Experimental evidence for stabilizing selection on virulence in a bacterial pathogen.

Area of Science:

• Evolutionary biology
• Pathogen dynamics
• Disease ecology

Background:

• The virulence-transmission trade-off hypothesis predicts pathogen fitness is maximized at intermediate virulence.
• Empirical evidence, especially during pathogen emergence, is limited.
• Testing this hypothesis requires pathogen isolates with varying virulence in naive hosts.

Purpose of the Study:

• To empirically test the virulence-transmission trade-off hypothesis at pathogen emergence.
• To determine the relationship between pathogen virulence and transmission rate.
• To identify the optimal virulence level for pathogen fitness.

Main Methods:

• Inoculated 55 Mycoplasma gallisepticum isolates of varying virulence into naive house finches.
• Collected isolates over 20 years from disease-exposed populations.
• Measured pathogen virulence and transmission rate to sentinel birds.

Main Results:

• A positive linear relationship was observed between Mycoplasma gallisepticum virulence and transmission rate.
• Pathogen isolates with intermediate virulence exhibited the highest fitness.
• The virulence-transmission relationship was not explained by pathogen load or replication rate.

Conclusions:

• Selection favors intermediate pathogen virulence during emergence in novel hosts.
• The virulence-transmission trade-off holds even when not linked to pathogen load.
• Findings challenge assumptions about the mechanisms driving pathogen evolution.