The geographic distribution and complex evolutionary history of the NX-2 trichothecene chemotype from Fusarium graminearum

•NX-2 F. graminearum may be endemic to southern Canada and the northern U.S.•TRI1 variation within FSAMSC-1 was shaped by trans-species evolution.•The NX-2 chemotype evolved relatively recently from a type B ancestor.•NX-2 evolved in association with a significant change in selection pressure on Tri1.•NX-2 strains may occupy an evolutionary niche distinct from type B F. graminearum. Fusarium graminearum and 21 related species comprising the F. sambucinum species complex lineage 1 (FSAMSC-1) are the most important Fusarium Head Blight pathogens of cereal crops world-wide. FSAMSC-1 species typically produce type B trichothecenes. However, some F. graminearum strains were recently found to produce a novel type A trichothecene (NX-2) resulting from functional variation in the trichothecene biosynthetic enzyme Tri1. We used a PCR-RFLP assay targeting the TRI1 gene to identify the NX-2 allele among a global collection of 2515 F. graminearum. NX-2 isolates were only found in southern Canada and the northern U.S., where they were observed at low frequency (1.8%), but over a broader geographic range and set of cereal hosts than previously recognized. Phylogenetic analyses of TRI1 and adjacent genes produced gene trees that were incongruent with the history of species divergence within FSAMSC-1, indicating trans-species evolution of ancestral polymorphism. In addition, placement of NX-2 strains in the TRI1 gene tree was influenced by the accumulation of nonsynonymous substitutions associated with the evolution of the NX-2 chemotype, and a significant (P