Fight not flight: Parasites drive the bacterial evolution of resistance, not escape

In the face of ubiquitous threats from parasites, hosts can evolve
strategies to resist infection or to altogether avoid parasitism, for
instance by avoiding behavior that could expose them to parasites or by
dispersing away from local parasite threats. 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 have the capacity to avoid moving towards
phage-infected cells, it is unknown whether bacteria can evolve dispersal
to escape from phages. In order to answer this question, we combined
experimental evolution and mathematical modeling. Experimental evolution
of the bacterium Pseudomonas fluorescens in environments with differing
spatial distributions of the phage Phi2 revealed that the host bacteria
evolved resistance depending on parasite distribution, but did not evolve
dispersal to escape 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 escape
via dispersal.