ARS | Publication request: Fragmentation of an aflatoxin-like gene cluster in a forest pathogen

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 26, 2012
Publication Date: May 13, 2013
Citation: Bradshaw, R.E., Slot, J.C., Moore, G.G., Chettri, P., De Wit, P., Ehrlich, K., Ganley, A., Olson, M.A., Rokas, A., Carbone, I., Cox, M.P. 2013. Fragmentation of an aflatoxin-like gene cluster in a forest pathogen. New Phytologist. 198:525-535.

Interpretive Summary: Aflatoxin is one of the most toxic chemicals known to man but how the molds that produce aflatoxins acquired the ability to make aflatoxins has been a puzzle to scientists. Recently, molds have been discovered that make some of the precursor molecules that are necessary for production. Precursors molecules are chemicals which usually do not accumulate in the growth medium of the mold unless one of the proteins involved in the production is defective. The mold Dothistroma spetosporum accumulates a modified precursor of aflatoxin. The genes that make the proteins necessary for formation of this modified precursor are present in 6 separate clusters whereas the genes that make aflatoxin are in a single cluster. This paper describes how the single cluster arrangement of genes may be the ancestor of the fragmented arrangement during the course of aflatoxin cluster formation. This helps to explain why certain species of A. flavus found naturally in crop-producing fields, by a partial fragmentation process, have lost the ability

Technical Abstract: Secondary metabolic pathway genes are typically clustered in fungi. An exception to this paradigm is seen for genes required for the production of dothistromin, an aflatoxin-like virulence factor produced by the pine needle pathogen Dothistroma septosporum. In contrast to the tight clustering of genes required for aflatoxin biosynthesis, dothistromin genes are dispersed and occur in six 'mini-clusters' across one .-Mb chromosome. Although many studies have addressed the origins of fungal SM gene clusters, only little attention has been paid to elucidating the cases where clustering is absent. We combined comparative genomics and population genetics approaches to elucidate evolutionary origins of the fragmented arrangement of dothistromin genes and concluded that most evidence points to fragmentation of a common large ancestral cluster. Orthologs of aflatoxin, sterigmatocystin and dothistromin (ASD) genes were found in multiple fungal genomes within both Dothideomycetes and Eurotiomycetes. Although extensive rearrangements hampered the inference of an ancestral gene order, conserved synteny of pairs of early-pathway genes such as HexA/HexB and, in some cases, co-localization of paralogs such as AvfA/OrdB, support the hypothesis that the dispersed clusters originate from a common ancestor. Among close relatives of D. septosporum, closer linkage of mini-clusters suggested intermediate fragmentation states among Dothideomycetes. Within D. septosporum, population analysis showed correspondence between blocks of allelic recombination and the mini-clusters that have been retained in the species. The most plausible explanation for these results is that fragmentation of a larger ancestral cluster occurred to give rise to the present-day dispersed arrangement of dothistromin genes seen in D. septosporum. This study challenges the widely held concept that clustering of SM genes is always advantageous for fungi and suggests that cluster fragmentation may facilitate metabolic re-tooling and subsequent niche adaptation.