Species limits were investigated within the Fusarium graminearum clade (Fg clade) through phylogenetic analyses of DNA sequences from portions of 11 nuclear genes including the mating-type (MAT) locus. Nine phylogenetically distinct species were resolved within the Fg clade, and they all possess contiguous MAT1-1 and MAT1-2 idiomorphs consistent with a homothallic reproductive mode. In contrast, only one of the two MAT idiomorphs was found in five other species, four of which were putatively asexual, and the other was heterothallic. Molecular evolutionary analyses indicate the MAT genes are under strong purifying selection and that they are functionally constrained, even in species for which a sexual state is unknown. The phylogeny supports a monophyletic and apomorphic origin of homothallism within this clade. Morphological analyses demonstrate that a combination of conidial characters could be used to differentiate three species and three species pairs. Species rank is formally proposed for the eight unnamed species within the Fg clade using fixed nucleotide characters.
Bibliographical noteFunding Information:
We thank Jody Robinson and Jean Juba for excellent technical assistance, Walter Gams for assistance with the Latin descriptions, Joe Bielawski for helpful discussions regarding maximum likelihood analyses, Steve Prather and Don Fraser for preparing the figures, Amy Morgan for synthesis of the primers, and the individuals and culture collections that generously supplied strains used in this study. K.O.D., T.J.W., and H.C.K. acknowledge the support of the US Wheat and Barley Scab Initiative. D.M.G. was supported by National Research Initiative Competitive Grant 2002-35201-12545. The mention of firm names or trade products does not imply that they are endorsed or recommended by the US Department of Agriculture over other firms or similar products not mentioned.
- Fusarium head blight
- Gene trees
- Histone H3
- Reciprocal monophyly
- Species limits
- Species trees