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Free keywords:
stochastic switching; origins; immune response; exclusion rule; bottleneck
Abstract:
Stochastic phenotype switching—often considered a bet hedging or risk-reducing strategy—can enhance
the probability of survival in fluctuating environments. A recent experiment provided direct evidence for
an adaptive origin by showing the de novo evolution of switching in bacterial populations propagated
under a selective regime that captured essential features of the host immune response. The regime
involved strong frequency-dependent selection realized via dual imposition of an exclusion rule and population
bottleneck. Applied at the point of transfer between environments, the phenotype common in the
current environment was assigned a fitness of zero and was thus excluded from participating in the next
round (the exclusion rule). In addition, also at the point of transfer, and so as to found the next bout of
selection, a single phenotypically distinct type was selected at random from among the survivors (the bottleneck).
Motivated by this experiment, we develop a mathematical model to explore the broader
significance of key features of the selective regime. Through a combination of analytical and numerical
results, we show that exclusion rules and population bottlenecks act in tandem as potent selective
agents for stochastic phenotype switching, such that even when initially rare, and when switching engenders
a cost in Malthusian fitness, organisms with the capacity to switch can invade non-switching
populations and replace non-switching types. Simulations demonstrate the robustness of our findings
to alterations in switching rate, fidelity of exclusion, bottleneck size, duration of environmental state
and growth rate. We also demonstrate the relevance of our model to a range of biological scenarios
such as bacterial persistence and the evolution of sex.
