Submitted to: Molecular Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/9/2009
Publication Date: 11/19/2009
Citation: Moore, G.G., Singh, R., Horn, B.W., Carbone, I. 2009. Recombination and lineage-specific gene loss in the aflatoxin gene cluster of Aspergillus flavus. Molecular Ecology 18(23):4870-4887. Interpretive Summary: Aflatoxins produced by the mold Aspergillus flavus are potent carcinogens that contaminate corn, peanuts, tree nuts, cottonseed and other crops. Aflatoxins are synthesized by a cluster of genes on one of the chromosomes. Many A. flavus strains that do not produce aflatoxins are missing genes in the gene cluster. Such nontoxic strains are often used for biological control in reducing aflatoxin contamination in crops. This research shows that sexual reproduction is likely responsible for gene loss and the inability to produce aflatoxins. The results will be important in assessing the genetic stability of nontoxic strains used for biological control.
Technical Abstract: Aflatoxins produced by Aspergillus flavus are potent carcinogens that contaminate agricultural crops. Recent efforts to reduce aflatoxin concentrations in crops have focused on biological control using nonaflatoxigenic A. flavus strains AF36 (= NRRL 18543) and NRRL 21882 (the active component of afla-guard®). However, the evolutionary potential of these strains to remain nonaflatoxigenic in nature is unknown. To elucidate the underlying population processes that influence aflatoxigenicity, we examined patterns of linkage disequilibrium (LD) spanning 21 intergenic regions in the aflatoxin gene cluster of A. flavus. We show that recombination events are unevenly distributed across the cluster in A. flavus. Six distinct LD blocks separate late pathway genes aflE, aflM, aflN, aflG, aflL, aflI and aflO, and there was no discernable evidence of recombination among early pathway genes aflA, aflB, aflC, aflD, aflR and aflS. The discordance in phylogenies inferred for the aflW/aflX intergenic region and two non-cluster regions, tryptophan synthase and acetamidase, is indicative of trans-species evolution in the cluster. Additionally, polymorphisms in aflW/aflX subdivide A. flavus strains into two distinct clades, each harboring only one of the two approved biocontrol strains. The clade with AF36 includes both aflatoxigenic and nonaflatoxigenic strains while the clade with NRRL 21882 is comprised of only nonaflatoxigenic strains and includes all strains of A. flavus with partial gene clusters. Our detection of LD blocks in partial clusters indicates that recombination may have played an important role in cluster disassembly, and multilocus coalescent analyses of cluster and non-cluster regions indicate lineage-specific gene loss in A. flavus. These results have important implications in assessing the stability of biocontrol strains in nature.