Genetic and epigenetic engines of diversity in pathogenic microbes

Mutation is traditionally thought of as the raw material for evolution, but it is not the only medium upon which selection acts. Microbial pathogens deploy a panoply of strategies to adapt and thrive when exposed to diverse environmental stresses. Previously, we reviewed some of these strategies [1], and here, we update and expand on these to include RNA interference (RNAi) loss and cytoplasmic elements, epimutations, biological noise, dispensable B chromosomes, and variation in ploidy states (Fig 1). Microorganisms are capable of traversing multiple routes to reach an evolutionary peak, even those that are often deleterious from the perspective of either an unstressed microbe or a multicellular organism.