Low virulence evolves as a bet-hedging strategy in fluctuating environment

Abstract

The effect of fluctuating environmental conditions (i.e. environmental stochasticity) on the evolution of virulence has been broadly overlooked, presumably due to a lack of connection between the fields of evolutionary epidemiology and insect ecology. Practitioners of the latter have known for a long time that stochastic environmental variations can impact the population dynamics of many insects, some of which are vectors of infectious diseases. Here we investigate whether environmental stochasticity affecting a vector's life history can have an indirect impact on the evolutionarily expected virulence of the parasite, using Chagas disease as an example. We model the evolution of virulence using the adaptive dynamics framework, showing that parasite virulence should decrease when the vector's dynamics randomly change in time. The decrease is even more pronounced when environmental variations are frequent and ample. This decrease in virulence can be viewed as a bet-hedging strategy: when a parasite is at a risk of not being transmitted (e.g. because vectors are scarce), its best option is to stay in the host longerthat is, to be less virulent. Lowering virulence is thus very similar to increasing iteroparity, a well-known risk-spreading strategy, and should be expected to evolve whenever parasite transmission varies randomly in time.

Pathogen evolution in finite populations: slow and steady spreads the best

“…We analysed the effects of demographic stochasticity induced by finite population size but environmental stochasticity may also affect evolution [ 36 , 61 , 62 ]. Environmental factors are known to have dramatic impacts on pathogen transmission and it would thus be particularly relevant to expand the current framework to account for the effects of random perturbations of the environment on pathogen evolution [ 63 ]. Indeed, although we focused our analysis on the stationary distribution at the endemic equilibrium of the classical SIR model, we can also explore the effect of demographic stochasticity on the transient evolutionary dynamics away from the endemic equilibrium, e.g.…”

Ecological and Evolutionary Benefits of Temperate Phage: What Does or Doesn't Kill You Makes You Stronger

“…It is notable that temperance is expected to be favored under conditions where host population sizes are likely to fluctuate. [8,9] This may explain comparative genomic evidence that prophages are especially common in pathogenic bacteria, which undergo small population bottlenecks when establishing infections. [7] However, population size fluctuations are also common for environmental bacteria experiencing seasonal growth rates, and correspondingly temperate phages are more abundant compared to lytic phages in high latitude, seasonal marine systems than more stable tropical regions.…”

Host-parasite coevolution can promote the evolution of seed banking as a bet-hedging strategy

“…A second question regards the potential for evolving bet-hedging strategy in parasites under coevolution. Indeed bet-hedging strategies also evolve in parasites, such as low virulence in parasites transmitted by vectors in fluctuating environments (Nguyen et al 2015). Thus we speculate that unstable cycles of coevolution could also generate bet-hedging in parasites, promoting the existence of dormant survival strategies within or outside hosts.…”