Gene genealogies reveal global phylogeographic structure and reproductive isolation among lineages of Fusarium graminearum , the fungus causing wheat scab

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 data set comprising 7,120 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 introgressive hybridization and intragenic recombination among lineages of the F. graminearum clade in nature.

Multilocus sequence analysis of Fusarium pseudograminearum reveals a single phylogenetic species

“…In the Fg clade, phylogenetic lineages are linked to the geographic origin of isolates (O'Donnell et al 2000;O'Donnell et al 2004;Ward et al 2002), but this is not the case with F. pseudograminearum as seen in the current study. In the tree topology generated from the MLS data set, clades Fps 2 (Australia and USA), Fps 4 (New Zealand and Canada) and Fps 6 (Australia, Canada and South Africa) contained isolates from multiple regions.…”

“…Conversely, the former F. graminearum Group 2 has been subject to in depth study in recent times probably because of its close association with FHB worldwide. Cumulative sequencing and phylogenetic comparison of 11 nuclear genes has demonstrated that F. graminearum (hereafter referred to as the Fg clade) consists of nine distinct lineages (O'Donnell et al 2000;O'Donnell et al 2004;Ward et al 2002), each of which have been proposed for formal species status (O'Donnell et al 2004). The presence of nine phylogenetic species within the Fg clade, begs the question of whether a similar lineage development/speciation has occurred within its close relative, F. pseudograminearum.…”

“…In the Fg clade, lineage structure was related to geographic origin, and was seen as a cause of lineage development (O'Donnell et al 2000). Relationships between genotypes and geography can also be indicative of factors such as climatic preference, and/or host genotype interactions.…”

“…This situation is the reverse of that observed in the closely related, and morphologically similar, Fg clade. In the Fg clade, GCPSR has been used to show congruence in phylogenetic signal over 11 genes (O'Donnell et al 2000;O'Donnell et al 2004;Ward et al 2002), and it has been formally proposed that the Fg clade be redefined into nine phylogenetic species (O'Donnell et al 2004).…”

“…Amplifications were carried out as described O'Donnell et al (2000), sequences obtained from NCBI database (http:// www.ncbi.nlm.nih.gov/). e PHO locus sequences were obtained separately for phosphate permease 1 and 2 genes from the NCBI database for F. pseudograminearum and F. graminearum complex isolates, and concatenated into a single sequence prior to alignment.…”