An experimental system using a bacteriophage virus and its bacterial host was used to test the hypothesis that increased virulence evolves in response to selection for increased transmission. This model assumes that virulence is correlated with traits that increase fecundity of the pathogen. Transmission rates to new susceptible hosts were experimentally manipulated, varying the relative intensity of selection against virulence. Phage with relatively high transmission rates benefit from increased phage production (fecundity) because their progeny represent a larger proportion of the population at the time of re-infection, while phage with low transmission rates increase their frequency in the population by allowing increased growth and cell division of the host. As predicted by the model, phage transmitted at high rates evolved higher virulence (indicated by a slower host cell growth rate) and significantly higher fecundity (determined by phage titers) than phage subjected to low transmission rates. However, other assays of virulence, such as phage impact on host intrinsic growth rate, and long-term survival of the infected host, showed no significant difference whether infected with high or low transmission phage indicating that measures of virulence are sensitive to the method of assay. These results reveal the complexity of the dynamics of virulence and underscore the need for more experimental data to tease apart this complexity to gain a better understanding of the evolution of virulence.
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