Ecological conditions determine extinction risk in co-evolving bacteria-phage populations.

Rosanna C T Wright,Ellie Harrison,Michael A Brockhurst

doi:10.1186/s12862-016-0808-8

Abstract

BackgroundAntagonistic coevolution between bacteria and their viral parasites, phage, drives continual evolution of resistance and infectivity traits through recurrent cycles of adaptation and counter-adaptation. Both partners are vulnerable to extinction through failure of adaptation. Environmental conditions may impose unequal abiotic selection pressures on each partner, destabilising the coevolutionary relationship and increasing the extinction risk of one partner. In this study we explore how the degree of population mixing and resource supply affect coevolution-induced extinction risk by coevolving replicate populations of Pseudomonas fluorescens SBW25 with its associated lytic phage SBW25Ф2 under four treatment regimens incorporating low and high resource availability with mixed or static growth conditions.ResultsWe observed an increased risk of phage extinction under population mixing, and in low resource conditions. High levels of evolved bacterial resistance promoted phage extinction at low resources under both mixed and static conditions, whereas phage populations could survive when phage susceptible bacterial genotypes rose to high frequency.ConclusionsThese findings demonstrate that phage extinction risk is influenced by multiple abiotic conditions, which together act to destabilise the bacteria-phage coevolutionary relationship. The risk of coevolution-induced extinction is therefore dependent on the ecological context.

Highlights

Antagonistic coevolution between bacteria and their viral parasites, phage, drives continual evolution of resistance and infectivity traits through recurrent cycles of adaptation and counter-adaptation
We explore how two ecological variables altering the intensity of bacteria phage coevolution, population mixing and resource level, affect the extinction risk of coevolving bacteria-phage populations in the Pseudomonas fluorescens SBW25 – lytic phage SBW25 2 model system [21]
Out of six replicate populations per treatment, phage extinction was observed in all low resource replicates under population mixing and half of those in static conditions

Summary

Introduction

Antagonistic coevolution between bacteria and their viral parasites, phage, drives continual evolution of resistance and infectivity traits through recurrent cycles of adaptation and counter-adaptation Both partners are vulnerable to extinction through failure of adaptation. Microbial communities are subject to a broad range of biotic and abiotic selection pressures and are capable of rapid evolutionary change in response [1] Antagonistic interactions, such as between bacteria and bacteriophages (phages), in particular drive continuous evolutionary change through recurrent cycles of adaptation and counter-adaptation, known as Red Queen coevolution [2–4]. This process may be driven by directional selection favouring ever broader ranges of resistance and infectivity achieved by recurrent selective sweeps (Arms Race dynamics (ARD)) [5], or proceed by sustained allele frequency oscillations driven by negative frequencydependent selection (Fluctuating Selection dynamics (FSD)) [6]. Strong directional selection under ARD coevolution may exacerbate asymmetries in evolutionary potential between the coevolving partners

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Conclusion