Fight not flight: parasites drive the bacterial evolution of resistance, not avoidance

Abstract

AbstractIn the face of ubiquitous threats from parasites, hosts often evolve strategies to resist infection or to altogether avoid contact with parasites. At the microbial scale, bacteria frequently encounter viral parasites, bacteriophages. While bacteria are known to utilize a number of strategies to resist infection by phages, and can physically navigate their environment using complex motility behaviors, it is unknown whether bacteria evolve avoidance of phages. In order to answer this question, we combined experimental evolution and mathematical modeling. Experimental evolution of the bacteriumPseudomonas fluorescensin environments with differing spatial distributions of the phage Phi2 revealed that the host bacteria evolved resistance depending on parasite distribution and infectivity, but did not evolve dispersal to avoid parasite infection. Simulations using parameterized mathematical models of bacterial growth and swimming motility showed that this is a general finding: while increased dispersal is adaptive in the absence of parasites, in the presence of parasites that fitness benefit disappears and resistance becomes adaptive, regardless of the spatial distribution of parasites. Together, these experiments suggest that parasites should rarely, if ever, drive the evolution of bacterial avoidance via dispersal.