Evidence of a tomato spotted wilt virus resistance-breaking strain originated through natural reassortment between two evolutionary-distinct isolates.

A Tsw resistance-breaking tomato spotted wilt virus field isolate (TSWV-p331) found in northern Italy originated via reassortment from two evolutionary distinct TSWV strains, as revealed by recombination and phylogenetic analysis. Compared to the closest isolate present in the database, p331 NSs protein carries an unusually high number of amino acid substitutions, but no differences in the nucleocapsid protein. Despite these substitutions, p331 NSs is a potent silencing suppressor. As shown by phylogenetic analyses of TSWV nucleocapsid sequences collected over fifteen years, one likely p331 parental lineage has never been detected in northern Italy, allowing speculations on the origin of TSWV-p331.