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Use of Pyrosequencing to Quantify Incidence of a Specific Aspergillus flavus Strain Within Complex Fungal Communities Associated with Commercial Cotton Crops

First and third authors: Agricultural Research Service, United States Department of Agriculture, Department of Plant Sciences, University of Arizona, Tucson 85721-0036; and second author: Agricultural Research Service, United States Department of Agriculture, New Orleans.

Atoxigenic strains of Aspergillus flavus have been used as aflatoxin management tools on over 50,000 hectares of commercial crops since 2000. To assess treatment efficacy, atoxigenic strain incidence is routinely monitored by vegetative compatibility analyses (VCA) that require culturing, generation of auxotrophs, and complementation with tester mutants. Two pyrosequencing assays (PA) that require no culturing were developed for monitoring incidences of atoxigenic strains on ginned cottonseed. The assays, which quantify frequencies of characteristic single nucleotide polymorphisms (SNPs) in the aflR and pksA genes, were validated against standard VCA on cottonseed collected from commercial gins in South Texas, Arizona, and Southern California where the atoxigenic strain AF36 is used to manage aflatoxin contamination. Cottonseed washings were subjected to both VCA and PA. PA was performed directly on DNA isolated from particulates pelleted from the wash water by centrifugation. Addition of CaCl₂ and diatomaceous earth prior to pelleting increased the amount of DNA isolated. Accuracy and reproducibility of the PA were contrasted with those for the VCA that has been used for over a decade. Correlation coefficients between VCA and PA indicated good correspondence between the results from the two assays (r = 0.91 for aflR assay and r = 0.80 for pksA assay). PAs were highly variable for samples with low incidences of A. flavus due to variability in the initial polymerase chain reaction step. This held for both DNA isolated from cottonseed washes and for mixtures of purified DNA. For samples yielding low quantities of A. flavus DNA, averaging of results from 4 to 5 replicates was required to achieve acceptable correlations with VCA. Pyrosequencing has the potential to become a powerful tool for monitoring atoxigenic strains within complex A. flavus communities without limitations imposed by traditional culturing methods.

Additional keywords:biocontrol, population genetics.