| Interactions between bacteria and conjugative plasmids were used to address two distinct questions. (1) The first question concerns the relationship between genetic variability and rates of adaptation. Can plasmid-mediated recombination be used to generate novel genotypes, and thereby accelerate the rate of adaptation in otherwise asexual populations of bacteria? To test this hypothesis, genetically distinct Hfr (high frequency recombination) cells were periodically introduced into experimental populations of Escherichia coli. Recombinant genotypes became numerous (or even fixed) in all treatment populations, but, surprisingly, these recombinants showed no significant increase in fitness relative to the recipient's ancestor, nor relative to non-recombining control populations. One possibility is that fitness measurements do not accurately reflect complex selection dynamics, such as frequency-dependent and even nontransitive interactions, brought on by recombinant genotypes. In at least one recombination treatment population, further experiments confirmed that frequency-dependent selection allows recombinant genotypes to coexist. This stable coexistence of two genotypes on a single limiting resource was promoted by a cross-feeding interaction, but some evidence of a demographic tradeoff was also found. (2) The second question concerns the evolution of virulence in pathogens and other infectiously transmitted elements. Does host density influence the evolution of plasmid virulence and mode of transmission? This hypothesis was tested by allowing associations of E. coli and plasmid pB15 to evolve in replicated environments with different inputs of susceptible hosts. The plasmids' effects on host fitness and their conjugation rates were both observed to evolve, with a consistent tradeoff between rates of horizontal and vertical transmission. However, manipulations of host density had no effect on the evolution of plasmid virulence and mode of transmission. One possible explanation is that conjugation of pB15 does not behave according to mass-action, and some evidence in support of this possibility is presented. |
