Dothistromin genes at multiple separate loci are regulated by AflR.

In fungi, genes involved in the production of secondary metabolites are generally clustered at one location. There are some exceptions, such as genes required for synthesis of dothistromin, a toxin that is a chemical analog of the aflatoxin precursor versicolorin A and made by the pine needle pathogen Dothistroma septosporum. The availability of the D. septosporum genome sequence enabled identification of putative dothistromin genes, including an ortholog of the aflatoxin regulatory gene AflR, and revealed that most of the genes are spread over six separate regions (loci) on chromosome 12 (1.3 Mb). Here we show that levels of expression of the widely dispersed genes in D. septosporum are not correlated with gene location with respect to their distance from a telomere, but that AflR regulates them. The production of dothistromin by D. septosporum in which the AflR gene was knocked out (ΔDsAflR) was drastically reduced, but still detectable. This is in contrast to orthologous ΔAflR mutants in Aspergillus species that lack any aflatoxin production. Expression patterns in ΔDsAflR mutants helped to predict the complete set of genes involved in dothistromin production. This included a short-chain aryl alcohol dehydrogenase (NorB), which is located on chromosome 11 rather than chromosome 12, but was 24-fold down regulated in ΔDsAflR. An orthologous set of dothistromin genes, organized in a similar fragmented cluster arrangement to that seen in D. septosporum, was found in the closely related tomato pathogen Cladosporium fulvum even though this species does not produce dothistromin. In C. fulvum, pseudogenization of key biosynthetic genes explains the lack of dothistromin production. The fragmented arrangement of dothistromin genes provides an example of coordinated control of a dispersed set of secondary metabolite genes; it also provides an example where loss of dothistromin production might have allowed adaptation to a new pathogenic lifestyle.

Fragmentation of an aflatoxin-like gene cluster in a forest pathogen.

Plant pathogens use a complex arsenal of weapons, such as toxic secondary metabolites, to invade and destroy their hosts. Knowledge of how secondary metabolite pathways evolved is central to understanding the evolution of host specificity. The secondary metabolite dothistromin is structurally similar to aflatoxins and is produced by the fungal pine pathogen Dothistroma septosporum. Our study focused on dothistromin genes, which are widely dispersed across one chromosome, to determine whether this unusual distributed arrangement evolved from an ancestral cluster. We combined comparative genomics and population genetics approaches to elucidate the origins of the dispersed arrangement of dothistromin genes over a broad evolutionary time-scale at the phylum, class and species levels. Orthologs of dothistromin genes were found in two major classes of fungi. Their organization is consistent with clustering of core pathway genes in a common ancestor, but with intermediate cluster fragmentation states in the Dothideomycetes fungi. Recombination hotspots in a D. septosporum population matched sites of gene acquisition and cluster fragmentation at higher evolutionary levels. The results suggest that fragmentation of a larger ancestral cluster gave rise to the arrangement seen in D. septosporum. We propose that cluster fragmentation may facilitate metabolic retooling and subsequent host adaptation of plant pathogens.