Submitted to: Fungal Genetics and Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/2000
Publication Date: 1/4/2001
Interpretive Summary: The mold Aspergillus flavus produces carcinogenic aflatoxin in agricultural commodities such as peanuts, corn and cottonseed. Various management strategies necessary to reduce aflatoxin levels to within regulatory limits for animal feed and human food products result in large economic losses. A. flavus varies greatly in its capacity to produce aflatoxin, with some strains producing large amounts of aflatoxin and other strains not producing any toxins. In addition, the closely related A. oryzae does not produce aflatoxin and is considered to be a domesticated form of A. flavus. Because A. oryzae is often similar in appearance to A. flavus, it is necessary to distinguish the two molds in the fermentation industry. By examining the genes of A. oryzae and A. flavus, it is found that strains of A. oryzae were genetically alike and most likely descended from nonaflatoxin-producing ancestors of A. flavus. This ancestry reinforces the econfidence in safely using A. oryzae in food and beverage fermentations. .
Technical Abstract: Aspergillus oryzae is a fungus used for the industrial production of food, enzymes and other products for human consumption. Previous studies have shown a tight phylogenetic relationship between A. oryzae and A. flavus, a major producer of aflatoxins, such that some authors have advocated reducing A. oryzae to a variant of A. flavus. To address the question of the origins of this fungus, we analyzed DNA sequences from portions of 3 nuclear genes in a wide sample of A. flavus isolates and 2 A. oryzae isolates from around the world. Strains were also analyzed for the production of the mycotoxins cyclopiazonic acid and B and G aflatoxins. A region of the methyltransferase gene (omt 12) showed the highest level of variation, and exhibited a phylogenetic pattern that was largely consistent with patterns of toxin production adn sclerotial morphology. Strains of A. oryzae had identical omt 12 sequences, and formed a strongly supported clade with 3 isolates of A. flavus in the group I clade. These A. flavus isolates produced little or no CPA and did not produce detectable levels of either B or G aflatoxins. We conclude that A. oryzae was domesticated from a nonaflatoxin-producing lineage of A. flavus at least 4000 years ago. In addition, 4 out of 6 strains a A. flavus group II were found to produce measurable levels of G aflatoxin, and showed strong phylogenetic connection with a G aflatoxin-producing undescribed taxon. Small sclerotial S isolates were found to be non-monophyletic; all group II isolates produced small sclerotia, but a subset of group I isolates did as well.