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Title: Degeneration of aflatoxin gene cluster in Aspergillus flavus from Africa and North America

Author
item Adhikari, Bishwo
item BANDHYOPADHYAY, RANAJIT - International Institute Of Tropical Agriculture (IITA)
item Cotty, Peter

Submitted to: Applied Microbiology and Biotechnology Express (AMB Express)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/2016
Publication Date: 8/31/2016
Citation: Adhikari, B.N., Bandhyopadhyay, R., Cotty, P.J. 2016. Degeneration of aflatoxin gene cluster in Aspergillus flavus from Africa and North America. Applied Microbiology and Biotechnology Express (AMB Express). 6:62. doi: 10.1186/s13568-016-0228-6.

Interpretive Summary: Aflatoxins are potent cancer-causing toxins produced by the fungus Aspergillus flavus. The aflatoxin-producing potential of A. flavus is known to vary among fungi from different fields, areas, and regions. Non-aflatoxin producing A. flavus isolates (“atoxigenics”) that are endemic have been used successfully to reduce aflatoxin contamination in different crops. The mechanisms responsible for atoxigenicity are diverse, and for most atoxigenic A. flavus isolates the specific mechanisms are unknown. By analyzing the complete sequence of the genes involved in aflatoxin production, we studied the mechanism of atoxigenicity and changes happening in the aflatoxin gene cluster. We found that the atoxigenicity could be either due to deletion of the genes responsible for aflatoxin production or due to presence of defects in the genes. We also found that the genes involved in aflatoxin production are degenerating but the process of such degeneration was not similar for all isolates. Analysis of the process of degeneration of the atoxigenic isolates will help us to develop and improve the biocontrol programs for the management of aflatoxins in the field.

Technical Abstract: Aspergillus flavus is the primary causal agent of food and feed contamination with the toxic fungal metabolites aflatoxins. Aflatoxin-producing potential of A. flavus is known to vary among isolates. The genes involved in aflatoxin biosynthesis are clustered together and the order of genes within the cluster is highly conserved among A. flavus isolates. Complete deletion of the aflatoxin gene cluster, partial deletions and presence of single nucleotide polymorphism (SNP) in particular aflatoxin biosynthesis genes have all been linked to lack of aflatoxin synthesis. In order to identify the mechanisms of atoxigenicity and understand the evolution of aflatoxin gene cluster in atoxigenic isolates, we sequenced and analyzed the aflatoxin gene cluster from 35 A. flavus isolates from North America, West Africa and East Africa. Our results showed that the inability of some isolates to produce aflatoxin resulted from the absence of the necessary biosynthetic genes, whereas in other isolates, it originated from the degeneration of the cluster either due to smaller deletions or presence of SNPs in the coding regions. We also found that the process of degeneration was different for parts of the gene cluster; some genes involved in the early stage of the toxin biosynthesis suffered from deletions while those involved in later part of toxin biosynthesis suffered from high levels of SNP mutations suggesting the fact that the process of degeneration is not similar for all atoxigenic isolates. Analysis of the diversity of atoxigenic isolates combined with the evolution of aflatoxin gene cluster leading to the degeneration should provide further insight into the mechanism of atoxigenicity in A. flavus and thus contribute to the diagnosis and monitoring of the biocontrol isolates applied in the field.