Left – tree showing phylogenetic relationships of the 23 Fusarium species complexes and placement of Fusarium sp. NRRL 66182 within the F. buharicum species complex. In the tree, species complex names are abbreviated using specific epithets of the species after which the complexes are named (e.g., the F. sambucinum species complex is abbreviated as sambucinum). Upper right – culture of Fusarium sp. NRRL 66182 growing on potato dextrose agar medium. Middle right – fumonisin biosynthetic gene cluster, which occurs in Fusarium sp. NRRL 66182 even though it is distantly related to other fumonisin-producing species of Fusarium. Lower right – chemical structure of fumonisin B1, one of the most economically important mycotoxins produced by Fusarium.
Image Credit: Robert Proctor, Amy McGovern and Crystal Probyn.

Fusarium (family Nectriaceae) is a species-rich fungal genus that poses a dual threat to agriculture because many species cause destructive crop diseases and/or contaminate infected crops with toxic secondary metabolites (mycotoxins) that are health hazards to humans and other animals. Some Fusarium mycotoxins are frequent contaminants of dried distillers’ grains, coproducts of grain-based ethanol production used as a protein-rich livestock feed. In addition, some species of Fusarium are pathogens of energy crops such as corn and sugar cane. Some species can also exist as endophytes in plants, including some bioenergy crops.

DNA-based phylogenetic analyses have resolved Fusarium into 23 multi-species lineages, or species complexes (Geiser et al. 2021). Fusarium sp. NRRL 66182 is a member of the Fusarium buharicum species complex, which is comprised of at least 7 phylogenetically distinct species (O’Donnell et al. 2022). During its evolutionary diversification, Fusarium has undergone multiple chromosomal fusions, and, as a result, members of early diverging species complexes tend to have more chromosomes (15 – 20) than members of later diverging complexes (4 – 7). The two members of the F. buharicum complex examined have 10 or 18 chromosomes. Members of this complex have been recovered from plants in the families Fabaceae and Malvaceae, from soil, and from human blood. Strain NRRL 66182 is a representative of a species that has not been formally described and was isolated from common marsh mallow (Hibiscus moscheutos) growing in the U.S. state of Washington. Although members of the F. buharicum complex are distantly related to Fusarium species that produce fumonisin and trichothecene mycotoxins, NRRL 66182 has the fumonisin biosynthetic gene cluster, and another undescribed species in the complex, represented by strain NRRL 66739, has the trichothecene biosynthetic gene cluster. The presence of these gene clusters in members of the F. buharicum species complex has likely resulted from horizontal gene transfer of the clusters from distantly related Fusarium species (Kim et al. 2020).

References:

• Geiser DM, Al-Hatmi A, Aoki et al. 2021. Phylogenomic analysis of a 55.1 kb 19-gene dataset resolves a monophyletic Fusarium that includes the Fusarium solani Species Complex. Phytopathology 111: 1064-1079.
• Kim HS, Lohmar JM, Busman M, et al. 2020. Identification and distribution of gene clusters required for synthesis of sphingolipid metabolism inhibitors in diverse species of the filamentous fungus Fusarium. BMC Genomics 21:510.
• O'Donnell K, Grafenhan T, Laraba I, et al. 2022. Fusarium abutilonis and F. guadeloupense, two novel species in the Fusarium buharicum clade supported by multilocus molecular phylogenetic analyses. Mycologia 114:682-696.