Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 15, 2005
Publication Date: February 1, 2006
Citation: Chang, P.-K., Ehrlich, K., Hua, S.T. 2006. Cladal relatedness among Aspergillus oryzae isolates and Aspergillus flavus S and L morphotype isolates. International Journal of Food Microbiology. 108:172-177. Interpretive Summary: Atoxigenic Aspergillus flavus strains are being developed as biocontrol agents to reduce aflatoxin contamination of crops. Populations of A. flavus in the fields are extremely diverse. On the basis of physiological and morphological characteristics, A. flavus can be divided into two types, S strain and L strain. S strain isolates produce, on average, numerous smaller sclerotia and more aflatoxin than L strain isolates. In this study, we examined phylogenic relationships among isolates of S- and L-morphotype of A. flavus and A. oryzae, a close relative of A. flavus, but has GRAS status. We found that a subgroup of atoxigenic L isolates present in many parts of the United States genetically resemble A. oryzae isolates. Another subgroup of atoxigenic L isolates, like A. flavus AF36, apparently descends from a toxigenic S ancestor, but the isolates have lost aflatoxin production. This work deciphers the genetic relationships of A. flavus subgroups. It provides a means for selecting potentially beneficial atoxigenic A. flavus strains, which can be used by scientists and researchers in future biocontrol strategies to eliminate aflatoxin contamination.
Technical Abstract: Aspergillus flavus is the main etiological agent for aflatoxin contamination of crops. Its close relative, A. oryzae, does not produce aflatoxins and has been widely used to produce fermented foods. We compared the phylogeny of A. oryzae isolates and L- and S-type sclerotial isolates of A. flavus using single nucleotide polymorphisms in the omtA gene in the aflatoxin biosynthesis gene cluster and deletions in and distal to the norB-cypA intergenic region as phylogenetic signals. Aflatoxin-producing ability and sclerotial size also were weighted in the analysis. Like A. flavus, the A. oryzae isolates form a polyphyletic assemblage. A. oryzae isolates in one clade strikingly resemble an A. flavus subgroup of atoxigenic L-type isolates. All toxigenic S-type isolates closely resemble another subgroup of atoxigenic L-type isolates. Because atoxigenic S-type isolates are extremely rare, we hypothesize that loss of aflatoxin production in S-type isolates may occur concomitantly with a change to L-type sclerotia. All toxigenic L-type isolates, unlike A. oryzae, have a 1.0 kb deletion in the norB-cypA region. Although A. oryzae isolates, like S-type, have a 1.5 kb deletion in the norB-cypA region, none were cladally related to S-type A. flavus isolates. Our results show that A. flavus populations are genetically diverse. A. oryzae isolates may descend from certain atoxigenic L-type A. flavus isolates.