Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/9/2000
Publication Date: N/A
Citation: Interpretive Summary: In recent years, the plant disease called scab or Fusarium head blight of small grains such as wheat and barley has reemerged in epidemic proportions in the United States and elsewhere in the world. Grains infected with the scab fungus, Fusarium graminearum, are often contaminated with toxins produced by the pathogen. Over three billion dollars have been nlost in the U.S. from scab epidemics during the past decade due to yield losses and price discounts resulting from reduced seed quality. In this study, we tested whether or not strains from a global collection of F. graminearum represent a single species by comparing genetic data obtained from six gene sequences. Genetic relationships inferred from the six DNA sequences were highly similar in that they all resolved the same seven cryptic (previously undetected) species within "F. graminearum". Toxin analyses and pathogenicity tests demonstrated that all seven species produce toxins and are capable of causing scab on wheat. Knowing that there are seven scab pathogens, rather than one, provides plant pathologists and plant breeders with critical information needed to develop more effective breeding programs and disease control efforts.
Technical Abstract: During the past decade, the plant disease called scab or Fusarium head blight of wheat and barley has reached epidemic proportions in North America and elsewhere in the world. Scab is an economically devastating plant disease not only because it causes significant reduction in seed yields and quality but also because infested seeds are often contaminated with trichothecene and estrogenic mycotoxins that pose a serious threat to animal health and food safety. To test whether the primary etiological agent of scab, the fungus Fusarium graminearum, is panmictic throughout its range, allelic genealogies were constructed from six single-copy nuclear genes from strains selected to represent the global genetic diversity of this pathogen. Excluding one hybrid strain, all six genealogies recovered the same seven biogeographically structured lineages, suggesting that they represent phylogenetically distinct species among which gene flow has been very limited during their evolutionary history. Parsimony analysis of the combined dataset comprising 7133 aligned nucleotide characters resolved most relationships among the seven lineages of the F. graminearum clade and related fusaria included in the study. Phylogenetic evidence is also presented for hybridization and intragenic recombination among lineages of the F. graminearum clade in nature. The phylogeographic hypothesis proposed from the molecular evidence is consistent with a pattern of allopatric speciation associated with multiple long-distance dispersal events together with evidence for sympatric speciation.