A cysteine/methionine auxotroph of the opportunistic fungus Aspergillus flavus is associated with host-range restriction: a model for emerging diseases.

The evolution of host specialization in pathogens is a topic of considerable interest, particularly since it can represent a decisive step in the emergence of infectious diseases. Aspergillus flavus is an opportunistic fungus capable of infecting a wide variety of hosts, including plants, insects and mammals, although with low virulence. Here the derivation of an A. flavus strain that exhibits severe host restriction is reported. This strain exhibited a severe diminution or a complete lack of conidial production on a variety of standard agar media and on various plant species. However, it retained its ability to infect insects from various orders and to re-emerge from and adequately conidiate on the insect cadavers as a culmination of the pathogenic life cycle. This strain, demonstrating insect-dependent conidiation, was discovered to be a cysteine/methionine auxotroph due to an inability to reduce sulfate to sulfite. However, other A. flavus auxotrophs tested for plant and insect host range failed to show insect-dependent conidiation. An association between this specific auxotroph and a decreased host range is shown, emphasizing the role of nutrition in the host-pathogen relationship with respect to host restriction and evolution towards obligate pathogenesis.