*Carbone1, Ignazio; Jakobek1, Judy L.; Ramirez-Prado1, Jorge H.; Horn2, Bruce W.
The evolution of aflatoxin biosynthesis. In: International Meeting on "Population and Evolutionary Biology of Fungal Symbionts", Ascona, Switzerland, 2007. AB-40.
The biosynthesis of aflatoxin (AF) involves over 20 enzymatic reactions in a complex polyketide pathway that converts acetate and malonate to the intermediates sterigmatocystin (ST) and O-methylsterigmatocysin (OMST), the respective penultimate and ultimate precursors of AF. Although ST, OMST, and AF are potent carcinogens in animals, the biological and evolutionary significance of these compounds in fungi is unknown. The goal of this work is to use a combination of macro- and micro-evolutionary approaches to identify the forces giving rise to and maintaining aflatoxin genes in clusters. These inferences have significant implications for biocontrol strategies, which introduce nonaflatoxigenic strains into native populations.
Our systematic searches for aflatoxin gene cluster homologs followed by correlation analysis of gene duplicates across five Aspergillus genomes revealed seven putative gene modules. Genes in modules show a similar pattern of duplication across species, which is consistent with vertical transmission and rules out horizontal transfer of aflatoxin genes. To determine whether gene modules were recruited from other genomic locations or reorganized from an ST ancestor we examined molecular variation in the cluster for a population of A. parasiticus. This analysis revealed five distinct recombination blocks each showing a pattern of significant linkage disequilibrium and shared evolutionary history, which is consistent with decent and reorganization of gene modules from an ST ancestor. We inferred gene genealogies for each block and for two noncluster regions and found an approximately twofold deeper coalescence for cluster genealogies compared to noncluster genealogies, our internal standard of neutral evolution. Significantly deeper cluster genealogies are indicative of balancing selection in the AF cluster and are further corroborated by the existence of trans-species polymorphisms and common haplotypes in the cluster for several species in Aspergillus section Flavi. Estimates of Ka/Ks across the entire cluster also provide evidence of selection for new species and chemotypes, in this case, OMST and AF from an ST ancestor, and indicate a possible role of chemotypes in ecological adaptation and speciation.
Keywords: gene duplication, recombination blocks, linkage disequilibrium, compatibility, coalescent, Aspergillus
*Institution: 1Center for Integrated Fungal Research, Department of Plant Pathology, North Carolina State University, Raleigh, NC 27695; 2National Peanut Research Laboratory, USDA, ARS, Dawson, GA 39842
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