Submitted to: Mycologia
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/28/2008
Publication Date: 1/5/2009
Citation: Schroers, H., Lamprecht, S.C., O Donnell, K., Kammeyer, P.L., Johnson, S., Sutton, D.A., Rinaldi, M.G., Summerbell, R.C. 2009. Taxonomy and Phylogeny of the Fusarium dimerum Species Group. Mycologia. 101(1):44-70. Interpretive Summary: Some broadly defined morphological species, such as Fusarium dimerum, have been reported to cause localized opportunistic infections in healthy individuals, especially ones associated with corneal trauma, as well as disseminated infections of immuno-compromised or immuno-suppressed patients. In addition, this filamentous mold has also been shown to be pathogenic to an endangered, medicinally-important, cactus-like plant indigenous to Africa, Hoodia gordonii. The objective of this study was to assess whether this human and plant pathogen comprises more than one morphologically and phylogenetically distinct species. Such knowledge is essential to understanding the environmental reservoir of this pathogen and its epidemiology. To address this question, we conducted phylogenetic analyses of DNA sequence data obtained from three genes and detailed observations of their microscopic morphology to assess species boundaries. Results of these analyses clearly demonstrate that F. dimerum, as originally defined, comprises at least 12 phylogenetically distinct species, and that over half of these species can be identified using morphology alone. Furthermore, while the results demonstrate for the first time that at least 5 species within this complex are capable of causing invasive infections of humans, F. dimerum sensu stricto and F. delphinoides are responsible for most of the mycoses caused by members of this species group. Results of this study should be of interest to and benefit medical health care professionals, especially clinical microbiologists and infectious disease specialists, by providing molecular and morphological markers for distinguishing the different human pathogenic species.
Technical Abstract: The morphospecies Fusarium dimerum, known only from its anamorph, comprises at least 12 phylogenetically distinct species. Analyses of the large subunit ribosomal DNA (LSU rDNA) show they are taxa of the Nectriaceae (Hypocreales) and form a phylogenetically distinct clade within Fusarium. According to the original description and illustration, F. dimerum (= F. dimerum var. pusillum), for which no herbarium material could be located, is characterized by macroconidia with a single, median septum. Fusarium lunatum (= F. dimerum var. violaceum) forms similar but longer macroconidia and purple, catenate or clustered chlamydospores. Fusarium delphinoides sp. nov., F. diseptatum sp. nov., F. penzigii sp. nov., F. nectrioides comb. nov. (= F. dimerum var. nectrioides), and two unnamed Fusarium spp., produce macroconidia with mostly 2 or rarely 3 septa. The name F. dimerum, which originally was applied to a fungus from a citron, is used for a taxon including isolates causing infections in immuno-competent and immuno-compromised patients. Strains of F. nectrioides, F. delphinoides, F. penzigii and F. diseptatum are known from soil and dead plant substrata or, rarely, as agents of trauma-related eye infections of humans. Fusarium lunatum is an inhabitant of the cladodes of species within the cactus genera Opuntia and Gymnocalycium. Its unnamed closest sister taxon, which also forms 1-septate macroconidia and purple, clustered chlamydospores, was isolated from a human sinus. Fusarium delphinoides is a pathogen of the cactus-like African species Hoodia gordonii (Apocynaceae). Phylogenetic analyses based on combined sequences of the internal transcribed spacer region, LSU rDNA and partial sequences of the elongation factor 1-alpha and beta-tubulin genes identified a clade of several species producing predominately 2-septate macroconida as the reciprocally monophyletic sister of F. dimerum. The basal sister group to the two aforementioned clades includes Fusarium lunatum, F. domesticum and another undescribed species, all of which form 1-septate macroconidia.