Submitted to: Mycotoxin Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/15/2006
Publication Date: 10/1/2006
Citation: Ehrlich, K.C. 2006. Evolution of the Aflatoxin Gene Cluster. Mycotoxin Research. 22:9-15. Interpretive Summary: Aspergillus species produce the carcinogenic aflatoxins on important cereal crops, but why they do this remains a mystery. In this report, we suggest that originally the fungus produced brightly colored compounds that attracted insects. This allowed the fungi to be dispersed to new places. After millions of years, the original purpose became less important and the fungi needed to defend against hostile competitors or to colonize plants. They did this by adding additional genes to the original genetic machinery. Such changes are called adaptive evolution. The fungi then probably were more successful at colonizing plants and fending off predators. This logical progression in evolutionary development fits the known chemistry of the aflatoxin precursor molecules.
Technical Abstract: Why Aspergillus species produce aflatoxin remains an unsolved question. In this report, we suggest that evolution of the aflatoxin biosynthesis gene cluster has been a multistep process. More than 300 million years ago, a primordial cluster of genes allowed production of anthraquinones that may have served as insect attractants to facilitate spore dispersal. Later adaptive evolutionary steps introduced genes into the cluster that encoded enzymes associated with fungal virulence. These genes may have allowed the otherwise saprophytic fungi to be better able to colonize living plants. Later, genes for production of aflatoxins B1 and G1 were added to the basal cluster. Loss of the ability to produce aflatoxin G1 occurred with the divergence of A. flavus, a species that, perhaps, was more successful than its ancestors at colonizing plants. This logical progression in evolutionary development of the aflatoxin biosynthetic cluster fits the phylogenetic data, as well as known chemical reactivity of the initially formed anthraquinone polyketide metabolites.