Coevolutionary arms races between bacteria and bacteriophage

Abstract

We propose a computational and theoretical framework for analyzing rapid coevolutionary dynamics of bacteriophage and bacteria in their ecological context. Bacteriophage enter host cells via membrane-bound surface receptors often responsible for nutrient uptake. As such, a selective pressure will exist for the bacteria to modify its receptor configuration and, in turn, for the phage to modify its tail fiber. A mathematical model of these trait adaptations is developed by using the framework of adaptive dynamics. Host strains differ in their efficiency of resource uptake and resistance to phage, whereas phage strains differ in their host preference for adsorption. We solve the evolutionary ecology model and find the conditions for coevolutionary branching and relevant dimensionless parameters leading to distinct quasispecies. We confirm these calculations using stochastic Monte Carlo simulations of populations evolving in a chemostat with fixed washout rate and inflow resource density. We find that multiple quasispecies of bacteria and phage can coexist in a homogeneous medium with a single resource. When diversification occurs, quasispecies of phage adsorb effectively to only a limited subset of the total number of quasispecies of bacteria, i.e., functional differences between quasispecies arise endogenously within the evolutionary ecology framework. Finally, we discuss means to relate predictions of this model to experimental studies in the chemostat, using the model organisms Escherichia coli and the virulent strain of phage.ver 40 years ago the influential ecologist G. E. Hutchinson proposed ''the paradox of the plankton'' (1). Many phytoplankton species are functionally equivalent and live in well mixed pelagic environments, or so the paradox contends. As such, their diversity should be limited by the inevitable competitive advantage possessed by a small number of types. However, phytoplankton diversity is observed to be many orders of magnitude greater in natural samples (2) than predicted by the theory of competitive exclusion (3). This gap between theory and empirical data has been debated widely in the literature, and Hutchinson himself offered a number of ecological scenarios that purport to resolve the paradox (1). These scenarios include spatial heterogeneity in the environment, symbiotic interactions and predation, temporal switching in competitive strategies, as well as the catalytic effect of predation. These scenarios constitute a suite of possible approaches for resolving the paradox of the plankton as well as the fundamental question: why are there so many species (4, 5)? The accelerating scientific interest in studies of biodiversity in the intervening decades reflects the importance of this (increasingly practical) problem in evolutionary ecology.In this paper, we develop a quantitative framework to address aspects of the generation and maintenance of diversity in microbial systems. Typical aquatic samples contain bacterial densities on the order of 10 7 ml Ϫ1 (6) and there is evidence that vi...

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A multitrophic model to quantify the effects of marine viruses on microbial food webs and ecosystem processes

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