Distribution, Function, and Evolution of a Gene Essential for Trichothecene Toxin Biosynthesis in .

Trichothecenes are terpenoid toxins produced by species in 10 fungal genera, including species of . The trichothecene biosynthetic gene () cluster typically includes the gene, which encodes a terpene synthase that catalyzes formation of trichodiene, the parent compound of all trichothecenes. The two species, and , that have been examined are unique in that is located outside the cluster in a genomic region that does not include other known genes. In the current study, analysis of 35 species representing a wide range of the phylogenetic diversity of revealed that 22 species had , but only 13 species had both and the cluster. was not located in the cluster in any species. Using complementation analysis of a deletion mutant, we demonstrated that some homologs from species that lack a cluster are functional, but others are not. Phylogenetic analyses suggest that was under positive selection following its divergence from homologs in other fungi but before species began diverging from one another. We propose two models to explain these diverse observations. One model proposes that the location of outside the cluster resulted from loss of from the cluster in an ancestral species followed by reacquisition horizontal transfer. The other model proposes that in species that have a functional but lack the cluster, trichodiene production provides a competitive advantage.