efficient and durable deployment of resistance in commercial cultivars. EarlierTy-6 mapping efforts were unsuccessful due to a lack of molecular markers that saturated the tomato genome, but recent genomic advances have overcome this problem. The Ty-6 gene was mapped on chromosome 10 near 63 Mb according to the SL2.50 tomato genome assembly. Whole genome re-sequencing of Ty-6 inbred breeding lines revealed a Ty-6 genic region with very few polymorphisms and no evidence of a typical wild species introgression. These findings shed new light on our previous difficulties locating Ty-6, and our data provides a valuable resource for robust molecular marker development and potential Ty-6 cloning efforts. The effect of Ty-6 alone and in combination with either Ty-3 or ty-5 against two different begomoviruses was investigated. Ty-6 alone provided moderate and high levels of resistance against TYLCV and ToMoV, respectively. The greater effect of Ty-6 against ToMoV suggests that this gene may be particularly useful for control of bipartite begomoviruses. In general, the highest levels of resistance were obtained when Ty-6 was combined with either Ty-3 or ty-5.