RELATIONSHIPS AMONG ISOLATES OF ASPERGILLUS SECT FLAVI THAT VARY IN AFLATOXIN PRODUCTION

Relationships among 47 fungal isolates belonging to Aspergillus flavus sect. flavi were inferred from restriction site variability in a portion of the Taka-amylase A gene. Portions (1,168 bp) of the gene were amplified by polymerase chain reaction from genomic DNA of A. flavus, A. oryzae, A. parasiticus, A. sojae, A. nomius, and A. tamarii. The resulting amplification products were subjected to restriction analysis. A UPGMA (unweighted paired group method with arithmetic averaging) dendrogram based on the analysis divided A. flavus into two clusters that were 98% similar. Both clusters included atoxigenic and toxigenic isolates, and one cluster contained all four isolates of A. oryzae examined. Isolates within the highly toxigenic S strain, which produced only B aflatoxins, were not differentiated from certain L strain isolates. However, two S strain isolates, which produced both B and G aflatoxins, differed by 18% from other A. flavus isolates and were almost as closely related to A. parasiticus isolates as to other A. flavus isolates. All isolates of both A. sojae and A. parasiticus were identical at all restriction sites tested. However, A. nomius isolates were highly variable. Conspecific variability was far greater within A. nomius species than within other aflatoxin-producing species. However, one isolate of A. tamarii differed from the three other A. tamarii isolates studied (which were identical at all tested sites) by 35%. Parsimony analyses of the restriction site data suggest that both A. flavus and A. parasiticus arose from an ancestor shared with A. nomius via an A. flavus S strain intermediate. The results support previous suggestions that A. sojae and A. oryzae are forms of A. parasiticus and A. flavus, respectively.