Aspergillus flavus is a widely distributed saprotrophic fungus and the most common causal agent of aflatoxin contamination on food crops. Despite the notoriety of A. flavus to produce aflatoxin, strains vary greatly in their levels of toxin production and atoxigenic strains are not uncommon in nature. Within Aspergillus flavus, there are two distinct morphologies, namely morphotypes: the S morphotype produces numerous small sclerotia and limited conidiospores and the L morphotype produces relatively fewer large sclerotia and abundant conidiospores. S strains are consistently toxigenic, whereas L strains vary greatly in toxin production ranging from atoxigenic to highly toxigenic. Morphological differences between the two morphotypes suggest the production of abundant sclerotia in S strains is advantageous for long-term survival in the soil, whereas the production of abundant conidia in the L strains is advantageous for aerial dispersal to the phyllosphere. The selection pressure to maintain aflatoxin production may be higher in the soil compared to the phyllosphere, leading to the retention of consistently toxigenic S strains in the soil. In contrast, the selection pressure may be lower in the phyllosphere, allowing the survival of atoxigenic or low toxin-producing L strains in that niche. To develop hypotheses on differential niche adaption, we compared the genomes of three S strains and three L strains to identify structural differences and genes unique to each morphotype. A 530 kb inversion was identified between the morphotypes that affects a secondary metabolite gene cluster and a cutinase gene on the margins. The L strain genomes contained 10 deletions, many of which involve secondary metabolite genes. Each morphotype had unique genes that play a role in carbon/nitrogen metabolism, secondary metabolism, and antimicrobial defense. These findings indicate that the genomes of the two morphotypes differ beyond developmental genes, and that they may have diverged as they adapted to their respective niches.
Full conference title:
- Asperfest 14 (2017)