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 port ion of the Taka-amylase A gene. Portions (1,168 bp) of the gene were a mplified by polymerase chain reaction from genomic DNA of A. flavus, A . oryzae, A. parasiticus, A. sojae, A. nomius, and A. tamarii. The res ulting amplification products were subjected to restriction analysis. A UPGMA (unweighted paired group method with arithmetic averaging) den drogram based on the analysis divided A. flavus into two clusters that were 98% similar. Both clusters included atoxigenic and toxigenic iso lates, and one cluster contained all four isolates of A. oryzae examin ed. 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 aflatoxin s, differed by 18% from other A. flavus isolates and were almost as cl osely related to A. parasiticus isolates as to other A. flavus isolate s. 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 spe cies than within other aflatoxin-producing species. However, one isola te of A. tamarii differed from the three other A. tamarii isolates stu died (which were identical at all tested sites) by 35%. Parsimony anal yses of the restriction site data suggest that both A. flavus and A. p arasiticus arose from an ancestor shared with A. nomius via an A. flav us S strain intermediate. The results support previous suggestions tha t A. sojae and A. oryzae are forms of A. parasiticus and A. flavus, re spectively.