Submitted to: National Fusarium Head Blight Forum Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 12/15/2003
Publication Date: 12/15/2003
Citation: O Donnell, K., Ward, T.J., Geiser, D.M., Kistler, H.C., Gale, L.R., Aoki, T. 2003. Global genetic diversity of fusarium graminearum clade species and their mycotoxin potential [abstract]. National Fusarium Head Blight Forum Proceedings. p. 149.
Technical Abstract: Although the primary etiological agent of FHB, Fusarium graminearum, has been regarded as a single, panmictic species worldwide, phylogenetic analyses of DNA sequences from 11 nuclear genes totaling 13.6 kb [i.e., genealogical concordance phylogenetic species recognition (GCPSR)] have shown that this morphospecies actually consists of 9 phylogenetically distinct and biogeographically structured species [FIG. 1] (hereafter referred to as the Fg clade) [PNAS97:7905-7910 (2000) and PNAS 99:9278- 9283(2002)]. GCPSR is based on the fact that population-splitting events associated with speciation eliminate shared neutral polymorphism over time, resulting in descendant species with reciprocally monophyletic genealogies of orthologs. Given their importance to world agriculture, species rank is formally proposed for the eight unnamed cryptic species within the Fg clade using fixed nucleotide characters and conidial characters. In addition to the unexpectedly high level of species diversity within the Fg clade, the virulence-associated trichothecene mycotoxin genes are under a novel form of balancing selection resulting in the maintenance of B-trichothecene chemotype polymorphism through multiple speciation events [FIG. 2], which may have important consequences for the fitness and aggressiveness of FHB pathogens on particular hosts or in particular environments. Taken together, these studies suggest that the combined species and mycotoxin diversity of FHB pathogens is remarkably high. However, it appears that only a fraction of this diversity is currently represented within North America [FIG. 3]. Therefore, the introduction of novel FHB pathogens or chemotypes via global trade in agricultural products has the potential to exacerbate the FHB problem in the U.S. We have developed protocols for the multiplex amplification of two sets of chemotype-specific primers, previously designed from genes within the trichothecene gene cluster (TRI3 and TRI12). Using these tests, chemotype diversity has been assessed in a collection of isolates from the U.S., China and Brazil [FIG. 4]. Chemotype frequencies were more balanced within Brazil (15ADON, 3ADON, NIV) and China (3ADON, NIV) compared with the U.S. [predominantly 15ADON, see Gale et al.poster], although the 15ADON chemotype was completely absent from the four Chinese populations surveyed to date. The development of robust molecular tools for FHB species identification and chemotype determination will significantly improve disease surveillance and global monitoring efforts, and will make available for the first time detailed information on the geographic and host distributions of FHB pathogens and their trichothecene chemotypes, enhancing current knowledge of the ecology, epidemiology and population dynamics of these mycotoxigenic cereal pathogens.