Molecular Characterization of Fusarium globosum Strains from South African Maize and Japanese Wheat

Evaluating three commonly used growth media for assessing fumonisin analogues FB1, FB2 and FB3 production by nine Fusarium verticillioides isolates

Maize is most often infected by the fumonisin-producing Fusarium verticillioides. Total fumonisins of natural infected grain is made up of FB₁, FB₂ and FB₃ with FB₁ occurring naturally at higher levels. A maize plant can be infected with more than one F. verticillioides isolate, and finding a reliable method to elucidate the toxigenic potential of these isolates is important to extrapolate the possible fumonisin risk to consumers of grain. It is not clear whether F. verticillioides produces similar fumonisin levels, as well as fumonisin analogue ratios, across media. In this study, nine F. verticillioides isolates were subjected to three methods of fumonisin testing using liquid media, maize patties and a field trial (silk inoculation of grain) in Potchefstroom, South Africa. Spore concentrations of 1 × 10⁶ conidia ml^-1 of each isolate were used to inoculate the different media and levels fumonisin analogues were measured using HPLC. Fumonisin production per isolate was highly variable and was influenced by the two-way interaction of F. verticillioides isolate × growth media. Total fumonisins produced in the liquid medium ranged from 0 to 21.3 ppm, on maize patties fumonisins they ranged from 0 to 21.5 ppm, and in the silk inoculation technique they ranged from 0 to 15.5 ppm. The fumonisin analogue FB₁ occurred at higher levels followed by FB₃ in both in vitro studies. In the silk inoculation technique, fumonisin analogue FB₂ was the second highest occurring analogue after FB₁. Isolate GCI 282 produced higher FB₂ and FB₃ levels than FB₁ in the patties and grain, respectively. In order not to miscalculate the fumonisin and analogue ratio levels per F. verticillioides isolate, the growth medium will have to be optimised for each isolate and more than one growth medium used.

Isolation and characterization of two mitoviruses and a putative alphapartitivirus from Fusarium spp

The filamentous fungus Fusarium spp. includes several important plant pathogens. We attempted to reveal presence of double-stranded (ds) RNAs in the genus. Thirty-seven Fusarium spp. at the MAFF collection were analyzed. In the strains of Fusarium coeruleum, Fusarium globosum and Fusarium solani f. sp. pisi, single dsRNA bands were detected. The strains of F. coeruleum and F. solani f. sp. pisi cause potato dry rot and mulberry twig blight, respectively. Sequence analyses revealed that dsRNAs in F. coeruleum and F. globosum consisted of 2423 and 2414 bp, respectively. Using the fungal mitochondrial translation table, the positive strands of these cDNAs were found to contain single open reading frames with the potential to encode a protein of putative 757 and 717 amino acids (molecular mass 88.5 and 84.0 kDa, respectively), similar to RNA-dependent RNA polymerases of members of the genus Mitovirus. These dsRNAs in F. coeruleum and F. globosum were assigned to the genus Mitovirus (family Narnaviridae), and these two mitoviruses were designated as Fusarium coeruleum mitovirus 1 and Fusarium globosum mitovirus 1. On the other hand, a positive strand of cDNA (1950 bp) from dsRNA in F. solani f. sp. pisi contained an ORF potentially encoding a putative RdRp of 608 amino acids (72.0 kDa). The putative RdRp was shown to be related to those of members of the genus of Alphapartitivirus (family Partitiviridae). We coined the name Fusarium solani partitivirus 2 for dsRNA in F. solani f. sp. pisi.

A single nucleotide polymorphism in the translation elongation factor 1α gene correlates with the ability to produce fumonisin in Japanese Fusarium fujikuroi

PCR-RFLP based on the translation elongation factor 1α (TEF) gene was developed to identify Fusarium fujikuroi in the Fusarium (Gibberella) fujikuroi species complex. Ninety-three strains, most of which were obtained from various sources in Japan, were identified as F. fujikuroi and their capability to produce fumonisin was investigated using an in vitro assay. Fumonisin production was detected in 50 strains isolated from maize, strawberry, wheat, and rice, whereas it was undetectable in 43 strains derived from rice seeds and rice seedlings carrying the bakanae disease, and from unknown sources. A single nucleotide polymorphism in the TEF gene (T618G) correlated with the ability to synthesize fumonisin.

Birth, death and horizontal transfer of the fumonisin biosynthetic gene cluster during the evolutionary diversification of Fusarium

Fumonisins are a family of carcinogenic secondary metabolites produced by members of the Fusarium fujikuroi species complex (FFSC) and rare strains of Fusarium oxysporum. In Fusarium, fumonisin biosynthetic genes (FUM) are clustered, and the cluster is uniform in gene organization. Here, sequence analyses indicated that the cluster exists in five different genomic contexts, defining five cluster types. In FUM gene genealogies, evolutionary relationships between fusaria with different cluster types were largely incongruent with species relationships inferred from primary-metabolism (PM) gene genealogies, and FUM cluster types are not trans-specific. In addition, synonymous site divergence analyses indicated that three FUM cluster types predate diversification of FFSC. The data are not consistent with balancing selection or interspecific hybridization, but they are consistent with two competing hypotheses: (i) multiple horizontal transfers of the cluster from unknown donors to FFSC recipients and (ii) cluster duplication and loss (birth and death). Furthermore, low levels of FUM gene divergence in F. bulbicola, an FFSC species, and F. oxysporum provide evidence for horizontal transfer of the cluster from the former, or a closely related species, to the latter. Thus, uniform gene organization within the FUM cluster belies a complex evolutionary history that has not always paralleled the evolution of Fusarium.

Mycotoxicological research in South Africa 1910-2011

The British mycologist, I.B. Pole-Evans, was appointed as the first South African government mycologist in 1905 following the Anglo-Boer War (1899-1902). The Onderstepoort Veterinary Research Institute was founded in 1908 with the Swiss veterinarian, Arnold Theiler, as the first director. Thus, the stage was set for the commencement of mycotoxicological research when the Union of South Africa came into being in 1910. The first accounts of this pioneering research appeared in the 'Seventh and eight reports of the Director of Veterinary Research, Union of South Africa. 1918' in which D.T. Mitchell reported on the experimental reproduction of the neurotoxic syndrome, diplodiosis, in cattle with pure cultures of Stenocarpella maydis (= Diplodia zea) isolated by P.A. Van der Bijl and grown on sterile maize kernels. This is the first report of the experimental reproduction of a veterinary mycotoxicosis with a pure culture of a fungus in South Africa and possibly in the world. This seminal research was followed by a great deal of multidisciplinary research on veterinary mycotoxicoses as well as human syndromes in which fungal toxins are suspected to be involved, taxonomy of mycotoxigenic fungi and chemistry of mycotoxins in South Africa. The mycotoxicoses studied in South Africa include the following (more or less in chronological order): diplodiosis, Paspalum staggers, aflatoxicosis, human hepatocellular carcinoma, ochratoxicosis, lupinosis, facial eczema, tremorgenic mycotoxicosis, hyperoestrogenism, stachybotryotoxicosis, ergotism, leukoencephalomalacia and human oesophageal cancer. A major breakthrough in mycotoxicological research was made in South Africa in 1988 with the isolation and chemical characterisation of the carcinogenic fumonisins produced by Fusarium verticillioides in maize. Current research at the PROMEC Unit of the South African Medical Research Council on the risk assessment of fumonisins and intervention methods to reduce fumonisin intake by rural populations on a maize staple diet is highlighted. This paper concludes with a selected list of mycotoxicological publications by South African mycologists/plant pathologists, veterinarians and chemists/biochemists.