Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/14/2002
Publication Date: 7/9/2002
Citation: WARD, T.J., BIELAWSKI, J.P., KISTLER, H.C., SULLIVAN, E., O DONNELL, K. EVIDENCE OF ANCIENT POLYMORPHISM AND ADAPTIVE EVOLUTION IN THE TRICHOTHECENE GENE CLUSTER OF PHYTOPATHOGENIC FUSARIUM. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES. 2002. V. 99 (14). P. 9278-9283. Interpretive Summary: Fungi within the Fusarium graminearum species complex cause a variety of diseases on cereal crops worldwide, including scab of wheat and barley, and ear and stem rots of maize. Taken together, these diseases result in billion dollar losses to agriculture each year. In addition, these fungi contaminate cereal grains with trichothecene mycotoxins that pose a serious threat to animal health and food safety. Differences in the types of trichothecene toxins (trichothecene profile) produced by individual strains are not well correlated with known genetic relationships. Therefore, it is not clear that these relationships provide a useful basis for understanding the role that trichothecenes play in plant pathogenesis or other aspects of the ecology of these fungi. To address this problem, genetic data were obtained from eight clustered genes involved in the production of trichothecene toxins. The results of this study indicate that differences in trichothecene profile are biologically significant, and that the genetic basis for these differences is not adequately represented by the known genetic relationships among species. These results provide an appropriate context for understanding the role trichothecene toxins play in the biology of plant-pathogenic fungi. In addition, molecular methods were developed for identifying the trichothecene profile of individual strains, making it possible to monitor their movement in agricultural commodities.
Technical Abstract: Plant pathogens within the Fusarium graminearum species complex cause a variety of economically devastating diseases of cereal crops worldwide. They also produce distinct profiles (chemotypes) of trichothecene mycotoxins that act as virulence factors on some hosts and pose a serious threat to animal health and food safety. Trichothecene chemotype differences are not well correlated with phylogenetic relationships within the Fusarium graminearum species complex, indicating that these chemotypes are evolutionarily unstable or that the evolutionary history of these chemotypes is different from that of the Fusarium graminearum species complex. To address this question, a 19 kb region of the trichothecene gene cluster, including eight trichothecene genes, was sequenced in 39 strains chosen to represent the global genetic diversity of species in the Fusarium graminearum species complex and their close relatives. Phylogenetic analyses of these sequences indicate that each trichothecene chemotype has a single evolutionary origin, and that chemotype polymorphisms have been maintained through multiple speciation events. Chemotype-specific differences in selective constraint, and evidence of adaptive evolution within trichothecene genes are also reported.