Ecological analysis of secondary metabolite production in Aspergillus spp.

Abstract

A complex and fascinating aspect of fungal development is the production of secondary metabolites. One of the best characterized secondary metabolite pathway is the aflatoxin (AF) and sterigmatocystin (ST) pathway, found in many Aspergillus spp. These compounds are among the most potent carcinogens known. Despite the significance of Aspergillus spp. as AF/ST producers and a thorough knowledge of the steps involved in the biosynthesis pathway, the effect of AF/ST on fungal fitness and the maintenance of this phenotype in natural Aspergillus populations has not been addressed. The conservation of the AF/ST biosynthesis pathway across a broad spectrum of fungal taxa implies an ecological role. As a first step in identifying the roles of AF/ST in the ecology of the fungus, we characterized an A. flavus population isolated from a corn field in Texas to look at the maintenance of the secondary metabolite phenotype in a natural population. We also examined the costs and benefits of expression of AF/ST and AF/ST pathway intermediates under a variety of environmental conditions including those mimicking field milieu using the genetic model A. nidulans. Twenty-seven genotype groups were identified based on the RFLP fingerprints generated. Both S and L strains were present in the population. There was no relationship between sclerotia size and fluorescence (used as a surrogate for AF). Our experiments with A. nidulans indicate that ST has an effect on the fitness of the fungus. Relative fitness of the A. nidulans strains generated during this study on a given substrate was robust across treatments tested. There was a definite interaction between light and substrate. The stepwise increase in fitness as more of the ST pathway was completed on media was not faithfully duplicated on corn.