title: Repression/depression of conjugative plasmids and their influence on the mutation-selection balance in static environments.
abstract: We study the effect that conjugation-mediated Horizontal Gene Transfer (HGT) has on the mutation-selection balance of a population in a static environment. We consider a model whereby a population of unicellular organisms, capable of conjugation, comes to mutation-selection balance in the presence of an antibiotic, which induces a first-order death rate constant [Formula: see text] for genomes that are not resistant. We explicitly take into consideration the repression/de-repression dynamics of the conjugative plasmid, and assume that a de-repressed plasmid remains temporarily de-repressed after copying itself into another cell. We assume that both repression and de-repression are characterized by first-order rate constants [Formula: see text]and [Formula: see text], respectively. We find that conjugation has a deleterious effect on the mean fitness of the population, suggesting that HGT does not provide a selective advantage in a static environment, but is rather only useful for adapting to new environments. This effect can be ameliorated by repression, suggesting that while HGT is not necessarily advantageous for a population in a static environment, its deleterious effect on the mean fitness can be negated via repression. Therefore, it is likely that HGT is much more advantageous in a dynamic landscape. Furthermore, in the limiting case of a vanishing spontaneous de-repression rate constant, we find that the fraction of conjugators in the population undergoes a phase transition as a function of population density. Below a critical population density, the fraction of conjugators is zero, while above this critical population density the fraction of conjugators rises continuously to one. Our model for conjugation-mediated HGT is related to models of infectious disease dynamics, where the conjugators play the role of the infected (I) class, and the non-conjugators play the role of the susceptible (S) class.
authors.forename: Yoav
authors.initials: Y
authors.lastname: Atsmon-Raz
authors.affiliation: Department of Chemistry, Ben-Gurion University of the Negev, Beér-Sheva, Israel.
authors.forename: Yoav
authors.initials: Y
authors.lastname: Raz
authors.affiliation: Department of Chemistry, Ben-Gurion University of the Negev, Beér-Sheva, Israel.
authors.forename: Emmanuel David
authors.initials: ED
authors.lastname: Tannenbaum
mesh_terms: D004777:Environment; D019143:Evolution, Molecular; D022761:Gene Transfer, Horizontal; D008957:Models, Genetic; D009154:Mutation; D010957:Plasmids / Q000235:genetics*; D012641:Selection, Genetic