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

Authors: Kelly, Amy; Proctor, Robert; BELZILE, FRANCOIS - University Of Laval; CHULZE, SOFIA - National University Of Rio Cuarto; CLEAR, RANDALL - Collaborator; Cowger, Christina; ELMER, WADE - Connecticut Agricultural Experiment Station; LEE, THERESA - National Institute For Agricultural Science & Technology; OBANOR, FRIDAY - Commonwealth Scientific And Industrial Research Organisation (CSIRO); WAALWIJK, CEES - Plant Research International - Netherlands; Ward, Todd

Submitted to: Fungal Genetics and Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/3/2016
Publication Date: 8/4/2016
Publication URL: https://handle.nal.usda.gov/10113/5695389
Citation: Kelly, A., Proctor, R.H., Belzile, F., Chulze, S.N., Clear, R.M., Cowger, C., Elmer, W., Lee, T., Obanor, F., Waalwijk, C., Ward, T.J. 2016. The geographic distribution and complex evolutionary history of the NX-2 trichothecene chemotype from Fusarium graminearum. Fungal Genetics and Biology. 95:39-48.

Interpretive Summary: Fusarium graminearum and related fungi are responsible for Fusarium head blight (FHB) and other economically destructive diseases of cereal crops world-wide. In addition, these fungi contaminate grain with mycotoxins that pose a significant threat to food safety and animal health. Strains of F. graminearum with a previously unknown mycotoxin type, termed NX-2, were recently identified from FHB infected wheat. However, the origin and distribution of this novel toxin type is unknown. In this research, we conducted a molecular survey providing the first data on the distribution and prevalence of NX-2 strains from a global collection of F. graminearum isolates. Our results indicate the NX-2 toxin type may have originated in, and is potentially restricted to, southern Canada and the northern U.S. These data also demonstrated that strains with the NX-2 toxin type occur on a broader set of cereal hosts (wheat, oat, and barley) than was previously recognized. A set of nine genetic changes within a mycotoxin biosynthetic gene were identified as unique to NX-2 strains, and are likely responsible for the difference in toxin production profiles that have been observed. In addition, our results demonstrate that the NX-2 chemotype evolved in response to a significant change in selective pressure suggesting this novel toxin type may provide an adaptive advantage to FHB pathogens in some environments or on some hosts. As such, the results reported here are critical to promoting food safety and cereal production through improved mycotoxin monitoring and improved understanding of the ecological and toxicological significance of the NX-2 toxin type that can inform efforts to breed cereals with broad resistance to FHB.

Technical Abstract: F. graminearum and 21 related species form a species complex (FSAMSC-1) characterized by production of type B trichothecenes. However, some F. graminearum strains were recently found to produce NX-2, a novel type A trichothecene, resulting from variation in the trichothecene biosynthetic enzyme Tri1. We used a PCR-RFLP assay targeting variation in the TRI1 gene to identify NX-2 alleles among a global collection of 2,526 F. graminearum. NX-2 isolates were identified from southern Canada and the northern U.S., where they were observed at low frequency (< 3.0%), but over a broader geographic range and a broader set of cereal hosts than previously recognized. The NX-2 allele was not observed among 540 F. graminearum sampled from other parts of the United States and 1,482 isolates from a global collection of F. graminearum. 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 nonsysnonymous substitutions associated with the evolution of the NX-2 chemotype, and a significant (P < 0.001) change in selection pressure was observed along the NX-2 branch (w = 1.16) in comparison to other branches in the TRI1 phylogeny (w = 0.17). Parameter estimates were consistent with positive selection for specific amino-acid changes during the evolution of NX-2, but direct tests of positive selection were not significant. Phylogenetic analyses of four-fold degenerate sites and intron sequences in TRI1 indicated the NX-2 chemotype had a single evolutionary origin and evolved recently from a type B ancestor. Our results indicate the NX-2 chemotype may be indigenous, and possibly endemic, to southern Canada and the northern U.S. In addition, we demonstrate that the evolution of TRI1 within FSAMSC-1 has been complex, with evidence of trans-species evolution and chemotype-specific shifts in selective constraint.