Abstract:
To study emerging diseases, I employed a model pathogen-host system involving
infections of insect larvae with the opportunistic fungus Aspergillus flavus, providing insight into
three mechanisms ofpathogen evolution namely de novo mutation, genome decay, and virulence
factoracquisition In Chapter 2 as a foundational experiment, A. flavus was serially propagated
through insects to study the evolution of an opportunistic pathogen during repeated exposure to a
single host. While A. flavus displayed de novo phenotypic alterations, namely decreased
saprobic capacity, analysis of genotypic variation in Chapter 3 signified a host-imposed
bottleneck on the pathogen population, emphasizing the host's role in shaping pathogen
population structure. Described in Chapter 4, the serial passage scheme enabled the isolation of
an A. flavus cysteine/methionine auxotroph with characteristics reminiscent of an obligate insect
pathogen, suggesting that lost biosynthetic capacity may restrict host range based on nutrient
availability and provide selection pressure for further evolution. As outlined in Chapter 6,
cysteine/methionine auxotrophy had the pleiotrophic effect of increasing virulence factor
production, affording the slow-growing auxotroph with a modified pathogenic strategy such that
virulence was not reduced. Moreover in Chapter 7, transformation with a virulence factor from a
facultative insect pathogen failed to increase virulence, demonstrating the necessity of an
appropriate genetic background for virulence factor acquisition to instigate pathogen evolution.
