![]() ![]() While the evolution of virulence has been studied extensively during the past two decades, both using selection experiments and observations of parasites evolved in nature, how hosts respond to virulence-mediated selection is less well-explored. Indeed, even closely related viruses, such as different strains of myxoma or coronaviruses, can differ greatly in virulence. ![]() Infectious organisms vary strikingly in their level of virulence and the resulting selection they impose on hosts. Thus, by accelerating the evolution of host resistance, more virulent phages caused shorter epidemics. Using a mathematical model of our system, we show that increasing virulence strengthens selection for SRM owing to the higher costs of infection suffered by SIE immune hosts. While SIE emerged rapidly against both phages, SRM evolved faster against the high- than the low-virulence phage. The bacterial host exhibited two alternative defence strategies: (1) super infection exclusion (SIE), whereby phage-infected cells were immune to subsequent infection at the cost of reduced growth, and (2) surface receptor mutations (SRM), providing resistance to infection by preventing phage attachment. To test this, we experimentally evolved the bacterium Vibrio alginolyticus in the presence of two variants of a filamentous phage that differ in their virulence. Increased virulence can increase selection for host resistance evolution if the benefits of avoiding infection outweigh resistance costs. While the evolution of different virulence levels is well studied, the evolution of host resistance in response to different virulence levels is less understood and, at present, mainly based on observations and theoretical predictions with few experimental tests. Pathogens vary strikingly in their virulence and the selection they impose on their hosts. ![]()
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