Phylogenetic Comparison of Swainsonine Biosynthetic Gene Clusters among Fungi

Swainsonine is a cytotoxic alkaloid produced by fungi. Genome sequence analyses revealed that these fungi share an orthologous gene cluster, SWN, necessary for swainsonine biosynthesis. To investigate the SWN cluster, the gene sequences and intergenic regions were assessed in organisms containing swnK, which is conserved across all fungi that produce swainsonine. The orders of fungi which contained orthologous swainsonine genes included Pleosporales, Onygenales, Hypocreales, Chaetothyriales, Xylariales, Capnodiales, Microthyriales, Caliciales, Patellariales, Eurotiales, and a species of the Leotiomycetes. SwnK and swnH2 genes were conserved across all fungi containing the SWN cluster; in contrast, swnT and swnA were found in a limited number of fungi containing the SWN cluster. The phylogenetic data suggest that in some orders that the SWN cluster was gained once from a common ancestor while in other orders it was likely gained several times from one or more common ancestors. The data also show that rearrangements and inversions of the SWN cluster happened within a genus as species diverged. Analysis of the intergenic regions revealed different combinations and inversions of open reading frames, as well as absence of genes. These results provide evidence of a complex evolutionary history of the SWN cluster in fungi.