Mycotoxin genetics and gene clusters

Fungi produce low molecular weight secondary metabolites such as antibiotics and mycotoxins. Antibiotics cure diseases whereas mycotoxins cause diseases in plants, animals and human beings. Species such as Aspergillus, Fusarium, Penicillium and Stachybotrys are known to produce mycotoxins that accumulate in processed foods and feeds, although the incidence of infection occurs before processing, during the active growth of the organism. Among the mycotoxins, aflatoxins produced by Aspergillus flavus and A. parasiticus have been extensively studied at the molecular level. A complex biosynthetic pathway involving sixteen steps is mediated by individual major genes. These fungi have eight linkage groups, but the aflatoxin/sterigmatocystin (AF/ST) metabolic pathway genes have been mapped to only three linkage groups; ten of them belong to linkage group VII, and one of each to linkage group II and VIII. These genes are involved in both the regulatory and biosynthetic pathways and are clustered on the respective chromosomes. Clustering of genes in fungi indicates an evolutionary trend among genes that orchestrate gene function. Being linked together they segregate 'as a unit', thereby conferring a selective advantage to the organism. The evolution of gene clusters takes place through vertical or horizontal gene transfer. In fungi, horizontal gene transfer is most effective. Functionally, the mechanism of evolution of mycotoxin gene clusters in fungi seems to be similar to the evolution of a super-gene. The possible implications of evolutionary parallelism of gene clusters and super-genes is briefly explored.