Genetic Analysis of the Aspergillus flavus Vegetative Compatibility Group to Which a Biological Control Agent That Limits Aflatoxin Contamination in U.S. Crops Belongs

Abstract

Some filamentous fungi in Aspergillus section Flavi produce carcinogenic secondary compounds called aflatoxins. Aflatoxin contamination is routinely managed in commercial agriculture with strains of Aspergillus flavus that do not produce aflatoxins. These non-aflatoxin-producing strains competitively exclude aflatoxin producers and reshape fungal communities so that strains with the aflatoxin-producing phenotype are less frequent. This study evaluated the genetic variation within naturally occurring atoxigenic A. flavus strains from the endemic vegetative compatibility group (VCG) YV36. AF36 is a strain of VCG YV36 and was the first fungus used in agriculture for aflatoxin management. Genetic analyses based on mating-type loci, 21 microsatellite loci, and a single nucleotide polymorphism (SNP) in the aflC gene were applied to a set of 237 YV36 isolates collected from 1990 through 2005 from desert legumes and untreated fields and from fields previously treated with AF36 across the southern United States. One haplotype dominated across time and space. No recombination with strains belonging to VCGs other than YV36 was detected. All YV36 isolates carried the SNP in aflC that prevents aflatoxin biosynthesis and the mat1-2 idiomorph at the mating-type locus. These results suggest that VCG YV36 has a clonal population structure maintained across both time and space. These results demonstrate the genetic stability of atoxigenic strains belonging to a broadly distributed endemic VCG in both untreated populations and populations where the short-term frequency of VCG YV36 has increased due to applications of a strain used to competitively exclude aflatoxin producers. This work supports the hypothesis that strains of this VCG are not involved in routine genetic exchange with aflatoxin-producing strains.

Global population structure and adaptive evolution of aflatoxin‐producing fungi

“…For example, in haplotype H39, an A. parasiticus isolate from Argentina groups with global A. flavus isolates. Interestingly, haplotype H47 includes the AF36 biocontrol strain, and despite being sampled from different geographic regions (Arizona, USA; Texas, USA; Karnataka, India), all four of the nonaflatoxigenic strains in this haplotype have the same nonsense mutation in aflC (data not shown), which suggests dispersal that transcends geographic boundaries as reported in other studies (Grubisha & Cotty, 2015; Ortega‐Beltran, Grubisha, Callicott, & Cotty, 2016). Moreover, many nonaflatoxigenic A. flavus L isolates, from various localities, group with A. oryzae .…”

Biological control of aflatoxins in Africa: current status and potential challenges in the face of climate change

“…However, atoxigenic phenotypes are highly stable. This has been demonstrated over decades in agroecosystems experimentally (Grubisha and Cotty, 2015;Ortega-Beltran et al, 2016). Stability even extends over evolutionary time (Adhikari et al, 2016).…”

“…This is because hybridisation between toxigenic and atoxigenic isolates of A. flavus in laboratory crosses was shown to produce progenies with either no aflatoxin-production or lower aflatoxin production than the aflatoxin-producing parent (Olarte et al, 2012). Although Olarte et al (2012) suggest sexual recombination is common in A. flavus populations, including VCG YV36, to which AF36 belongs, after more than a decade of commercial use of AF36, dangerous recombinants have not been observed (Grubisha and Cotty, 2015). Farmers have used AF36 on more than a million hectares without adverse effect in Arizona, California, and Texas.…”

“…Atoxigenic VCGs are used as biopesticides in areas where the active ingredients are native and have co-evolved with both other VCGs in the A. flavus population and native host plants. Results to date demonstrate that the VCGs used as active ingredients have high genetic stability across vast areas (Adhikari et al, 2016;Grubisha and Cotty, 2015;Ortega-Beltran et al, 2016). As biocontrol programs to reduce aflatoxin contamination of crops are pursued globally, development of biopesticides based on atoxigenic A. flavus VCGs native to target agroecosystems need to be pursued.…”

“…Members of certain specific VCGs produce large quantities of aflatoxins (>10,000 µg/kg), while members of specific different VCGs produce low concentrations of aflatoxins and members of some VCGs produce no aflatoxins at all (Cotty, 1989;Joffe, 1969). VCGs composed entirely of individuals with no aflatoxin-producing ability are known as atoxigenic (Atehnkeng et al, 2016;Grubisha and Cotty, 2015). Inability of members of some VCGs to produce aflatoxins is linked to the presence of several lesions in the genome including deletions and single nucleotide polymorphisms or SNPs (Ehrlich and Cotty, 2004;Adhikari et al, 2016).…”