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

Despite the near-ubiquity of plasmids in bacterial populations and the prof ound contribution of infectious gene transfer to the adaptation and evoluti on of bacteria, the mechanisms responsible for the maintenance of plasmids in bacterial populations are poorly understood. In this article, we address the question of how I,plasmids manage to pcr-sist over evolutionary time. Empirical studies suggest that plasmids are not infectiously transmitted at a rate high enough to be maintained as genetic parasites. In PART I, eve p resent a general mathematical proof that if this is the case, then plasmids will not be able to pcr sist indefinitely solely by carrying genes that ar e beneficial or sometimes beneficial to their, host bacteria. Instead, such genes should, in the long run, be incorporated into the. bacterial chromos ome. If the mobility of host-adaptive genus imposes a cost, that mobility w ill eventually be lost. In PART II, we illustrate a Pail of mechanisms bt w hich I,plasmids can be maintained indefinitely when when their rates of tra nsmission are coo lo for them to be genetic parasites. First, plasmids may. persist because they can transfer locally adapted genes to newly arriving strains bearing evolutionary innovations, and thereby preserve the local ad aptations in the face of background selective sweeps. Second, plasmids may persist because of their ability to shuttle intermittenly favored genes bac k and forth between various (noncompeting) bacterial strains, ccotypes, or even species.