Natural selection, infectious transfer, and the existence conditions for bacterial plasmids

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Natural selection, infectious transfer, and the existence conditions for bacterial plasmids

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Title: Natural selection, infectious transfer, and the existence conditions for bacterial plasmids
Author: Bergstrom, Carl T.; Lipsitch, Marc; Levin, Bruce R.
Abstract: Despite the near-ubiquity of plasmids in bacterial populations and the profound contribution of plasmid-borne genes and infectious gene transfer to the adaptation and evolution of bacteria, the mechanisms responsible for the maintenance of plasmids in bacterial populations are poorly understood. In this report, we address the question of how plasmids manage to persist over evolutionary time. Previous explanations have typically relied upon the ability of plasmids to deliver occasionally-useful genes to the right place at the right time. In contrast, we present a general mathematical proof that if (as suggested by several empirical studies) plasmids are not infectiously transmitted at a rate high enough to be maintained as genetic parasites in single populations, they will not be able to persist indefinately in these populations by carrying genes that are beneficial or sometimes beneficial to their host bacteria. Using more specific mathematical models, along with computer simulations, we illustrate a pair of mechanisms by which plasmids can be maintained indefinately even when their rates of transmission are too low to maintain them as genetic parasites. First, we show that plasmids may be maintained because of their ability to transfer locally adapted genes to newly-arriving strains bearing evolutionary innovations, and thereby preserve the local adaptations in the face of background selective sweeps. Second, we demonstrate that plasmids can persist because of their ability to shuttle intermittantly favored genes back and forth between various (non-competing) bacterial strains, ecotypes, or even species.
URI: http://hdl.handle.net/1773/2001

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