Ecology, Distribution, Toxigenicity and Diversity of Aflatoxin-Producing Fungal Communities in Maize Fields of Mexico and Interactions of these Fungi with Native Maize Landraces

Abstract

Aflatoxins are carcinogenic mycotoxins most frequently associated with the filamentous fungus, Aspergillus flavus. These potent toxins pose serious health threats and their concentrations in foods are widely regulated. Maize, a critical staple of billions, is frequently contaminated with aflatoxins. Development of commercial maize hybrids with superior resistance to aflatoxin contamination has been sought for over 30 years without success. Analyses of native maize land races (MLRs) from Mexico revealed several accessions with significant resistance to both aflatoxin contamination and fungal reproduction. Physical barriers are important components of MLRs resistance. Traditional use of MLRs may reduce human exposure to aflatoxins. MLRs may contribute resistant genes of significant value in breeding for aflatoxin resistance. In Mexico, maize is produced from<10 to over 2,000 masl. Elevation had only minor influence on community compositions of aflatoxin-producing fungi associated with maize over three years in Sonora, Mexico. Most variation in community structure occurred between years. Dominant vegetative compatibility groups (VCGs) differed among years but were detected in similar frequencies across four agroecological zones. Multiple locations and multiple years must be sampled to obtain realistic assessments of the most successful VCGs. Elevated frequencies of VCG YV150 throughout Sonora during 2006 led to investigate genetic diversity within this VCG using microsatellite loci. The 2006 increases were attributed to rapid increase of a single clone. Examination of YV150 isolates collected over 20 years in the US and Mexico revealed several haplotypes and two genetically distinct populations, which were composed of isolates containing only a population specific mating-type idiomorph. Microsatellite loci in each population were in gametic equilibrium. Gene flow between isolates with different idiomorphs was not detected. VCG YV36, to which the biocontrol agent AF36 belongs, was found to be endemic to Mexico. Microsatellite loci revealed diversity within YV36 from Mexico, but all isolates harbored the single nucleotide polymorphism in the aflatoxin polyketide synthase gene, pksA, that confers atoxigenicity to AF36. Three YV36 isolates also had deletions in pksA suggesting continued degradation. Presence of endemic YV36 isolates in Mexico may facilitate rapid regulatory approval of AF36 for use in prevention of aflatoxin contamination of maize in Mexico.